15
Field Crops Research, 4 (1981) 297--311 297 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands RESPONSE OF CASSAVA TO WATER SHORTAGE III. STOMATAL CONTROL OF PLANT WATER STATUS D.J. CONNOR* and J. PALTA Centro Internacional de Agricultura Tropical (CIAT), A.A. 6713, Cali (Colombia) *Permanent address: School of Agriculture, La Trobe University, Bundoora, Vic. 3083 (Australia) (Accepted 25 May 1981) ABSTRACT Connor, D.J. and Palta, J., 1981. Response of cassava to water shortage. III. Stomatal control of plant water status. Field Crops Res., 4: 297--311. Diurnal measurements of leaf water potential and the diffusive conductance of the abaxial surface of two cassava cultivars, M Col 22 and M Mex 59, were made on three occasions on field grown plants during a 10-week period of rainfall exclusion. Conductances of about 10 mm s-' were observed in the rainfed plots but generally the mean conductance was in the range 3--5 mms -1. The minimum water potential of --1.8 MPa was observed in the rainfed plots. Water shortage caused reduction in mean conductance to < 1 mms-' at which level the control of water loss maintained leaf water potential > --1.5 MPa at all times. Stress plots recovered more slowly during the late afternoon but during the day had higher leaf water potentials than the controls. At the same levels of leaf water potential the con- ductance of M Mex 59 was less than that of M Col 22 in both control and stress plots. Measurements are also reported of the stomatal distribution, density and pore size for both fully expanded leaves and those whose expansion was seriously restricted by the water shortage. INTRODUCTION Plants may restrict water loss by either a reduction in the extent of their evaporating surfaces or by a reduction in the rate of water loss per unit evaporating area. The latter can be achieved by an increase in the resistance to water flow in the catena between soil and atmosphere and in this pathway the most actively variable resistance is that of the stomatal pathway. Unlike leaf loss, stomatal closure does not involve the sacrifice of growth reserves previous- ly assimilated and currently elaborated as potentially active leaf area. Stomatal closure does reduce the current carbon assimilation capacity of the plant but under conditions of water shortage the important consideration is not high growth rate but rather the most efficient use of the available water, in the maintenance of existing yield and in its increase. For active plants this requires 0378-4290/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

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Page 1: Response of cassava to water shortage III. Stomatal control of plant water status

Field Crops Research, 4 (1981) 297--311 297 Elsevier Scientific Publishing Company, Amsterdam - - Printed in The Netherlands

R E S P O N S E O F C A S S A V A T O W A T E R S H O R T A G E I I I . S T O M A T A L C O N T R O L O F P L A N T W A T E R S T A T U S

D.J. CONNOR* and J. PALTA

Centro Internacional de Agricultura Tropical (CIAT), A.A. 6713, Cali (Colombia)

*Permanent address: School of Agriculture, La Trobe University, Bundoora, Vic. 3083 (Australia)

(Accepted 25 May 1981)

ABSTRACT

Connor, D.J. and Palta, J., 1981. Response of cassava to water shortage. III. Stomatal control of plant water status. Field Crops Res., 4: 297--311.

Diurnal measurements of leaf water potential and the diffusive conductance of the abaxial surface of two cassava cultivars, M Col 22 and M Mex 59, were made on three occasions on field grown plants during a 10-week period of rainfall exclusion. Conductances of about 10 mm s - ' were observed in the rainfed plots but generally the mean conductance was in the range 3--5 m m s -1. The minimum water potential of --1.8 MPa was observed in the rainfed plots. Water shortage caused reduction in mean conductance to < 1 m m s - ' at which level the control of water loss maintained leaf water potential > --1.5 MPa at all times. Stress plots recovered more slowly during the late afternoon but during the day had higher leaf water potentials than the controls. At the same levels of leaf water potential the con- ductance of M Mex 59 was less than that of M Col 22 in both control and stress plots. Measurements are also reported of the stomatal distribution, density and pore size for both fully expanded leaves and those whose expansion was seriously restricted by the water shortage.

