Effects of drought on O2 evolution and stomatal conductance of beans at the pollination stage

Environmental and Experimental Botany 42 (1999) 155–162

Effects of drought on O2 evolution and stomatalconductance of beans at the pollination stage

C. Pimentel a,*, G. Hebert b, J.V. da Silva c

a Departmento de Fitotecnia, I.A., Uni6ersidade Federal Rural do Rio de Janeiro, 23851-970 Seropedica, Rio de Janeiro, Brazilb Laboratoire de Biologie Vegetale, Uni6ersite Paris 7, Fontainebleau 77300, France

c Laboratoire de Biochimie de l’adaptation a la secheresse, Uni6ersite Paris 7, Paris 75251, France

Received 7 December 1998; received in revised form 11 May 1999; accepted 2 June 1999

Abstract

The predawn leaf water potential (Cl), O2 evolution (Ac) and stomatal conductance (gs) of three bean genotypesgrowing in a greenhouse were evaluated in order to compare the effect of water stress induced at different ages,especially during reproductive ontogeny. There was a peak of Ac for all genotypes 30 days after sowing (DAS) at thepollination stage, and the greatest effects of drought on this parameter occurred at this stage, with a decline in Acfrom a higher Cl (−0.35 MPa) than at the other stages (below −1.0 MPa). The Cl and final yield were also morereduced when drought was imposed at the pollination and flowering stages than at the vegetative or pod filling stages.During water stress, the gs of the genotypes was reduced by drought, but Ouro negro showed a different behavior inits stomatal control under drought. Firstly, with water deficit imposition, it maintained a higher gs level in themorning (9:00 h), when the vapor pressure deficit is the lowest of the day, than at midday (12:00 h) and in theafternoon (15:00 h). Secondly, after rehydration, this genotype had a higher gs than non stressed plants. © 1999Elsevier Science B.V. All rights reserved.

Keywords: Water stress; O2 evolution; Stomatal conductance

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1. Introduction

More than 60% of common bean crops grownin developing countries of Latin America, Africa,and Asia suffer from drought at some stage ofgrowth (Singh, 1995), with more than 50% reduc-

tion in yield when drought occurs during thereproductive stage (Halterlein, 1983). If drought isimposed at this stage, there are some markeddifferences in the responses of bean genotypes,which can be used to select for drought tolerance(Bascur et al., 1985; Pimentel et al., 1990; Fageriaet al., 1991). Plant water deficits at the pollinationstage commonly cause flower abortion due to anirreversibly arrested embryo development (Zin-selmeier et al., 1995). This is caused in part by thereduction of photosynthesis and limited sucrose

Abbre6iations: Cl, pre-dawn leaf water potential; Ac, O2

evolution; gs, stomatal conductance.* Corresponding author. Fax: +55-21-6821120.E-mail address: [email protected] (C. Pimentel)

S0098-8472/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved.

PII: S0098 -8472 (99 )00030 -1

C. Pimentel et al. / En6ironmental and Experimental Botany 42 (1999) 155–162156

flux from leaves to ovaries (Schussler and West-gate, 1995). Therefore, there is increasing evidencefor metabolic and growth regulator effects in var-ious crops, and some direct dehydration effectsthat might account for the susceptibility to waterlimitation of the pollination stage. CO2 and ABAare involved, and photosynthesis can also play arole but these factors might act in concert orseparately depending on the crop and stage ofdevelopment (Kramer and Boyer, 1995).

