REVIEW ARTICLE

Linking drought-resistance mechanisms to drought

avoidance in upland rice using a QTL approach:

progress and new opportunities to integrate stomatal and

mesophyll responses

Adam H. Price1,5, Jill E. Cairns1, Peter Horton2, Hamlyn G. Jones3 and Howard Griffiths4

1Department of Plant and Soil Sciences, University of Aberdeen, Aberdeen AB24 3UU, UK2Department of Molecular Biology and Biotechnology, University of Sheffield, Sheffield S10 2TN, UK3School of Life Sciences, University of Dundee, Dundee DD1 5EH, UK4Department of Plant Sciences, University of Cambridge, Cambridge CB2 3EA, UK

Received 20 August 2001; Accepted 10 December 2001

Abstract

The advent of saturated molecular maps promisedrapid progress towards the improvement of cropsfor genetically complex traits like drought resistancevia analysis of quantitative trait loci (QTL). Progresswith the identification of QTLs for drought resistance-related traits in rice is summarized here with theemphasis on a mapping population of a crossbetween drought-resistant varieties Azucena andBala. Data which have used root morphological traitsand indicators of drought avoidance in field-grownplants are reviewed, highlighting problems anduncertainties with the QTL approach. The contribu-tion of root-growth QTLs to drought avoidanceappears small in the experiments so far conducted,and the limitations of screening methodologies andthe involvement of shoot-related mechanisms ofdrought resistance are studied. When compared toAzucena, Bala has been observed to have highlysensitive stomata, does not roll its leaves readily, hasa greater ability to adjust osmotically, slows growthmore rapidly when droughted and has a lower water-use efficiency. It is also a semi-dwarf variety andhence has a different canopy structure. There is aneed to clarify the contribution of the shoot todrought resistance from the level of the biochemistryof photosynthesis through stomatal behaviour andleaf anatomy to canopy architecture. Recentadvances in studying the physical and biochemical

processes related to water use and drought stressoffer the opportunity to advance a more holisticunderstanding of drought resistance. These includethe potential use of infrared thermal imaging to studyenergy balance, integrated and online stable iso-tope analysis to dissect processes involved in carbondioxide fixation and water evaporation, and leaffluorescence to monitor photosynthesis and photo-chemical quenching. Justification and a strategyfor this integrated approach is described, which hasrelevance to the study of drought resistance inmost crops.

Key words: Infrared thermography, Oryza sativa, photo-

synthesis, root growth, stable isotopes, water-use efficiency.

Introduction

Since the development of molecular markers first allowedthe construction of saturated linkage maps (Botsteinet al., 1980), it has been clear that the technology ofquantitative trait locus (QTL) analysis could be usefullyemployed to analyse the genetics of complex traits. Byproducing mapping populations based on crosses ofparental varieties contrasting for the trait of interest, itshould prove possible to identify which parts of thegenome improve the trait. It should also prove possibleto identify the genomic regions that influence component

5 To whom correspondence should be addressed. Fax: [44 (0)1224 272703. E-mail: pss@abdn.ac.uk

� Society for Experimental Biology 2002

Journal of Experimental Botany, Vol. 53, No. 371, pp. 989–1004, May 2002

traits theoretically linked to the main trait and approx-imately quantify the contribution of these componenttraits. Once achieved, the targeting of genomic regionsfor varietal improvement would be possible throughmarker-assisted selection (Stuber et al., 1999). CombiningQTL mapping with other biotechnological advancessuch as physical mapping and whole genome sequencingopens the opportunity for the identification of theresponsible gene(s) by map-based landing (Tanksleyet al., 1995) or a candidate gene approach (Pfiegeret al., 2001).

The application of the QTL approach to the geneticdissection of drought resistance in rice (Oryza sativa L.)is certain to be difficult. The challenge is worthwhileconsidering the very substantial impact of drought onrice production (Evenson et al., 1996) and the relativelylimited progress previously made in improving thedrought resistance of high yielding varieties (Price andCourtois, 1999; Fukai and Cooper, 1995; Nguyen et al.,1997). There are two main reasons for the slow progressin this area. Firstly, the phenomenon of drought is com-plex in itself. To a plant, drought is an interactionbetween precipitation, evapotranspiration, irradiation,soil physical properties (including the soil hydrologicaland strength properties), soil nutrient availability, andbiological interaction with pests, pathogens and neigh-bouring riceuweed plants). This makes it difficult to definewith any precision a ‘typical drought’ for screening pur-poses which is likely to represent the conditions of thetarget environment. Secondly, rice germplasm displays adiverse range of sometimes genetically complex mech-anisms of drought resistance, including mechanisms ofdrought escape (short duration), drought avoidance(e.g. deep rooting) and drought tolerance (e.g. osmoticadjustment). Because the drought phenomenon and themechanisms of drought resistance interact (i.e. the samemechanism of drought resistance will not work for alldrought environments) it will naturally prove difficultto identify regions of the rice genome which contributeto resistance to drought in a broad range of droughtenvironments. The ultimate goal of breeders is to identifyQTLs that increase yield under drought or at leastincrease yield stability under drought. Since yield in theabsence of drought is itself a complex trait influenced bymany component traits, and since under drought, thesetraits will interact with the drought environment and theresistance mechanisms, the true complexity of breedingfor yield under drought is clear. In other cereal species,good progress has been made where a single droughtresistance mechanism has been shown to be of greatimportance. For example, there is great promise in workon QTL mapping anthesis to silking interval in maize(Ribaut et al., 1997; Frova et al., 1999) and the stay greentrait in sorghum (Crasta et al., 1999). Rice has provedmore difficult.

Mechanisms of drought resistance in rice

In this paper, drought adaptation is defined followingLevitt, which distinguishes drought resistance fromdrought escape (flowering to complete life cycle beforedrought), divides drought resistance into drought (stress)avoidance (maintenance of tissue water potential) anddrought (stress) tolerance and further divides droughttolerance into dehydration avoidance and dehydrationtolerance (Levitt, 1980). Most of the traits that contributepotentially to the drought resistance of rice have beenthoroughly reviewed (Fukai and Cooper, 1995; Nguyenet al., 1997, Price and Courtois, 1999). A brief summaryis given below, with greater detail given to the study ofroots, given their demonstrated importance in droughtresistance and their contribution to other potential con-straints such as nutrient deficiency. One important pointto note here is that many of the observed responses todrought in plants obey Le Chatelier’s Principle (whensubject to a perturbation a system tends to respond insuch a way as to minimize the effect of the perturbation).For example, drought responses such as stomatal closure,leaf rolling, enhanced root growth, enhanced ABA pro-duction, and so on act to minimize water deficits. It fol-lows that in any particular case one of these responsescould indicate particular sensitivity to drought or mayindicate a highly responsive drought tolerance mech-anism. It is often difficult or impossible to distinguishthese two and this must be remembered when choosingindicators of drought stress.