INTRODUCTION

P l a n t s m a y r e s t r i c t w a t e r loss b y e i t h e r a r e d u c t i o n in t h e e x t e n t o f t h e i r e v a p o r a t i n g s u r f a c e s o r b y a r e d u c t i o n in t h e r a t e o f w a t e r loss p e r u n i t e v a p o r a t i n g a r e a . T h e l a t t e r c a n b e a c h i e v e d b y a n i n c r e a s e in t h e r e s i s t a n c e t o w a t e r f l o w in t h e c a t e n a b e t w e e n so i l a n d a t m o s p h e r e a n d in t h i s p a t h w a y t h e m o s t a c t i v e l y v a r i a b l e r e s i s t a n c e is t h a t o f t h e s t o m a t a l p a t h w a y . U n l i k e l e a f loss , s t o m a t a l c l o s u r e d o e s n o t i n v o l v e t h e s a c r i f i c e o f g r o w t h r e s e r v e s p r e v i o u s - ly a s s i m i l a t e d a n d c u r r e n t l y e l a b o r a t e d as p o t e n t i a l l y a c t i v e l e a f a r ea . S t o m a t a l c l o s u r e d o e s r e d u c e t h e c u r r e n t c a r b o n a s s i m i l a t i o n c a p a c i t y o f t h e p l a n t b u t u n d e r c o n d i t i o n s o f w a t e r s h o r t a g e t h e i m p o r t a n t c o n s i d e r a t i o n is n o t h i g h g r o w t h r a t e b u t r a t h e r t h e m o s t e f f i c i e n t u s e o f t h e a v a i l a b l e w a t e r , in t h e m a i n t e n a n c e o f e x i s t i n g y i e l d a n d in i t s i n c r e a s e . F o r a c t i v e p l a n t s t h i s r e q u i r e s

0378-4290/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

Page 2: Response of cassava to water shortage III. Stomatal control of plant water status

298

that the most favourable internal water status should be maintained under existing environmental conditions.

Stomatal control of water loss can thus play an important long term as well as short term role in the regulation of water reserves. Furthermore, it is closely tuned to the water status of the important carbon assimilation organs because it operates as the leaf--atmosphere interface. As an "end of pa thway" resis- tance, stomatal control sets a lower limit to the water status of the entire plant system and is able to respond to the capacitance of the total system which is not always negligible.

The stomatal system and its response in cassava is not well known. Williams (1971) accounted for his difficulties in using a pressure drop porometer on three Malaysian cultivars by suggesting the absence of adaxial stomata, but Viegas (1976) reporting wide ranging studies on Manihot spp. comments, wi thout elaboration, that stomata do occur on both leaf surfaces. Apart from isolated measurements (CIAT, 1972) there are no field data available for leaf diffusive conductance of cassava although there are a few recent laboratory studies (Aslam et al., 1977; Mahon et al., 1977) in which an analysis of the physical pathway of diffusion is made. These measurements support the con- clusion of Williams (1971) that the leaf conductance of cassava is low relative to other crop species but his equipment and the problems he encountered in using it on cassava do not allow either an absolute definition of the conduc- tance pathway and its response, nor an easy comparison with other species.

In this paper we report observations on the diurnal patterns of leaf water potential and leaf diffusive conductance of two cultivars, M Col 22 and M Mex 59, both before and during a period when water was withheld from the crops.

METHODS

The experimental design and field site are described in detail in the first paper of the series (Connor et al., 1981). Briefly, two cultivars, M Col 22 and M Mex 59, were planted on 25 April 1979 and during the 72-day period from 12 August to 23 October 1979 were subjected to a period of water stress, achieved by the exclusion of rainfall with covers of black plastic sheeting over the soil surface. On one occasion before the plastic was placed, 25 July, and on three occasions during the period of rainfall exclusion, 29 August, 12 Sep- tember and 26 September, continuous measurements were maintained around three replicate treated plots and their controls of leaf water potential and of leaf diffusive conductance. Measurements were made on recently fully ex- panded leaves in the upper canopy and generally duplicate measurements of water potential and the diffusive conductance of the abaxial surface, to which stomata are restricted, of four central lobules were measured before passing onto the next plot. To estimate leaf water potential, the xylem pressure potential of the central lobule was measured using pressure chambers. By using the lobule the difficulty arising from latex vessels common in this species was minimized. A comparison between xylem pressure potential using lobules

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299

and entire leaves showed tha t no difference existed. A recent s tudy (Ike et al., 1978) used psychrometry to demonstrate that the pressure chamber accurate- ly determines leaf water potential in cassava. Diffusive conductance was mea- sured with Lambda autoporometers. The sensors were calibrated before and after each occasion of measurement.