During the past years, many workers have beeninterested in understanding how drought limitsphotosynthesis. This process can be evaluated bythe measurement of O2 evolution (Ac), which isstoichiometrically equivalent to fixed CO2 (Delieuand Walker, 1981). Nevertheless, for plants desic-cated at the vegetative stage, the effect on Ac onlyoccurs in the presence of a severe water deficitcorresponding to a relative water content of 70–50% (Kaiser, 1987; Cornic et al., 1989; Cas-tonguay and Markhart, 1991; Chaves, 1991).Chaves (1991) postulated that stomatal closure isresponsible for the decline in net photosynthesisof leaves submitted to mild desiccation, and thatthe photosynthetic apparatus appears to be re-markably resistant to water stress. In this case, thepartial pressure of CO2 inside the leaf (ci) will below. Thus, to evaluate photosynthesis by O2 evo-lution, a saturated concentration of CO2 in themeasurement chamber is needed to overcomestomatal limitation and oxygenase function ofRubisco (Cornic et al., 1989). It was also arguedthat under drought an increased electron transferfrom photochemical reactions is allocated to theO2 reduction by photorespiration, protecting thephotosynthetic apparatus (Cornic et al., 1992).However, Brestic et al. (1995) demonstrated thatphotorespiration does not protect the photosyn-thetic apparatus during drought.

In turn, Lauer and Boyer (1992) showed anearly complete loss of photosynthetic biochemi-cal activity in sunflowers, soybeans and beansduring a water deficit, and they concluded thatstomatal closure does not limit photosynthesisbecause ci remains the same or increases underdesiccation. Also, measurements of CO2 ex-changes indicate an arrest of photosynthesis andphotorespiration (Boyer, 1978; Pham Thi and

Vieira da Silva, 1978; Pham Thi et al., 1982),which is due in part to the control of gs, butTashakorie et al. (1979) reported decreased activ-ity of the first enzyme in the photorespiratorycycle, glycolate oxidase, under a water deficit.Therefore, the effects of drought on photosynthe-sis are not yet elucidated, and less is known aboutthese effects at the reproductive stage. At thisstage, the metabolic and growth regulator activi-ties of leaves are altered by the high flux ofsucrose to reproductive organs (Schussler andWestgate, 1995), and drought can cause a markedeffect on photosynthesis compared to otherstages.

Most of the work involving measurements ofO2 evolution under water stress, was done at thevegetative stage, and showed a decline of O2

evolution only at a severe water stress. However,photoassimilate production and export are moresensitive to drought when water stress occurs atthe pollination stage. Therefore, our objective wasto evaluate the sensitivity of the O2 evolution andyield components to water deficit at the pollina-tion stage, compared to the vegetative, floweringand pod filling stages. To address this objective,we tested the hypothesis that O2 evolution andyield are more sensitive to water deficits at polli-nation stage than at other stages, and such sensi-tivity reflects metabolic inhibition ofphotosynthesis.

2. Materials and methods

Studies were conducted with three bean (Phase-olus 6ulgaris L.) genotypes, cultivated in 3 l potsunder greenhouse conditions, with 27°C day/17°Cnight, 13 h light with irradiance of 1500 mmolm−2 s−1/11 h dark. The potting mixture con-tained equal parts of neutral peat and vermiculiteand the stress was imposed slowly (8 days). Plantswere watered daily, and once a week they received50 ml of a nutrient solution containing 2.5 mMMgSO4, 20 mM Na2EDHA, 5.0 mM MnSO4, 1.0mM CuSO4, 2.0 mM ZnSO4, 10 mM H3BO4, 5.0mM (NH4)6 Mo7O24, plus 3 g l−1 of a chemicalfertilizer (Plantafora, Hoertghys, 4% N, 6% P, 4%K) and 2 g l−1 of iron chelate.

C. Pimentel et al. / En6ironmental and Experimental Botany 42 (1999) 155–162 157

The selected genotypes were: Xodo, an ancientblack seed genotype, BAT 477, recommended bythe International Center of Tropical Agriculture(CIAT) in Colombia as having a good waterstress tolerance (White et al., 1990), and Ouronegro, a new black seed genotype with high pro-ductivity. The black seed genotypes are exten-sively cultivated in Brazil. The three genotypeshave the same cycle, and therefore the water stresstreatment can be applied at the same time foreach stage.

In this experiment, 144 pots were laid out in acompletely randomized design (three genotypes×four age groups× four sample plants: one well-watered and three water-stressed× threereplicates). The water stress consisted of withhold-ing water for 8 days, followed by rehydration for2 days in plants of the three genotypes 20 daysafter sowing (DAS), and at 30, 40, and 50 DAS.These ages correspond to the vegetative, pollina-tion, flowering, and pod filling stages, respec-tively.