The root system

Since a plant obtains its water and mineral requirementsfrom its roots and the availability of these resourcesoften imposes a limit to plant productivity, it is difficult tooverstate the importance of roots to plant productivity.Root development is fundamentally involved in theresponse to many plant stresses, in particular, droughtand mineral deficiency. The possession of a deep andthick root system which allows access to water deep in thesoil profile is considered crucially important in determin-ing drought resistance in upland rice and substantialgenetic variation exists for this (Ekanayake et al., 1985b;Fukai and Cooper, 1995; O’Toole, 1982; Yoshida andHasegawa, 1982). This trait may be less important inrainfed lowland rice, where hardpans may severely restrictroot growth. Here, the ability to penetrate a hard layer isconsidered important and genetic variation in the abilityto penetrate a layer of hard wax has been demonstrated(Yu et al., 1995). This trait may also be useful in uplandrice where high penetration resistance may limit root-ing depth and where soils will harden as they dry. It isimportant to note two points here. First, the penetrationof roots through uniform hard layers like wax is probablyachieved through the possession of large root diameter

990 Price et al.

which resist buckling (Cook et al., 1997), while when theimpedance is due to a coarse textured sandy or stonyhorizon, it may be that thin roots would penetrate moreeasily. Second, the investment of carbon in a deep rootsystem may have a yield implication because of lostcarbon allocation to the shoot.

It is vitally important to appreciate that root growth isprofoundly influenced by the environment. Adverse con-ditions (chemical or physical) directly inhibit root growth(e.g. low water potential or highulow temperature). Bio-logical factors in the root environment can also havea major influence on root distribution, but they aregenerally poorly understood. Particularly important isthe presence of root feeding organisms such as nemat-odes, termites, mites, and aphids that can severely reduceroot proliferation or rooting depth and thereby affectdrought resistance (Audebert et al., 2000). The shootenvironment can also indirectly influence root growtheither via carbon supply or signalling processes (e.g. lightinterception, water status, nutrient status). It has beensuggested that plants respond to shifts in resource supplyby allocating carbon to the organ involved in captur-ing the limited resource (Thornley, 1972; Dewar, 1993).When light is limiting, plants invest in shoot biomass.When nitrogen is limiting, they invest in root production.At the mechanistic level, theories implicating sucrosesupply (Farrar, 1992), hormonal action (Jackson, 1993)or a combination of both (Van der Werf and Nagel, 1996)have been advanced to explain this phenomenon. It seemsclear that the root tip is an important component inthe sensing and signalling of environmental cues to thewhole plant (Aiken and Smucker, 1996). Responses todrought or temperature are probably complex due to amultiplicity of physical or biochemical processes directlyaffected. At the genetic level, the response of roots to theenvironment is poorly understood because roots areintrinsically difficult to study, particularly in the naturalenvironment.

Shoot-related traits of drought resistance

Several mechanisms of drought resistance are associatedwith the shoots of rice. These are listed in Table 1, whichincludes references for the evidence of genetic variationin each trait. These are not discussed in any furtherdetail because most are adequately dealt with in thereviews cited at the beginning of this section, whilstsome are discussed in the later section describing dif-ference between Azucena and Bala. Three recent devel-opments need expansion. Firstly, a report (Tripathy et al.,2000) highlights the potential value of membrane stabil-ity as a component of drought tolerance in rice. Theseauthors indicate that maintaining ion balance undertissue water deficit is important in drought resistanceand show that genetic variation exists in rice. Secondly,

the most recent development in the use of stable isotopemeasurements in the assessment of water use efficiency(WUE) is to link carbon isotope discrimination to oxygenisotope discrimination, which can distinguish betweencarbon isotope discrimination due to stomatal resistanceanduor photosynthetic activity (Barbour and Farquhar,2000). This new development may improve the cost-effective determination of water use efficiency in cropsincluding rice. It has been pointed out, however, that adrought-resistant plant is one that maximizes the use ofavailable water, rather than maximizing WUE (Jones,1981, 1993). Thirdly, the value of improving the use ofabsorbed light, resistance to photoinhibition and capacityfor non-photochemical quenching to improve droughtresistance of rice has been described (Horton, 2000).In addition, a genetic basis for difference in resistanceto photoinhibition in rice has been demonstrated (Jiaoand Ji, 2001). Each of the traits highlighted in Table 1 arephysiologically, biochemically and genetically complex inthemselves and interact with each other. Unfortunately,rice scientists are still some distance away from beingable to identify the ideal trait or traits to use in selectionfor drought resistance or to use in the identification ofgenomic regions contributing to it.

A strategy for QTL mapping

This paper does not intend to describe the detail of doingQTL analysis. The reader is directed to the review by

Table 1. Shoot-related traits that may be important in thedrought resistance of rice, indicating whether there is evidenceof significant genetic variation in the rice germplasm and thereference for that evidence (see Price and Courtois, 1999, formore details)

Potential droughtresistance-related

Significant geneticvariation observed?

Reference

characteristic

Rapid leaf rolling Yes Dingkuhn et al., 1989Price et al., 1997bTurner et al., 1986aTurner et al., 1986b

Rapid stomatal Yes Dingkuhn et al., 1989closure Dingkuhn et al., 1991a

Price et al., 1997bTurner et al., 1986aTurner et al., 1986b

High water useefficiency

Yes Dingkuhn et al., 1991b

Thick epicuticularwax

Yes O’Toole and Cruz, 1983

Osmotic adjustment Yes Lilley and Ludlow, 1996Dehydration

toleranceYes Lilley and Ludlow, 1996

Stay green ability Not testedMembrane stability Yes Tripathy et al., 2000Photochemical

quenchingNot tested

Photoinbitionresistance

Yes Jiao and Ji, 2001

Rice drought resistance 991

Kearsey for the general principles of QTL analysis(Kearsey, 1998) and to Price and Courtois for the appli-cation to drought resistance in rice (Price and Courtois,1999). In outline, however, the QTL approach is asfollows; (a) choose a trait with value or potential value forbreeding; (b) identify parental lines displaying extremephenotypes for this trait (or parents that will produce asegregating population through transgression, despitetheir similarity); (c) cross these lines to produce progenieswhich segregate for the trait (e.g. F2, back-cross, recom-binant inbred or doubled haploid population) of 70–300plants or lines (100 lines is generally considered thelower limit); (d) phenotype the population for the trait;(e) screen the parents of the population for genetic poly-morphism using restriction fragment length polymorph-isms (RFLPs) or PCR based markers (commonly 70–200markers); (f ) determine the genotype of all the progeniesfor the selected markers; (g) construct a genetic map frommarker data using computer programs such as Map-Maker (Lander et al., 1987) or JoinMap (Stam, 1990); (h)identify markers associated with the trait using analysisof variance or, more powerfully, use interval mappingtechniques such as employed by the program MapMakeruQTL, QTLCartographer (CJ Basten, BS Weir, and ZBZeng, Department of Statistics, North Carolina StateUniversity) or PLABQTL (HF Utz and AE Melchinger,Institute of Plant Breeding and Genetics, University ofHohenhein, Germany).