Observations were also made on the stomatal characteristics of the cultivars. Leaf surface impressions were collected by applying a thin coating of plastic spray (artists fixative) to the leaf surface and then transferring it on transparent cellulose tape to a glass microscope slide (Clemens and Jones, 1978). Impres- sions were taken of both leaf surfaces from fully expanded leaves both before and during the stress t reatment. Measurements of stomatal density and pore length were made at 400× magnification. The areas of the leaves from which they were taken were also measured to describe the extent of stress on leaf development (Connor and Cock, 1981).

In order to establish the perspective of the stomatal responses reported here, calculations were made of crop water-use using the conductance patterns, leaf temperatures, crop leaf area and free water evaporation appropriate to 26 Sep- tember, the day of measurement in the second half of the stress period. For this a combination equation (Slatyer, 1967) was used. It was assumed that the measured daffy total free water evaporation occurred sinusoidally during day- light hours, that the aerodynamic resistance of the crops was 0.3 s cm -1 and that for hypostomatous leaves the canopy resistance is estimated by leaf resistance/leaf area index (Szeicz et al., 1973).

RESULTS

General climatic data are available from a site 500 m from the experimental plots. Data for the four days of measurement are presented in Table I.

The diurnal observations of diffusive conductance and of leaf water potential are presented in Figs. 1 and 2--4. Fig. 1 portrays the response before the rainfall exclusion period, 25 July, and for this reason contains only a cultivar comparison. Figs. 2--4 are identical in construction and compare, during the stress period, the gradual response of the crops to water shortage. Air temperature and quantum flux density included in these figures were mea- sured at the experimental site. The leaf temperature measurements are those from the porometers as used in the calculation of leaf conductance. Since the porometers were used routinely on the abaxial (under) leaf surface there was little difficulty in avoiding overheating due to sunlight striking the sensor. In Table II a summary has been made of the mean leaf diffusive conductance ob- tained during the morning and during the afternoon in each cultivar--treatment comparison.

The relationship between diffusive conductance and leaf-air vapour pressure difference was investigated for observations taken when quantum flux density exceeded 900 pE m -2 s -1. No evidence of a threshold response was seen so that the linear trends that are evident in some of the data sets can be satisfactorily

Page 4: Response of cassava to water shortage III. Stomatal control of plant water status

4C

220G 200C

b 1800 c 160C ~_--. x ~ 1400 E 'E 120C

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8 6oc

40C

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Hour of Day

2200

1600

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400

40

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Fig. 1. Diurnal l eaf water potent ia l , l eaf di f fus ive c o n d u c t a n c e , leaf temperature and air temperature , and q u a n t u m f lux dens i ty o f t w o cassava cultivars on 25 July 1 9 7 9 , 12 w e e k s after planting.

Fig. 2. Diurnal q u a n t u m f lux dens i ty and air temperature and the e f f e c t o f water stress on the leaf water potent ia l , l eaf dif fusive c o n d u c t a n c e and leaf temperature o f t w o cassava cultivars. Measurements taken 29 Augus t 1 9 7 9 , 16 w e e k s after plant ing and 2 weeks after the start o f the per iod o f rainfall exc lus ion . (The vertical bars represent 1 standard error.)

Page 5: Response of cassava to water shortage III. Stomatal control of plant water status

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Page 6: Response of cassava to water shortage III. Stomatal control of plant water status

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Page 7: Response of cassava to water shortage III. Stomatal control of plant water status

TABLE I

Climatic data for the four days of measurement

303

Day Temperature (°C) Shortwave Pan Vapour radiation evaporation pressure

Maximum Minimum (ca] cm -2) (mm) (kPa)

25 July 31.0 17.0 438 7.8 1.76 29 August 28.0 17.0 456 4.8 1.92 12 September 29.0 18.0 426 3.9 2.01 26 September 31.0 18.0 433 3.5 1.96

TABLE II

Mean conductance (mm s -1 ) of the abaxial surface of leaves of two cultivars of cassava during the morning (08.30--12.00 h) and the afternoon (12 .30-16 .30 h) in response to water stress (each value is the mean of at least 100 measurements)

Cultivar Treatment Date

25 July 79 29 August 12 September 26 September

am pm am pm am pm am pm

M Col 22 Control 7.0 4.5 4.3 3.4 5.4 5.4 3.4 2.8 Stress -- -- 3.2 2.0 2.7 1.7 1.2 1.4