During water stress, the predawn leaf waterpotential (Cl, in MPa) was measured in a pressurechamber (Chas. W. Cook and Sons, UK), every 2days on the fourth trifoliolate leaf per plant foreach replicate. The same plant was used to evalu-ate the other parameters. The O2 evolution (Ac, inmmol O2 m−2 s−1) measurements were performedon a 10 cm2 leaf disc placed in a Hansatech LD2O2-electrode unit (Hansatech Ltd., Norfolk, UK).The chamber was equipped with a water jacketfor temperature maintenance at 25°C, and a capil-lary matting on the base of the chamber carried abicarbonate/carbonate buffer, allowing the atmo-sphere of the chamber to be saturated with CO2

close to 5%. The O2 evolution was measured every

2 days on the third trifoliolate leaf per plant foreach replicate, which was the first to be fullyexpanded. During the drying and rehydrationtreatments at the pollination stage (30 DAS), thestomatal conductance (gs, in mol m−2 s−1) of thetwo black seed genotypes cultivated in Brazil,Xodo and Ouro negro, was measured daily withan Li-65 porometer (LI-COR, Lincoln, NE), at9:00 h (3 h after the onset of the light period), andat 12:00 and 15:00 h. The third genotype, BAT477, is used for breeding programs, but it is notcultivated, and therefore, its gs was not evaluated.

The measurements of O2 evolution, stomatalconductance and predawn leaf water potentialwere made with three replicates on differentplants; the data were subjected to analysis ofvariance, and means were compared and segre-gated by the Tukey test, with the level of signifi-cance set at 0.05%.

3. Results

The well-watered plants of the genotypes Xodo,BAT 477, and Ouro negro, which flowered at 40DAS, showed a high Ac at the pollination stage,30 DAS (Table 1), when the flower buds begin tobe formed. However, the Ac value for Xodo atpollination stage was not significantly differentfrom the pod filling stage; for BAT 477, it was notdifferent from the vegetative and flowering stages;and for Ouro negro, the Ac value at the pollina-tion stage was not significantly different from thevalues at the vegetative and pod filling stages.

During exposure to water stress, the reductionof Cl (Fig. 1A–C) was lower at the vegetative (20DAS) or pod filling (50 DAS) than at the pollina-

Table 1O2 evolution (Ac, mmol O2 m−2 s−1) on well-watered plants (without water stress) of three bean genotypes at 20, 30, 40 and 50 daysafter sowing (DAS)a

Vegetative (20 DAS) Pod filling (50 DAS)Pollination (30 DAS) Flowering (40 DAS)Genotype

14.0ab10.0b17.3a10.7bXodo13.4ab 12.1b16.7a13.6abBAT 477

16.1a12.3ab 10.9b 12.4abOuro negro

a Data are means of three replicates. Within each column, there are no significant differences at 5% (Tukey test); and within eachrow, different letters represent significant differences at 5% (Tukey test) for each genotype.


Fig. 1. Leaf water potential (Cl) changes in 8 days of waterstress and after 2 days of rehydration at the vegetative (),pollination (�), flowering (�), or pod filling (2) stages: (A)Xodo; (B) BAT477; (C) Ouro negro. Data are means of threereplicates.

creased significantly for Xodo, BAT 477 andOuro negro at high Cl (−0.35 MPa) from the2nd day of water deficit. In contrast, for floweringplants at 40 DAS, Ac was reduced only from the4th day of stress for BAT 477 and Ouro negro ata low Cl (−1.15 and −1.0 MPa, respectively),and Xodo showed a decrease only from the 6thday of water stress. At the vegetative (20 DAS)and pod filling (50 DAS) stages, Ac was reducedonly from the 6th day of water deficit.