The likelihood of success in this endeavour is affectedby many factors, but it is increased by choosing as diver-gent a set of parents as possible (at least for the trait ofinterest), by producing and screening as large a popula-tion as possible, by reducing environmental error in traitmeasurement to as low as possible and by achieving agenetic distance between markers of about 20 cM or less(Churchill and Doerge, 1994; Gallais and Rives, 1993).

Given the difficulties associated with drought resist-ance highlighted in the introduction, the logical way to usea QTL approach in the exploitation of drought resistance-related genes was to produce several mapping popula-tions using parental varieties of contrasting droughtresistance, and conduct relevant phenotyping to identifythe regions of the rice genome that contribute to droughtresistance and its component traits (Price and Courtois,1999). By combining field evaluations of plant perform-ance under drought with physiological experiments on theunderlying mechanisms of drought resistance, it shouldprove possible to quantify the value of different droughtresistance mechanisms. Studies to date have thereforefollowed two main approaches. The first is to screenpopulations for indicators of drought resistance in fieldtrials. In these, plants are grown in the dry seasonand drought is imposed by withholding water. The secondapproach is to conduct laboratory or greenhouse experi-ments to analyse component traits of drought resistance,

thereby reducing the variability in the environment whichhampers their measurement in the field. The individualcompetent traits that have received the most attention todate are root morphology (Champoux et al., 1995; Yadavet al., 1997; Price and Tomos, 1997), root penetrationability (Ray et al., 1996; Ali et al., 2000; Price et al., 2000;Zheng et al., 2000) and osmotic adjustment (Lilley et al.,1996; Zhang et al., 1999). Having received less attentionis leaf rolling ability and stomatal sensitivity (Price et al.,1997b) and membrane stability (Tripathy et al., 2000).There is certainly an opportunity to study carbon iso-tope discrimination as an indicator of water use effici-ency (Dingkuhn et al., 1991b), non-stomatal resistance(O’Toole and Cruz, 1983) and possibly both stay-greenability and photosynthetic response to high light understress (Horton, 2000).

The objective in studying component traits is to be ableto identify genomic regions contributing to a trait whichtheoretically will improve drought resistance. In order to beof convincing value the QTLs must be shown to contri-bute in a range of environmental conditions, at leastthose comparable to the target environment. Therefore,there has been and will continue to be an emphasis onconducting experiments in a range of conditions in orderto assess QTL stability across environment. Once stableQTLs for component traits of drought resistance areidentified, the next step will be to introduce these QTLsinto a near isogenic background in order to conductmore sophisticated experiments to give a understandingof the underlying physiological and molecular nature ofthe QTL and to evaluate the contribution to yield in thetarget environment.

There are five populations that have been used to studyeither drought resistance or traits related to droughtresistance for which information is published. The firstused was a population of recombinant inbred lines froma cross between CO39 and Moroberekan which has beenused to study drought avoidance in the field and rootgrowth in soil-filled tubes (Champoux et al., 1995), rootpenetration through wax (Ray et al., 1996) and osmoticadjustment (Lilley et al., 1996). A double haploid popu-lation based on a cross between IR64 and Azucena hasbeen used to study field drought avoidance (Courtoiset al., 2000), root growth in tubes (Yadav et al., 1997),both of these together (Hemamalini et al., 2000) and rootpenetration (Zheng et al., 2000). Zhang et al. report theuse of a double haploid population based on a crossbetween varieties CT9993-5-10-1-M and IR62266-42-6-2(Zhang et al., 1999). For that population a range oftraits have been measured but few details are availableas yet. The identification of root penetration QTLs in apopulation from the cross IR58821-23-B-1-2-1ZIR5261-UBN-1-1-2 has been reported (Ali et al., 2000). Thispaper concentrates on just one other population, thatbeing a set of recombinant inbred lines based on a cross

992 Price et al.

between two drought-resistant upland varieties, Bala andAzucena. Comparative mapping between all these popu-lations which have only used a few of the same RFLPmarkers to produce their maps, is possible using theextensive maps of other authors (Causse et al., 1995;Kurata et al., 1994). Comparisons between RFLPs fromthe two common sources (Cornell University and theRice Genome Project, Japan) is more difficult, but canbe achieved either by reference to data on compar-ative mapping presented at the Third Rice Geneticssymposium at IRRI in 1995 (S McCouch personalcommunication), data on the Oryzabase web site(http:uushigen.lab.nig.ac.jpuriceuoryzabaseu) or to mapswhich use combinations of both sets of probes (Lu et al.,1997; Price et al., 2000).

Progress in QTL mapping droughtresistance and related traits in theBalaZAzucena population

A cross was made between the varieties Bala and Azucenain 1992 and an F2 population was used to identify QTLsfor simple root traits of plants grown in hydroponics(Price and Tomos, 1997; Price et al., 1997a). That pre-liminary experiment identified a number of genomicregions affecting root thickness and maximum rootlength. Using a leaf excision test, the rate of leaf rollingand stomatal closure was also mapped in that F2 popu-lation (Price et al., 1997b). Subsequently, the populationwas advanced by single seed descent to an F6 of 205recombinant inbred lines (Price et al., 2000). A new maphas been produced that now has 102 RFLP, 34 AFLPand 6 microsatellite markers (total 142 markers) giving agenome length of 1732 cM and an average space betweenmarkers of 12.2 cM.

