Novel Evidence for a Lack of Water Vapour Saturation Within the Intercellular Airspace of Turgid Leaves of Mesophytic Species

Ward, D. A. and Bunce, J. A. 1986. Novel evidence for a lackof water vapour saturation within the intercellular airspaceof turgid leaves of mesophytic species—J. exp. Bot. 37:504– By utilizing a dual-surface leaf chamber evidence was obtainedsuggesting that the water vapour pressure within the intercellularairspace of turgid leaves of mesophytic species can deviatesignificantly from the saturation vapour pressure at the leaftemperature. When the water vapour pressure of the air surroundingthe lower leaf surface of sunflower was maintained constantand high, suddenly exposing the upper leaf surface to air witha low water vapour content caused the lower leaf surface toexhibit a negative rate of transpiration (i.e. an apparent uptakeof water vapour). Since the vapour pressure of the air surroundingthe lower (moist) surface was less than the saturation vapourpressure at the leaf temperature, the occurrence of negativetranspiration indicated that the vapour pressure of the leafairspace deviated from saturation under the conditions of measurementused. For both soybean and sunflower it was also found that if thehumidity around the upper surface was maintained high and constant,a stepwise decrease in lower surface humidity caused substantialreductions in the transpiration rate and apparent conductanceof the upper surface without any concomitant change in its photosyntheticrate. In contrast, both the photosynthetic rate and conductanceof the lower surface were greatly reduced. The relative reductionsof photosynthetic rate and conductance at the lower surfacewere the same. These responses are most easily explained interms of a deviation from water vapour saturation within theintercellular airspace, which gives rise to spurious valuesof conductance. Key words: Intercellular space, water vapour pressure, turgid, leaves, mesophyte     

Humidity Pretreatment Affects the Responses of Stomata and CO2 Assimilation to Vapor Pressure Difference in C3 and C4 Plants

Experiments were carried out to investigate the long-term influenceof humidity on the short-term responses of stomata and CO₂ assimilationto vapor pressure difference in Oryza sativa (rice, C₃ species)and Panicum maximum (green panic, C₄ species). Plants were grownfor four weeks in growth chambers set at 35% and 85% relativehumidity at 25C air temperature, 38+2 Pa CO₂ partial pressureand 1,700µmol m^-2s^-1 photon flux density. Soil was saturatedwith water in both humidity treatments. Low humidity pretreatmentscaused low leaf conductance and low rates of transpiration andCO₂ assimilation in O. sativa, but small changes in stomatalresponses to humidity and in CO₂ assimilation were found inP. maximum. From the short-term gas exchange experiments, itwas noted that the responsiveness of leaf conductance to vaporpressure difference were affected by humidity pretreatmentsin O. sativa, whereas unaffected in P. maximum. In O. sativameasurements of CO₂ assimilation as a function of internal CO₂partial pressure (A-Ci curve) indicated that low humidity pretreatmentsreduced the CO₂ assimilation at high internal CO₂ partial pressure,but the initial slope of the A-Ci curve was unaffected. Furthermore,plant characteristics such as total dry weight and leaf areaof plants subjected to low umidity were lower than plants subjectedto high humidity. The reductions in O. sativa, however, werelarger than in P. maximum. Stomatal frequency from low humiditygrown plant was higher than that from high humidity grown plantsin both species although there is no significant difference.The data indicated that if the short term inhibition of netCO₂ assimilation at a high vapor pressure difference was imposedduring vegetative growth, the photosynthetic biochemistry andthe resultant plant growth were largely depressed in O. sativa,a C₃ species. (Received May 26, 1992; Accepted November 2, 1992)     

Photosynthesis and Transpiration in Rice as Influenced by Soil Moisture and Air Humidity