At the pollination and flowering stages, theplants have their maximal leaf area (Norman etal., 1995), and the reduction observed in Ac (Fig.2A–C) could be attributed to the maximal leaftranspiration and a more drastic effect of water

Fig. 2. O2 evolution capacity (Ac) changes in 8 days of waterstress and after 2 days of rehydration at the vegetative (),pollination (�), flowering (�), or pod filling (2) stages: (A)Xodo; (B) BAT477; (C) Ouro negro. Data are means of threereplicates.

tion (30 DAS) and flowering (40 DAS) stages. Onthe 8th day of dehydration for plants at 30 DAS,Cl of Xodo was significantly lower than for plantsat 20 and 50 DAS; and for BAT 477 and Ouronegro, the Cl value was lower only when com-pared to plants at 20 DAS. For plants experienc-ing water deficits at 40 DAS, in comparison withplants at 20 and 50 DAS, the Cl was significantlylower on the 2nd, 4th and 8th days for Xodo, andon the 8th day for Ouro negro, whereas BAT 477showed a decrease from the 4th day ofdehydration.

The greatest effect of water deficit, in terms of asignificant decrease in Ac, occurred at 30 DAS,the pollination stage for Xodo, BAT 477 andOuro negro (Fig. 2A–C). At this stage, Ac de-


Fig. 3. Stomatal conductance (gs) changes in 8 days of waterstress and after 2 days of rehydration at the pollination stage(30 DAS) at 9:00 (), 12:00 (�) and 15:00 h (�), on the twocultivated genotypes: (A) Xodo; (B) Ouro negro. Data aremeans of three replicates.

stressed plants, especially at the pollination stage.There was a more drastic reduction in the numberof seeds and in seed weight per plant for allgenotypes when water deficit was imposed at theflowering stage. The effect of dehydration on pro-ductivity was smaller when drought was appliedat the vegetative and pod filling stages.

4. Discussion

The high value of Ac, which is equivalent tophotochemical reaction activity, at the pollinationstage (Table 1) will ensure the ATP and NADPHproduction necessary for the high CO2 fixationoccurring at this stage (Jamro et al., 1993; Pi-mentel, 1998). During the pollination stage, theplants need to have a high accumulation of pho-toassimilates to be used for the next stages ofdevelopment, i.e. pod and seed growth, which arecharacterized by a lower photosynthesis (Ward-law, 1990). Also, in this species maximum mineralN assimilation or biological N2 fixation occursjust before the flowering stage, requiring also alarge quantity of the photoassimilates producedby shoots (Foyer and Galtier, 1996).

An effect of water stress on Ac has been shownonly in the presence of severe water stress, and itwas concluded that the initial decline in photosyn-thesis in bean leaves was not the result of damageto the light reactions of photosynthesis (Cornic etal., 1989; Castonguay and Markhart, 1991;Chaves, 1991). Our results are consistent with thisconclusion for the vegetative, flowering and podfilling stages. However, during the pollinationstage, Ac is more sensitive to water stress (Fig.2A–C). At this stage and at the flowering stage,Cl was also reduced to lower values than at otherstages (Fig. 1A–C). The high sensitivity of Ac atthis stage can be due in part to the effects of Cl

and growth regulator controlling the changes insource–sink relationships at this initial reproduc-tive stage, and in part to the higher cytoplasmicphotoassimilate accumulation on leaves than atother stages (Schussler and Westgate, 1995). Thisaccumulation is due to the higher CO2 fixation(Table 1) and consequent triose-P production atthis stage, associated with the arrested sucrose

stress on plants. However, at 30 DAS, the pollina-tion stage, the genotypes maintained values of Cl

not significantly different from the values at theother ages, until the 8th and last day of stress. Atthis age the effect on Ac was significant from the2nd day of drought, while at the other ages thiseffect began later.

The stomatal behaviour under water stress forthe two genotypes is presented in Fig. 3. With theimposition of drought gs was reduced for bothgenotypes, but Ouro negro had a high gs at 9:00 hduring the first 6 days of stress and during rehy-dration, with a lower gs value at 12:00 and 15:00h (Fig. 3B). For Xodo, gs values were the same at9:00 and 12:00 h almost every day (Fig. 3A).When rehydrated, on the 9th and 10th days oftreatment, the gs value at 9:00 h for Ouro negrowas significantly higher than for the controlplants, but this was not the case for Xodo.