Progress in QTL mapping root traits in theBalaZAzucena population

This population has been used for extensive root growthstudies in glasshouse or controlled environment rooms inthe UK. A total of 170 lines have been tested in soil-filledboxes for 4 weeks under well watered or drought (non-watered) conditions (Price et al., 1999). In that screen, abox 2Z1Z1 m (lengthZdepthZwidth) was constructedfrom plywood. The roots of each plant were held within aporous nylon fabric bag (0.9 m deepZ0.08 m diameter)containing sandy loam soil. After 4 weeks, the soil waswashed from each plant and maximum root length andthe thickness of the thickest root taken from the baseof each plant were measured. Using thin glass-sidedchambers (1Z0.3Z0.015 m) filled with a different sandyloam soil and again well-watered or non-watered treat-ments, 140 lines were evaluated over two summers forroot length during growth and for root thickness,rootushoot ratio and deep root weight (below 0.9 m)

after 56 d (Price et al., 1999, 2002a, b). The whole popu-lation has been screened in hydroponics (AH Price,unpublished data) while for 110 lines the ability of theroots to penetrate a simulated hard layer of wax petro-leum has been conducted (Price et al., 2000). These resultsindicate three important points. Firstly, under eachexperiment there are a large number of relatively smallQTLs detected, distributed over the genome. Secondly,the pattern of QTLs varies between the type of rootingenvironments and between different treatments (andeven between replicate runs to some extent). Thirdly, someconsistent pattern does emerge from studying com-bined data on this population and other data obtainedin other populations. Figure 1 shows a summary of theregions identified as controlling some root traits in theBalaZAzucena population. Based on these findings andthose of the other populations, five regions stand out, onchromosomes 2, 5, 7, 9, and 11.

At a region of chromosome 2, at about 125 cM fromthe top of the chromosome (at marker C601 in BalaZAzucena population), QTLs for root penetration abilityhave been revealed in all four of the mapping populationsin which root penetration has been tested (Price et al.,2000; Ray et al., 1996; Zhang et al., 1999; Zheng et al.,2000). In the BalaZAzucena population, there were alsoQTLs for maximum root length in both well-wateredand water-limited conditions in the thin chamber experi-ments (Price et al., 2002b) and for root length in hydro-ponics measured in both the F2 (Price and Tomos, 1997)and F6 (AH Price, unpublished data). Yadav et al. (1997)also found a QTL for maximum root length and rootthickness in this region.

On chromosome 5, at about 85 cM (at marker C43in this population) is a region with QTLs in the BalaZAzucena population for root penetration ability (Priceet al., 2000), root length and thickness in hydroponics ofboth the F2 (Price and Tomos, 1997) and F6 (AH Price,unpublished data). In each case it is the Bala alleles whichincrease root growth, even though Azucena has abetter root system than Bala. In the same region, Yadavet al. (1997) showed that Azucena alleles increase rootthickness.

On chromosome 7 at about 80 cM is a region whichhas effects on root morphology most strongly in otherpopulations. Thus in the BalaZAzucena population, aQTL at marker RG650 for maximum root length wasdetected in the thin chamber experiment (Price et al.,2002b). Yadav et al. (1997) reported QTLs for maximumroot length and deep root weight, Zheng et al. (2000)reported a QTL for root penetration ability and Zhanget al. (1999) reported a QTL for rooting depth in thesame place.

One of the most striking observations gained fromcomparing QTL mapping of root traits is that chromo-some 9 has two strong clusters of QTLs detected in all

Rice drought resistance 993

populations. The lower one is near 80 cM and is asso-ciated with marker G1085 in the BalaZAzucena. QTLsfor rootushoot ratio, root thickness, deep root weight, andmaximum root length at marker G1085 under both well-watered and water-limited conditions have been revealedin this population (Price et al., 2002b). In this region,Yadav et al. (1997) have reported QTLs for maximumroot length and deep root weight, Hemamalini et al.(2000) have reported QTLs for root thickness, Zhenget al. (2000) have reported a QTL for root thickness, andChampoux et al. (1995) reported QTLs for root thicknessand rootushoot ratio.

Above this region, at about 60 cM, is another regionaffecting root morphology. At marker G385 in the

BalaZAzucena population, QTLs were detected in thethin chamber screen for root thickness and rootushootratio (the latter only in water-limited treatment). Inthe same place Yadav et al. (1997) reported a QTL fordeep root weight while Champoux et al. (1995) reportedQTLs for root thickness and rootushoot ratio.

At about 85 cM on chromosome 11 is anotherregion that appears to affect root growth. At markerC189 in the BalaZAzucena population, QTLs formaximum root length in hydroponics were identifiedin the F2 (Price and Tomos, 1997) and for rootpenetration ability in the F6 (Price et al., 2000).Champoux et al. (1995) identified QTLs for maximumroot length, root thickness and rootushoot ratio in the

Fig. 1. Molecular linkage map with RFLP and microsatellite markers indicated (AFLP marker names omitted) of the BalaZAzucena populationshowing quantitative trait loci (QTLs) for root length, thickness or penetration ability. Indicated are those QTLs that were over the threshold value forthe analysis conducted. In the case of the glass chamber experiments (indicated GC), only QTLs which were apparent when data from well-watered andnon-watered plants were averaged are presented. QTLs on the left of the chromosome indicate that Azucena alleles increased the trait value.

994 Price et al.

same place. Also here, Ray et al. (1996) identified a QTLfor root penetration.

Progress in QTL mapping drought avoidance traits

Between 110 and 176 F6 lines have been screened forindicators of drought avoidance in sites in the Interna-tional Rice Research Institute (IRRI), Philippines and theWest Africa Rice Development Association (WARDA),Cote d’Ivoire over two dry seasons. QTLs for the visualscores leaf rolling and leaf drying and for relative watercontent have been located (Price et al., 2002c).

As with root traits, drought avoidance traits give manyQTLs which differ between sites and between years atthe same site. Again, by studying these results with thoseobtained on the other two populations that have beentested in a similar manner (Champoux et al., 1995;Courtois et al., 2000) a pattern does emerge from thecomplexity indicating some strong evidence of consistentdrought avoidance QTLs. What is particularly striking,however, is poor alignment of drought avoidance QTLswith the regions where there is good evidence for an effecton root thickness or depth (i.e. on chromosomes 2, 5, 7,9, and 11 described above). There is some evidence thatthe regions of chromosomes 5, 7, 9, and 11 that affectroot growth also affect performance under drought.On chromosome 5 QTLs in which the Azucena alleleincreases relative water content and reduces leaf rollingat WARDA were detected, but in the same place a QTLin which the Azucena allele increased leaf rolling at IRRIwas revealed (Price et al., 2002c). (Note that the Azucenaallele here decreases root thickness, length and penetra-tion ability in this population.) Courtois et al. (2000)identified QTLs in this region for leaf rolling in threescreens and both leaf drying and RWC in one screen,in each case the Azucena allele increasing droughtavoidance, in agreement with the increase in rootthickness associated with the Azucena alleles in theirpopulation (Yadav et al., 1997). Champoux et al. (1995)identified QTLs for leaf rolling just above RG351on chromosome 7 in two of the growth stages, whileCourtois et al. (2000) reported QTLs for leaf dryingin one screen and relative growth rate under droughtstress in another near RG351. Courtois et al. (2000)reported QTLs for leaf rolling in all screens at RZ12(maps to the same place as G1085) on chromosome 9,while Champoux et al. (1995) reported leaf rolling QTLsin all growth stages in the same location. Only Champouxet al. report evidence for QTLs for drought avoidanceat the root growth QTL on chromosome 11, with leafrolling QTLs in three growth stages (Champoux et al.,1995). There are two important points to note. Firstly,there is no convincing co-location of drought avoidanceQTLs with the root growth QTL on chromosome 2 in anypopulation. Secondly, only on chromosome 5 is there