M Mex 59 Control 6.6 2.2 3.7 4.4 4.3 5.8 3.1 2.8 Stress -- -- 3.0 1.3 2.0 1.6 1.2 1.0

s u m m a r i z e d as t h e s i m p l e c o r r e l a t i o n c o e f f i c i e n t s o f T a b l e I I I . T h e r e l a t i o n s h i p

h o l d s s t r o n g l y in t h e stress p l o t s a n d e spec i a l l y in t h e cu l t i va r M Mex 59. S t o m a t a l d e n s i t y a n d size a re p r e s e n t e d in T a b l e IV t o g e t h e r w i t h m e a s u r e -

m e n t s o n f o u r o t h e r cu l t i va r s f r o m a para l l e l e x p e r i m e n t m a i n t a i n e d at t h e

s a m e t i m e . T h e c a l c u l a t i o n s o f d i u r n a l c r o p t r a n s p i r a t i o n are p r e s e n t e d in Fig. 5. T h e s e c o m b i n e t h e e f fec t s o f r e d u c e d leaf a rea ( spec i f i ed in Fig. 5) a n d

r e d u c e d l ea f c o n d u c t a n c e d u e t o t h e stress t r e a t m e n t .

Fig. 3. Diurnal quantum flux density and air temperature and the effect of water stress on the leaf water potential, leaf diffusive conductance and leaf temperature of two cassava cultivars. Measurements taken 12 September 1979, 18 weeks after planting and 4 weeks after the start of the period of rainfall exclusion. (The vertical bars represent 1 standard error. )

Page 8: Response of cassava to water shortage III. Stomatal control of plant water status

304

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Page 9: Response of cassava to water shortage III. Stomatal control of plant water status

305

TABLE III

Corre la t ion b e t w e e n leaf diffusive c o n d u c t a n c e and leaf-air vapour pressure d i f ference for two cassava cultivaxs at a m b i e n t q u a n t u m flux dens i ty above 900 #E m -2 s-1

Cultivar T r e a t m e n t Date

25 Ju ly 29 August 12 S e p t e m b e r 26 S e p t e m b e r

M Col 22 Cont ro l 0.64* - -0 .22 0.26 --0.35 Stress - - 0.57* - -0 .65* - -0 .58

M Mex 59 Con t ro l - 0 . 9 0 * * * - -0 .28 - -0 .20 - -0 .50 Stress - - - 0 . 6 3 * * - - 0 . 7 9 * * * 0 .94***

0.40

E

.&

0.20

LO

0.00

Control M Col 22 •

M Mex 59 •

0800 ~000 t200 1400 ~600' i800 •

Stress 0

0 LAI : 4.0

i

0800 I000 1200 1400 lEO0 1800

Hour of Day

Fig. 5. C o m b i n e d e f fec t o f leaf area index and leaf diffusive c o n d u c t a n c e on the diurnal t ranspi ra t ion of t w o cassava cultivars. The p lan t and cl imatic data axe those col lec ted on 26 S e p t e m b e r and the calculat ions were made wi th a c o m b i n a t i o n equat ion .

Fig. 4. Diurnal q u a n t u m flux dens i ty and air t e m p e r a t u r e and the e f fec t of water stress on the leaf water po ten t ia l , leaf diffusive c o n d u c t a n c e and leaf t empe ra tu r e of two cassava cultivaxs. Measuremen t s taken 26 S e p t e m b e r 1979, 20 weeks af ter p lant ing and 6 weeks af ter the s tar t o f t he per iod o f rainfall exclusion. (The vertical bars r ep resen t 1 s t andard error. )

Page 10: Response of cassava to water shortage III. Stomatal control of plant water status

306

DISCUSSION

In previous papers reporting aspects of this exper iment it was shown that the exclusion of rainfall had significant effects on the product ion and distri- but ion of dry mat ter (Connor et al., 1981) and upon leaf product ion and senescence (Connor and Cock, 1981). Without except ion the data support the view that the response was to water shortage. For this reason it is impor tant to commence the present discussion with a consideration of the leaf water potential data since leaf water potential is generally accepted as the definitive parameter of plant water status.