The yield components of the genotypes areshown in Table 2. Generally, the number of podsper plant was significantly smaller in water


export from leaves to reproductive organs(Kramer and Boyer, 1995). Thus, an increase inphotosynthetic phosphorylated intermediates mayoccur in the cytoplasm and vacuoles, reducing theavailability of cytoplasmic Pi to be transportedback to chloroplasts by the Pi/triose-P transloca-tor, which controls chloroplast photoassimilatemetabolism (Leegood, 1996). Therefore, theremay be a decline of Pi in the stroma, limiting therate of ATP synthesis and pH control in thechloroplast (Hall and Rao, 1994), and conse-quently the photochemical reaction and Ac at thisstage may be more sensitive to water deficit thanat other stages.

Chaves (1991) and Plaut (1994) argued that theparameter most closely related to the decline inphotosynthesis during water stress is gs. In ourstudy with Xodo and Ouro negro, two genotypescultivated in Brazil, gs was reduced duringdrought too (Fig. 3(A–B)). Therefore, Ouro ne-gro genotype presented an interesting gs response

not shown by Xodo under water stress. Its highestgs values were observed at 9:00 h, decreasing at12:00 and 15:00 h. This response by Ouro negromight be advantageous in withstanding droughtsince in the morning (9:00 h), the vapour pressuredeficit of the air is low, and the plant can open thestomata to assimilate CO2 without any seriousloss of water by transpiration.

The yield components of the three genotypes(Table 2) were more reduced when drought oc-curred during the pollination (30 DAS) and flow-ering (40 DAS) stages, when the effects of waterstress on Cl are considerable (Fig. 1A–C). Atthese stages, photosynthesis is well correlated withthe final yield of reproductive crops (Kramer andBoyer, 1995), and because Ac is more affected bywater deficit at the pollination stage, the reductionin yield must be due in part to this inhibition ofAc. This accentuated decline in Ac and yieldcomponents, observed in bean genotypes underwater stress at the pollination stage, support the

Table 2Yield components of three bean genotypes, on well-watered plants (without water stress) for each age or under water stress appliedat 20, 30, 40 and 50 days after sowing (DAS)a

GenotypeStage Number of pods Number of seeds Productivity: g per plantSeed weight per plantper plant (%control)per plant (g per plant)

Xodo 7a 20a 3.9a 100ControlBAT 477 6a 27b 5.1b 100

20a 3.7a 100Ouro negro 6a

Vegetative 6aXodo 23c 4.7b 121(20 DAS)

4aBAT 477 14ab 3.1a 611003.7a18bc5aOuro negro

55Xodo 3aPollination 15a 2.1a

(30 DAS)422.1a12a3aBAT 477

Ouro negro 4a 11a 2.8a 74

XodoFlowering 6a 14b 2.0a 51(40 DAS)

BAT 477 4a 11ab 1.3a 26Ouro negro 4a 9a 1.2a 31

752.9a17a5aPod filling Xodo(50 DAS)

BAT 477 6a 23a 3.3a 65Ouro negro 5a 18a 3.6a 97

a Data are means of three replicates. Within each column, different letters represent significant differences at 5% (Tukey test) forgenotype at each age.


theory that the sensitivity of Ac and yield compo-nents to drought at this stage was greater than atthe other stages.

Our results with bean genotypes agree withthose of Schussler and Westgate (1995) and Zin-selmeier et al. (1995) with maize, and showed ahigher reduction of photosynthesis at the pollina-tion stage, which will cause a stronger yield effectthan at any other stages. However, more researchinto the regulation of photoassimilate synthesisand transport are needed to understand the effectsof water stress at the pollination stage.

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Effects of drought on O2 evolution and stomatal conductance of beans at the pollination stage