any co-location between drought avoidance and rootmorphology QTLs in the BalaZAzucena population (andeven here, only in the IRRI drought screen does the effecton drought avoidance agree with the effect on rootmorphology).

Possible explanations for lack of co-locationof root and drought avoidance traits

The poor co-location of drought avoidance and rootmorphological QTLs in populations in general, and in theBalaZAzucena population specifically, casts doubt onthe notion that improving the roots of rice will improvedrought resistance. Specifically, it is possible to offerseveral theories as to why there are no drought avoidanceQTLs at root growth QTLs in the BalaZAzucena popu-lation, each of which have profound implications forattempts to improve drought resistance in crops.

Roots do not contribute to drought resistance

There seems to be ample evidence that rice varieties oreven progenies of segregating populations which havebetter root systems assessed in controlled environmentexperiments do perform better in the field under droughtin terms of visual drought avoidance (Ekanayake et al.,1985b; Champoux et al., 1995; Price et al., 1997). Field-based studies of root distribution confirm that varietiesidentified as being deep rooting in the laboratoryugreenhouse are indeed better at depth in the field(Puckridge and O’Toole, 1981; Yoshida and Hasegawa,1982). It has been shown that deeper roots increase theability to extract soil water from depth (Mambani andLal, 1983a, b, c; Yoshida and Hasegawa, 1982), improveshoot water status (Yoshida and Hasegawa, 1982)and may increase yield under drought (Mambani andLal, 1983a). Root pulling force, a supposed integratorof rooting capacity, has also been shown to correlatewith shoot water status under drought stress (Ekanayakeet al., 1985a). It seems unlikely to be true that rootsare unimportant, since the evidence that deeper andthicker roots should contribute to drought resistance, atleast in some environments, seems convincing.

Root growth QTLs identified in controlledenvironments are not expressed in the field

Although a primary aim of root screening has been to getthe best compromise between practicality (ease of experi-ment) and relevance to the field situation, it must beaccepted that both the root and shoot environment arelikely to be different to that experienced in the realdrought situation. The plant root growth is known tobe influenced by rooting environment and probably theshoot environment. This means that artefactual results

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from root confinement, soil chemical (especially nutrientdistribution) or physical properties (water distribution,penetration resistance, temperature fluctuation) or shootenvironment will probably occur. The fact that the rank-ing of varieties for root morphologial characteristicsbetween different experiments using different methodsgenerally agrees (Price et al., 1997) suggests that thegenetic control of root thickness and depth is under quiteenvironmentally-robust genetic control. The success ofChampoux et al. at linking root thickness in particular(measured in soil-filled tubes) to drought avoidancestrongly suggests that root morphology QTLs measuredin a screenhouse are important in conferring droughtresistance in the field (Champoux, 1995).

The environment also affects individual QTLs and theQTLs with small effects are the more likely be influencedby variation across replicates, sites or repeated runs.QTLs which have R2 values around or below 10% maybe difficult to pick-up repeatedly for this reason or due tothe high statistical threshold used in QTL analysis, unlessthe population size is high (�100). However, several ofthe root growth QTLs reported do have R2 values around20% or above when measured in the greenhouse (e.g. anR2 of 29% for hydroponic root length on chromosome 11in Price and Tomos, 1997; 18% for root penetration ratioon chromosome 2 in Price et al., 2000; 18% for rootthickness of soil grown plants on chromosome 9 inPrice et al., 2002b). These are relatively large QTLs andone might reasonably expected them to influence rootmorphology in at least some field situations.

Limitations of field trials

A meaning for drought resistance?: Drought resistancecan have different meanings for different workers. Some,for example, consider drought resistance solely in termsof yield under drought conditions, while others are moreconcerned with stability of yield over a range of envir-onmental conditions. Yet others consider drought resis-tance in terms of survival of drought conditions, ratherthan productivity. It follows from these observations andcontrasting requirements that rankings of genotypes fordrought resistance depends on the method of assessmentused. It is likely that characters that favour droughtescape (such as early floweringuharvest) will be foundin different genotypes than characters that favour theavoidance of internal water deficits in dry conditions(such as deep roots, rapid stomatal closure or high cutic-ular resistance) or characters associated with biochemicaltolerance of internal water deficits. Furthermore, othervarieties may be apparently drought resistant through anenhanced ability to recover after a dry period. It seemslikely, therefore, that no single characteristic can be usedas a screen for drought tolerance in genetically diversecollections.

A particular problem with drought resistance screen-ing, especially under field conditions, is that the rankingof genotypes is very dependent on the specific environ-mental conditions of the trial (i.e. there is a high genotypeby environment interaction in drought screens). This iswell illustrated by the assessment of the BalaZAzucenapopulation in different sites (Price et al., 2002c). Only leafdrying scores correlated between IRRI and WARDA(r\0.373, P< 0.001), while leaf rolling scores and relativewater contents did not. Indeed, there were very poorcorrelations even between years at WARDA (r\0.068for leaf rolling, 0.124 for leaf drying and 0.206 (P\0.05)for RWC) although there was much better consistency atIRRI. Reasonably good correlations were found betweenleaf rolling scores in IRRI and an Indian field site in theAzucenaZIR64 population (r\0.470 and 0.66, P< 0.001)(Courtois et al., 2000).