The data presented in Figs. 1--4 demonstrate , at least during the day-time hours when measurements were made, that the plants of both cultivars from both t reatments showed remarkably consistent diurnal patterns of leaf water potential . There is evidence of slower and incomplete (Figs. 2--4) evening recovery under stress but overwhelmingly the impression is one of the mainte- nance of internal water status in the face of a t rea tment which had severe effects on plant form and productivi ty. Thus leaf water potential cannot be used here as an index of the differential stress experienced by the cultivar-- t rea tment combinations but rather as a s ta tement of the success of the stomatal response in maintaining stable water status. This was perhaps generally over- done, since dayt ime leaf water potential is if anything higher in the stressed plants. In this discussion we therefore deal principally with stomata and their contr ibut ion towards the maintenance of internal plant water status but we conclude with an analysis of the combined and interacting effects of stomatal and leaf area modificat ions on the pat tern of crop water use.

The porometers used in this study were calibrated over the range 0.3--20 mm s -1 and on no occasion was the conductance of the upper (adaxial) leaf surface seen to rise into this range. Stomata were not observed on the upper surface in these, or other , cultivars so that in rout ine measurement of leaf diffusive conductance emphasis was concentra ted on the behaviour of the lower leaf surface. Thus Figs. 1--4 present the diffusive conductance of one leaf surface acting in parallel with a complementary surface of effectively zero conductance. Determinat ion of the cuticular conduct ivi ty of the adaxial sur- faces was no t possible with the available instruments. However, it is certainly

0.3 mm s -1. Thus abaxial conductance and leaf conductance are equivalent for cassava but the distinction must be maintained in comparisons between conductances of hypos tomatous and amphis tomatous leaves.

The mean leaf diffusive conductance of cassava may, under favourable con- ditions, approach 10 mm s -1 (Figs. 1--3). Thus on the basis of the stomatal pa thway at least, field grown cassava should have the potential for high rates of photosynthesis and hence also of transpiration. However it is also clear f rom the data tha t even the visually unstressed cassava of the control plots, generally operated at considerably lower conductance (3--5 mm s -~) and this is probably the range of maximum conductance encountered in a rainfed environment. At this level, the conductance of the physical pathway is likely to limit photo- synthetic rates as well as provide a control on transpiration.

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307

Under stress the conductance patterns were considerably altered. Mean con- ductances show tha t the plants operate at extremely low exchange rates with average conductances for large samples frequently less than 1 mm s -1 (Figs. 2--4). Under stress conditions the high variability of leaf conductance, a characteristic of the stomatal behaviour under well watered conditions, is considerably reduced. Low conductances are consistent with an effective limi- tat ion to water loss. However, the comparison of these values with conduc- tances of lower leaf surfaces during the night and upper leaf surfaces at any time {~ 0.3 mm s -1) show that the stomata are not closed and that they remain responsive. The continuing growth of the storage roots and the limited but continuing development of the leaf system that was observed during this stress cycle (Connor et al., 1981; Connor and Cock, 1981) suggest that a positive carbon balance was maintained under these conditions but measurements of photosynthesis under field conditions are needed to define carbon assimilation under stress. A positive carbon balance was maintained down to a leaf water potential of - 1 2 bar with pot ted plants in the laboratory (J. Palta, unpub- lished). As in the field experiment reported here, the plants responded to in- duced stress by a restriction of stomatal aperture which was not associated with notable changes in the diurnal pattern of leaf water potential. Cassava is not alone with this form of response. Similar behaviour has been found in siratro (Macropunctilium atropurpureum) (Ludlow and Ibaraki, 1979; Wilson et al., 1980), cowpea (Vigna unguiculata) (Hall and Shultze, 1980), kenaf (Hibiscus cannabinus) (Muchow et al., 1980) and even with soybean under- going a slow drying cycle under field conditions (Reicosky and Deaton, 1979).

The summary of mean leaf conductance during morning and af ternoon periods presented in Table II highlights important features of the data presented in Fig. 1--4. Firstly the conductance of M Mex 59 is generally lower than that of M Col 22, indicating a potentially important genotypic difference. M Mex 59 is a more vigorous variety than M Col 22 and, although it maintained a higher leaf area in both the control and stress treatments, it was not possible to detect a more extensive root system (Connor et al., 1981). Low conductance compensates for high leaf area in the control of transpira- tion. The second feature is that the afternoon conductance in both cultivars, and especially under stress, is less than that of the morning. Two anatomical features of cassava could contribute to this. Firstly it has been shown (Connor et al., 1981) that cassava has a remarkably sparse root system in which root densities are commonly < 10 -3 mm -2, a characteristic likely to lead to supply difficulties later in any day as the soil in proximity to the roots dries and its hydraulic conductance falls, but which would become a more serious problem as the profile dries and its general hydraulic conductance falls. Secondly the plant itself has at least the potential for a significant internal capacitance in- volving both storage roots and stems. The significance of the internal capaci- tance in the water relations of cassava has yet to be evaluated but certainly a plant which can contain up to 8 1 of water (D.J. Connor and J. Palta, un- published) and be faced, under the conditions reported here, with a maximum