Drought screens in dry season: Because of the absoluteneed to have a drought in each screen (project fundingis rarely able to accommodate loss of whole seasonsof data due to uncertainty in climate), drought trialsare generally conducted in the dry season when low wateravailability can be guaranteed, and is combined withsupplementary irrigation. Unfortunately, these conditionsare generally very harsh for the plants and probably donot reflect well the conditions of natural droughts whichmore commonly occur as low water availability in a norm-ally reasonably wet period. Particularly important here iswhether the irrigation allows water to soak the whole soilprofile fully (i.e. does it keep the deep soil wet) and the verybright (often cloudless), hot and dry climatic conditionswhich may cause substantial physiological stress in theshoots even if there is adequate access to water.

Soil physical properties: The ability of roots to accesswater at depth is dependent on their growth through thesoil medium. If the soil has a high strength (has a highresistance to penetration) or if its strength increasesgreatly as it dries out (as all but the sandiest soils do to agreater or lesser extent) then the roots may fail to con-tribute to drought avoidance. It has been discovered thatthe site where field trials at WARDA were conducted wasvery hard. This is illustrated in Fig. 2. It shows the depthto a penetration resistance of 3.0 MPa (a resistance whichshould very substantially inhibit root growth) in the fieldat WARDA where drought screens were conducted andindicates that, for much of the area of the drought trialthe roots could not have been expected to grow below20–30 cm despite a demonstrated ability to grow wellover 1 m in glasshouse experiments. The relationshipbetween soil hardening, root growth and drought in thisand other WARDA sites is currently being investigated.

Soil biological properties: Field screens are prone todamage by pests and pathogens. Dry season screens

996 Price et al.

appear to be particularly prone to root-damaging pestssuch as mites, termites and nematodes. This is partly dueto the greater abundance of these aerobic organismsin drier soils (hence not a major problem in flooded ricein any season). Recent experiments at WARDA haveencountered mite and termite damage and nematode-induced root damage at IRRI has also been a problem(B Courtois, personal communication).

Shoot-related mechanisms which differ betweenthe two parents

The variety Bala was chosen as a parent in the crossingprogramme specifically because it displays a high degreeof drought resistance using standard measures of leafrolling and leaf drying, yet it does not have a good rootsystem. It must, therefore, have shoot-related mechan-isms of drought resistance which contribute to droughtresistance. Experiments using Bala indicate several mech-anisms of drought resistance that might be expected todiffer with Azucena. However, few of these mechanismshave been effectively evaluated for their contributionto yield under stress in rice, and most have not beenanalysed in the mapping population. It may be that in thedrought screens so far conducted that it is these traitsthat have dominated drought avoidance, either becausethey are very effective in conferring drought avoidanceor because the root system does notuwas not able tocontribute. The authors believe that there is considerablescope for investigating these mechanisms in more detail.

Leaf rolling and stomatal sensitivity: There is variation inthe degree to which the leaves of rice roll in response tolow water potential (Dingkuhn et al., 1989; Turner et al.,

1986a). The rate of leaf rolling upon leaf excision alsovaries in rice varieties (Price et al., 1997b) and the varietyBala was shown to roll unusually slowly (compared to 11other varieties including Azucena). A leaf rolling QTL hasbeen identified on chromosome 1 in the F2 (Price et al.,1997b) that coincides with a large effect QTL for leafrolling in the field (Price et al., 2002c). There is alsoevidence that the stomata of Bala are more sensitive thanthose of Azucena (Price et al., 1997b).

Shoot morphology and mass, canopy structure and responseto drought: Bala is a semi-dwarf variety while Azucenais tall. Bala tillers more than Azucena and producesnarrower and shorter leaves. The thickness of the leavesappears to depend on the environment since there was asignificant interaction between genotype and treatment(irrigated or droughted) at IRRI in 1996 for specific leafarea (P< 0.01). Bala had the thinner leaves under irriga-tion, but the thicker under drought when compared toAzucena. Increased leaf thickness should increase wateruse efficiency and therefore seems a useful response todrought. What seems certain is that the canopy structureof the two varieties will be markedly different in termsof light and heat interception, heat and water loss andprobably gas diffusion inside the leaf, all parameterswhich may be expected to affect water use efficiency anddrought-induced biouphotochemical damage to the leaf.

Under well-watered conditions, Bala has a smallershoot biomass than Azucena (about 25% smaller in UKgreenhouse experiments and 45% smaller in the field atIRRI) although this is not apparent in droughted plants(indeed, if anything, Bala are bigger under prolongeddrought in the greenhouse or field). There is a contra-diction here with (unpublished) data which indicates that,

Fig. 2. Results of a wet season soil penetration resistance survey of field used for drought screening at WARDA. Penetration was assessed in a gridof 5 m separation using a cone penetrometer (Remik, Australia). Presented here is a map based on depth to a resistance of 3 MPa. Droughted plotswere sown in the leftmost 25 m of the field.

Rice drought resistance 997

under drought the increase in plant height slows markedlyin Bala when droughted in comparison to Azucena.

Water-use efficiency as measured by D13C: There isgenetic variation for water use efficiency which is reflectedin difference in D13C between upland rice genotypes(Dingkuhn et al., 1991). These data suggest that Azucenahas a high water use efficiency although unfortunatelyBala was not tested. One of the two varieties thatwere used as parents to produce Bala at the CentralRice Research Institute, Cuttack, India, namely N22(Chaudary and Rao, 1982), was tested, however, and wasmuch less water use efficient than Azucena. Previouslyunpublished data collected on irrigated control plantsgrowing in the dry season at IRRI and WARDA con-firm that there is a difference in D13C between Bala andAzucena of about 1.2 mlY1 (Table 2). The large standarddeviation of the segregating population compared to theparents (IRRI only) in these data clearly indicate thatthere is a genetic component to the carbon isotope dis-crimination. QTL mapping, however, shows (Fig. 3) thatalthough genomic regions controlling these values can beidentified, in no case does the region detected at IRRI

coincide with one at WARDA. Further investigation intothis discrepancy could be useful.

Osmotic adjustment: It has been shown that, in general,the tropical japonica rice varieties have a poor ability toadjust osmotically under drought, something that indicarice varieties can do relatively well (Lilley and Ludlow,1996). It is believed that this represents an adaptation todrought in rainfed lowlands where roots cannot easilycontribute to drought avoidance. Azucena was shown tobe the 12th worst variety of the 61 tested while Balawas the 10th best. An osmotic adjustment QTL has beenmapped to chromosome 8 in the CO39ZMoroberekanpopulation (Lilley et al., 1996) and the same locationdoes appear to contribute to drought avoidance in theBalaZAzucena population (Price et al., 2002c).

Summary of shoot mechanisms: Several pieces ofinformation suggest that the shoots of Bala have anability to respond to drought rather more than Azucena.Thus stomata are more sensitive, osmotic adjustmentis more pronounced, shoot elongation growth is moresensitive, leaf thickness increases (rather than decreases inAzucena). On the other hand, Azucena rolls its leavesmore rapidly and D13C data indicate a more efficientwater use efficiency under low to moderate water stress.There appears to be many differences in the way in whichthe shoots of Bala and Azucena may be expected tointeract with the drought. These may well interfere withthe detection of co-located drought avoidance and rootgrowth QTLs, but they probably reflect the expressionof traits that would be valuable for improving drought

Table 2. D13C values for youngest fully expanded leaves fromfive plants of varieties Azucena, Bala and the F6 progeny grownunder irrigation in the dry season of IRRI in 1996 and WARDAin 1997 (] standard deviation)

Variety IRRI WARDA

Azucena Y27.15]0.13 Y27.26 (no replication)Bala Y28.27]0.02 Y28.12 (no replication)F6 Y27.71]0.58 Y27.14]0.51

Fig. 3. Molecular linkage map (with marker names omitted) of the BalaZAzucena population showing QTLs for D13C measured on five youngestfully expanded leaves of plants grown under irrigated conditions during dry season screens at IRRI in 1996 and WARDA in 1997. The valuesabove each box give the LOD value obtained with QTLCartographer using composite interval mapping, where a LOD of 3.1 represents the 0.05%genome wide threshold value. The boxes represent the Y1 and [1 LOD interval and the number above or below the box indicates the LOD scorefor the QTL. A positive LOD value indicates that Azucena alleles increase the trait value (reduced water use efficiency), a negative LOD indicates thatAzucena alleles reduced the trait value.

998 Price et al.

resistance. The techniques and approaches that will allowthe physiological processes underlying these contrast-ing drought responses to be investigated in detail willbe outlined next. Using advances in the understandingof photosynthetic physiology and biochemistry of rice(Horton and Murchie, 2000) together with additionalmethods to differentiate stomatal from mesophyll limita-tions, will resolve the contributions of shoots and rootsin conferring drought resistance and yield stability underdrought. This, it is hoped, will open the way to theidentification of new strategies for improving the droughtresistance of rice.

New approaches to study the physics,biochemistry and physiology of drought

Infrared thermography to analyse energy balanceand stomatal function

One potential approach to the screening of large numbersof lines of a crop such as rice for stomatal characteristicsuses an advance on the infrared thermometry (as used byGarrity and O’Toole, 1995), which is the use of thermalimaging (Jones, 1999a). The basic principle of thisapproach is that the energy balance of plant leaves isvery strongly dependent on the rate of evaporation, whichin turn is strongly dependent on stomatal conductance(Jones, 1999b). Indeed, screening for stomatal behaviourusing infrared imaging techniques has previously beenused in laboratory screens for ABA-insensitive mutantsin barley (Raskin and Ladyman, 1988) and for studyingthe stomatal functionality of Arabidopsis mutants (Grayet al., 2000). The technique can also be used as an indic-ator of plant stress, the precision of which can beimproved by the use of wet and dry reference surfacesfor calibration (Jones, 1999a, b). This allows a linear, 1 : 1,relationship between stomatal conductance measured byporometry and that calculated from thermograms to bedemonstrated in the laboratory (Fig. 4). Thermal imagingcould usefully be applied to Bala and Azucena to char-acterize their different relationships between plant waterstatus (and importantly soil water status) and stomatalfunction.

A problem with the application of thermal imagingtechniques in the field is that the relationship between leafor canopy temperature and stomatal conductance is verydependent on the other environmental conditions (humid-ity, radiation, windspeed, etc.). Preliminary field experi-ments, however, suggest that such an approach could alsobe applicable in the field. This is because in a comparativescreening trial with repeated controls, absolute calib-ration of the images is not necessary, one is primarily con-cerned with differential responses. A major advantage ofthermal imaging as opposed to the use of infrared thermo-metry or porometry to measure stomatal conductance

in the field is the shorter time required to obtain largenumbers of data. Relative canopy temperatures of dif-ferent plants in a mapping population could be measuredin a few minutes, thereby avoiding the confounding effectsof rapid changes of stomatal conductance in response tochanges in microclimate such as windspeed or radiationwhich can occur when using slower techniques on largenumbers of lines.

Stable isotopes and the interaction betweenwater use and mesophyll conductance

Given the problems described above in relating rootgrowth characteristics to QTL markers, one methodallowing the source of water to be distinguished is the useof 18O (or 2H, D) as a tracer of water abstraction withinthe soil profile (Ehleringer and Dawson, 1992, see alsoreviews in Griffiths, 1998). However, this is dependent ona discernible signal between groundwater and surface,meteoric water, and that at any given field trial, ground-water is available and accessible (see problems atWARDA, above). The 18O signal is directly affected byevaporation, whether at the soil surface or at the leaf,and how this leaf signal may be used to augment carbonisotope discrimination measurements is discussed.

Whilst providing an excellent marker for water useunder semi-arid conditions, the carbon isotope signal inleaf organic material reflects a combination of stomataland mesophyll processes (Table 2; Griffiths et al., 1999).In simplest terms, as long as the proportional draw-down of CO2 from ci (substomatal CO2 concentration)

Fig. 4. The 1 : 1 linear relationship obtained between leaf conductanceof Phaseolus vulgaris estimated from porometry measurements and thatcalculated from thermograms immediately prior to the use of theporometer. The calculations were based on calibration values obtainedfor wet benchcoteTM (Whatman Ltd), dry benchcote and benchcotecovered with surgical dressing ($ and n) or calculated frommeasurements made on another day under the same conditions usingwet and dry benchote only (s); (from Jones, 1999a).

Rice drought resistance 999

to cc (CO2 concentration at Rubisco) is constant, then ci

represents a good proxy for comparative water use meas-ured by carbon isotope discrimination (D), and thatcalculated from stomatal conductance (Di). If, however,some of the morphological and biochemical constraintsvary between the parent lines (such as leaf thicknessurolling, or N contentuRubisco activity; Table 3), themesophyll conductance will alter the additional draw-down to Rubisco and hence affect carbon isotope dis-crimination. The greater is the drawdown from ci to cc,the lower the discrimination expressed in organic material,or measured in real-time, when CO2 can be collectedduring on-line discrimination (Dobs). Under high tem-peratures, the impact of respiration and photorespirationbecome quantitatively greater at low rates of net CO2

uptake (Griffiths et al., 1999). A number of studies havesuggested that leaf N is a good proxy for Rubisco activity,and the importance of evaluating Rubisco allocation andlight use for rice in the field is discussed.