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308

dai ly w a t e r loss o f a r o u n d 4 1 does have a s ignif icant c apac i t y to bu f f e r its w a t e r loss p e r h a p s especia l ly in the ear ly pa r t o f the day .

C o n d u c t a n c e p a t t e r n s changed u n d e r stress b u t so also did the a n a t o m y o f t he s t o m a t a l appa ra tus . Even u n d e r re la t ively well wa te red cond i t i ons w h e n leaf e x p a n s i o n is n o t ser iously res t r i c ted , cassava s t o m a t a are small and dense re la t ive to o t h e r c r o p species (Table IV) . Under stress, howeve r , w h e n leaf expans ion is ser iously r educed , the s t o m a t a are b o t h smaller and m o r e dense pe r un i t area . T h e d a t a in Tab le IV d e m o n s t r a t e t h a t the e f fec t is no t s imp ly a r e d u c t i o n in leaf expans ion , because unde r stress t o t a l s t o m a t a per leaf as well as leaf size are r educed . T h e s t o m a t a l census da ta a l low i m p o r t a n t conclus ions .

TABLE IV

The effect of water stress on the stomatal characteristics of six cultivars of cassava

Plant age 3 months 7 months

Cultivar Control Plots Control Plots Stress Plots

Density Pore Density Pore Leaf Density Pore Leaf (mm -2) length (mm -2) length area (ram -2) length area

(.m) (~m) (cm 2) (~m) (cm 2)

M Col 22 509 15.8 620 16.7 150 856 8.5 42 M Mex 59 474 15.0 643 17.1 180 812 11.7 71 M Ven 218 516 12.9 648 19.6 295 919 13.8 40 M Col 72 602 15.6 614 15.5 199 880 9.7 54 M Col 638 538 13.6 626 17.8 110 812 10.9 39 M Col 1684 497 14.7 593 17.5 168 863 7.0 69 LSD (P < 0.05) 26 0.3 36 0.3 43 57 0.4 16

Sample number 20 160 24 48 12 24 48 12

Firs t ly , ca lcu la t ions of leaf c o n d u c t a n c e f r o m s toma ta l d imens ions (Monte i th , 1973) d e m o n s t r a t e t h a t fo r well e x p a n d e d cassava leaves the re is suf f ic ien t conduc t i ve capac i t y to expla in t he m eas u red high c o n d u c t a n c e s if on ly a r o u n d 10% of t he s t o m a t a are o p e n to 0.3 of the i r p o t e n t i a l m a x i m u m size. M a x i m u m size was ca lcu la ted as t he circle wi th c i r c u m f e r e n c e of twice t he length of t he closed pore . The value o f 0.3 is based u p o n the grea tes t ape r tu re s obse rved in the s t o m a t a l impress ions . Large n u m b e r s o f small s t o m a t a are a f e a tu r e o f those p lan ts t h a t exercise e f fec t ive s t om a t a l con t ro l b u t the s ignif icance o f this a n a t o m i c a l charac ter i s t ic is n o t u n d e r s t o o d . Ind iv idua l cassava leaves are k n o w n to r e m a i n p h o t o s y n t h e t i c a l l y act ive fo r per iods in excess o f 100 days (J .H. Cock , unpub l i shed da ta ) so pe rhaps dur ing this t i m e the con t ro l o f gas exchange is n o t a lways t he t ask o f all or the same subse t o f s t oma ta . The census da t a f u r t he r suggest t h a t even t h o u g h the s t o m a t a l dens i ty increases unde r stress, the r e d u c t i o n in p o r e size m o r e t h a n c o u n t e r a c t s it. Thus fo r leaves f o r m e d unde r stress, the c o n d u c t a n c e fo r a n y p r o p o r t i o n o f the to t a l s t o m a t a l c o u n t

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open to the same degree is less than for leaves formed under relatively well watered conditions. This anatomical modification under stress could be inter- preted as a significant adaptive mechanism since the maintenance of lower conductance of the same physiological status has direct significance to growth and survival under stress.