One means of distinguishing the interplay betweenstomatally-led evaporative demands and internal meso-phyll conductance is to analyse the 18O signal in leaforganic material or leaf water. The problem of analysingorganic material is that the signal reflects primarily theenvironmental conditions during leaf expansion, ratherthan more immediately during an episodic drought, andso analyses of leaf water 18O content have been under-taken for the two parent lines under field conditions inWARDA. Despite the limited replication, there is evid-ence of a consistent difference between the two varieties(Fig. 5), with greater enrichment generally found in Balarather than Azucena. Under laboratory conditions, adetailed study of the variation between on-line, instant-aneous carbon and oxygen isotope discriminationwould help to evaluate the different drought stress

characteristics of parent lines at the leaf level (Harwoodet al., 1998), while a survey of leaf water, anduor leaforganic material of the entire mapping population in afield trial, would help to distinguish the interplay betweenstomatal and mesophyll processes.

Photosynthetic capacity and light utilization

Under field conditions, irrigated rice shows a markedmidday depression of photosynthesis, with leaf temperat-ures reaching 35–37 �C for much of the day (Murchieet al., 1999). While there are implications for both lossesof net C gain and increased photorespiration at this time,there was no evidence of long-term photoinhibition inthis irrigated crop. However, it was speculated that inmore marginal habitats, such as those where upland riceis grown, the loss in overall yield attributable to photo-inhibition could be considerable (Horton and Murchie,2000). Thus screening of Photosytem II fluorescence

Fig. 5. 18O discrimination measured in water extracted from theyoungest fully expanded leaves from Azucena (open bar) and Bala(hatched bar) plants grown under irrigation on three sites at WARDAin the dry season of 2001 (Bar\ standard error, n\3).

8>>>>>>>>>>><>>>>>>>>>>>:

8>>>>>>>>><>>>>>>>>>:

8>>>>>>><>>>>>>>:

cc

Di

cc

ci

gi

O

O

N

Q

Q

organicd13C(d18O,VPD-led)

g

----

h

WUE

Dobs

13C,18Oin CO2

Q

Augs

8>>>>>>>>>>><>>>>>>>>>>>:

8>>>>>>>>><>>>>>>>>>:

8>>>>>>><>>>>>>>:

cc

Di

cc

ci

gi

O

O

N

Q

Q

organicd13C(d18O,VPD-led)

g

----

h

WUE

Dobs

13C,18Oin CO2

Q

Augs

8>>>>>>>>>>><>>>>>>>>>>>:

8>>>>>>>>><>>>>>>>>>:

8>>>>>>><>>>>>>>:

cc

Di

cc

ci

gi

O

O

N

Q

Q

organicd13C(d18O,VPD-led)

g

----

h

WUE

Dobs

13C,18Oin CO2

Q

Augs

Table 3. The link between stable isotope measurements 13C and 18O in organic material and real-time instantaneous discriminationduring gas exchange and coupling to stomatal and mesophyll conductance, as well as environmental conditions and leaf ultrastructureas determinants of isotope composition and link to Rubisco activity (from Griffiths et al., 1999)

Control of D CO2 supply and demand Physiologicalumorphological correlates Environmental conditions

Long term Instantaneous

VPDLife formustrategy

PPFDdiffusive exchange

SLM TEMPERATURE

RUBISCOCAPACITY

chlorophyll RAINFALL

SOIL WATER DEFICIT(Photo)respirationrefixation and exchange

Leaf N

Ash contentSOILSTRENGTHuNUTRIENTS

1000 Price et al.

characteristics, and the relationship between maintenanceof electron transport and capacity for non-photochemicalquenching of the mapping population under water-limited conditions may provide another importantmarker of leaf-level drought tolerance characteristics.

Photosynthetic capacity was also implicated as a keylimitation to overall productivity in irrigated rice(Murchie et al., 1999; Horton and Murchie, 2000), andas such would contribute to the overall mesophyllconductance (Table 3). Despite this, during grain-fillinga significant portion of chlorophyll and leaf N was lostfrom leaves, without a loss of photosynthetic capacity(Murchie et al., 1999), supporting the notion of leafRubisco as a store of mobilizable N. Rubisco content ofa new plant type (NPT) rice variety was much higher thanin IR72 (Fig. 6) apparently in excess of that needed tosustain photosynthesis, since NPT Pmax was actuallylower than that of IR72. Varying the content of Rubiscomay be an important strategy for ensuring adequate Nreserves for grain fill in certain rice varieties (Hortonand Murchie, 2000).

In summary, the efficiency of light use will in partbe determined by the canopy architecture (which differmarkedly between Bala and Azucena), with leaf angle andorientation both important to reduce self shading andoptimizing the use of available Rubisco, but perhaps atthe expense of photoinhibitory losses under drought-stressed conditions. In determining the trade-off betweenmaintaining yield and tolerance to drought, it wouldbe important to include additional measurements ofchlorophyll fluorescence, photosynthetic capacity andN allocation to evaluate the mapping population ofupland rice.

Conclusion

New developments in plant biology open the opportunityto investigate the mechanisms of drought resistance ina more holistic way than in the past. If advances in theuse of stable isotopes and infrared thermometry can becombined with advances in understanding of the funda-mental biochemical processes underlying photosyntheticactivity there are opportunities for great advancement.If this research is conducted in the context of geneticmapping on the model monocotyledon species, anduorcombined with gene array technology, the outcomes maybe even greater. The BalaZAzucena population offersexciting challenges to both plant physiologists and genet-icists to collaborate in elucidating the importance ofdifferent mechanisms of drought resistance in the contextof contrasting droughting environments and to make realprogress in identifying genomic regions (for breeders) oreven candidate genes involved in these crucial processes.Progress made using this approach for rice, will lead toequivalent opportunities in other crops.

Acknowledgements

Most of the work on Bala and Azucena presented here has beenconducted under projects funded by the UK Department forInternational Development (DFID), Plant Sciences Programme.This paper uses some previously unpublished data for whichA Price would like to acknowledge the following; BrigitteCourtois (IRRI), Alain Audebert and Monty Jones (WARDA)for the collaboration on the field evaluation of the population;John Townend (Aberdeen), Katherine Steele, John Gorham,Julian Bridges (Bangor, UK), and Lawrence Clarke (IACRRothamsted, UK) for the collaboration on root screening;Alain Audebert and Chris Mullins (Aberdeen) for the ongoingwork on soil physical properties and root growth. We aregrateful to Gary Lanigan and Nick Betson for analysing the18O data.

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