Crop water use is controlled not only by stomatal closure which can modify energy and gaseous exchange per unit leaf area, but also by modification to the extent of the evaporating surface itself. It has been shown (Connor and Cook, 1981) that leaf product ion and expansion in cassava are particularily susceptible to water shortage, so that crop water-use under stress is controlled by significant modifications to both parts of this water-use equation. The calculations in Fig. 5 demonstrate this important interaction using the physiol- ogical and environmental data collected for 26 September, late in the stress cycle. It was ment ioned earlier that M Mex 59 matches its higher transpiring surface with a lower leaf conductance. This is clearly seen in the calculated transpiration behaviour of the stressed plants in Fig. 5. The lower conductance of M Mex 59 was able to offset its higher leaf area to provide a comparable daily transpiration pattern, which it is tempting to suggest, more closely corre- sponds to the capacity of a similar root system to supply water.

These data make it possible to define important characteristics of stomatal form and funct ion in cassava. The stomata have the physical capacity to present relatively high conductances (10 mm s -1) to the pathway between the atmo- sphere and mesophyll cells but even under relatively well watered conditions the commonly encountered conductances were generally lower than this (3--5 mm s -1). Under stress, the pat tern of conductance is substantially modified. With significant intercultivar differences, the conductances are greatly reduced, less variable, and capable of causing significant reductions in crop water use. The expansion of leaves formed under stress was greatly reduced and the anatomical characteristics of the stomata were considerably altered also. The resulting pat tern of more dense bu t smaller s tomata itself contr ibutes to the maintenance of lowered conductances under stress. The diurnal behaviour of the plant suggests that it is regularly source limited in its water relations by its sparse root system but the quantitative significance of root density and of the hydraulic capacitance of the storage root- -s tem system have yet to be evaluated.

The data, however, throw limited light on the basis of the stomatal response. Even with some allowance for possible osmotic adaptat ion (Acevedo et al., 1979), it seems unlikely that feed back control by leaf water potential (Raschke, 1975) could explain the observed patterns of leaf conductance and leaf water potential. In view of the correlations that exist within these data be tween conductance and leaf-air vapour pressure difference (Table III) a feed forward control (Farquhar, 1978; Cowan, 1977) mediated by peristomatal transpiration is a more likely candidate. A number of species are considered to behave in this way (Hall et al., 1976; Muchow et al., 1980; Ludlow and Ibaraki, 1979; Sheriff and Kaye, 1977). There are, however, other alternatives including the possible involvement of stomatally active compounds (Loveys

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a n d K r i e d e m a n n , 1 9 7 3 ; B e a r d s e l l a n d C o h e n , 1 9 7 5 ) . A d d i t i o n a l l y i t s h o u l d b e s t r e s s e d t h a t a n y s t o m a t a l m e c h a n i s m t h a t m a i n t a i n s s t a b l e i n t e r n a l w a t e r s t a t u s in t h e f a c e o f v a r y i n g e v a p o r a t i v e d e m a n d w o u l d s h o w a s im i l a r c o r r e l a - t i o n b e t w e e n c o n d u c t a n c e a n d t h e d r i v i n g f o r c e f o r w a t e r loss , t h e l ea f - a i r v a p o u r p r e s s u r e d i f f e r e n c e .

ACKNOWLEDGEMENTS

D . J .C . w i s h e s t o t h a n k L a T r o b e U n i v e r s i t y f o r t h e o p p o r t u n i t y t h r o u g h i t s O u t s i d e S t u d i e s P r o g r a m m e , t o u n d e r t a k e t h e s e s t u d i e s a n d w i s h e s t o t h a n k C I A T f o r i t s e n c o u r a g e m e n t , a s s i s t a n c e a n d h o s p i t a l i t y . We b o t h w i sh t o e x p r e s s o u r a p p r e c i a t i o n t o t h e m e m b e r s o f t h e Cassava P h y s i o l o g y T e a m f o r t h e i r c a r e f u l a s s i s t a n c e in t h i s w o r k . Drs . F i s c h e r , H s i a o , L u d l o w a n d W h i t f i e l d o f f e r e d v a l u a b l e c o m m e n t s o n t h e m a n u s c r i p t .

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