The paper describes the effect of soil moisture content andair humidity on CO₂ exchange (P_N), CO₂ diffusion resistance(Cr) and transpiration (E) in four varieties of japonica rice(Oryza sativa L.). A decrease in soil moisture content reducedthe rate of photosynthesis to a varying degree in the varieties.Reduction in photosynthesis was attributed to increase in Cr.The effect of low soil moisture on photosynthesis and CO₂ diffusionwas further intensified by decrease in air humidity. By maintaininga high humidity in the air around the leaves however, the effectof soil moisture deficiency was reduced considerably, exceptin Rikuto Norin 21 which was very sensitive to soil-moisturedeficiency alone. Dryness of the air enhanced the transpirationrate, although the increase was relatively less in the plantsfacing a simultaneous water crisis at the root surface. In plantsgrowing under flooded conditions, a decrease in air humiditycaused a slight depression in P_N despite the simultaneous decreasein Cr. Oryza sativa L., rice, photosynthesis, transpiration, diffusion resistance, soil moisture, air humidity     

Seasonal and diurnal changes in photosynthesis and carbon partitioning in Vitis vinifera leaves in vines withand without fruit

In Vitis vinifera L. cv. Chardonnay maintained in a greenhouse,the maximum rate of photosynthesis, the measured rates of denovo sucrose and starch synthesis and the total leaf sucroseand starch contents were relatively constant throughout theperiod from April to July although the partitioning of newlyfixed carbon was modified in favour of sucrose synthesis half-waythrough the growing period. In these experimental conditions,no significant differences in these parameters were observedin plants from which the fruit had been removed in comparisonto the controls. In field-grown vines, photosynthesis rose toa maximum in the early morning consistent with the increasein ambient irradiance and then subsequently progressively decreased.This occurred every day. On clear days the mid-morning depressionin the rate of CO₂ assimilation was closely linked to decreasein stomatal conductance, but there was no correlation betweenthese parameters on days when the sun was overcast. There wasno correlation between leaf sucrose content and the depressionin photosynthesis. The calculated rate of non-cyclic electronflow did not decline in parallel with the mid-morning depression and the quantum efficiency of photosystem II was constantfor the whole of the period when the CO₂ assimilation was decreasing.The mid-morning depression of photosynthetic CO₂ assimilationwas related to both stomatal and non-stomatal effects. In neithersituation did it have any measurable feedback effect on theelectron transport rate or on the carbo hydrate contents ofthe leaves. Key words: Vitis vinifera L., source-sink interactions, sucrose, starch, photosynthesis     

Photosynthesis and Diffusion Conductance of the Valencia Orange Fruit under Field Conditions

CO₂ uptake and diffusion conductance of Valencia orange fruits(Citrus sinensis L. Osbeck) were measured in the field duringthe growing season of 1977/78 to ascertain if, as in the leaf,stomata control photosynthesis and transpiration under changingenvironmental conditions. Measurements were made on 15 yearold trees grown in a sandy loam soil and receiving either adry or a wet treatment. Fruit diffusive conductance was measuredwith a modified water vapour diffusion conductance meter andgross photosynthesis was measured with a ¹⁴CO₂ uptake meter.Photosynthetically active radiation (PAR) was measured witha quantum sensor. Fruits exposed to light assimilated CO₂ ata rate which was 25–50% of that assimilated by leaves.The uptake was dependent on fruit size, PAR, chlorophyll content,and on diffusive conductance of the fruit epidermis. Epidermalconductance showed a diurnal trend which was similar in shapeto that of the leaf except in the late afternoon. Cuticularconductance of the fruit was calculated and ranged between 0.22and 0.30 mm s^–1. It was speculated that the CO₂ uptakeby the fruit could support the growth of flavedo cell layerswhen exposed to light. Dry soil caused an increase in the ¹⁴CO₂uptake by fruit possibly caused by the increased potential areaof the stomatal opening per unit of fruit surface area.     

采煤塌陷影响下土壤含水量变化对柠条气孔导度、蒸腾与光合作用速率的影响

采煤塌陷引起的土壤环境因子的变化对矿区植物生长的影响越来越受到人们的关注,气孔导度、蒸腾与光合作用作为环境变化响应的敏感因子,研究植物气孔导度、蒸腾与光合作用的变化是揭示荒漠矿区自然环境变化及其规律的重要手段之一。研究采煤塌陷条件下植物光合生理的变化是探究煤炭开采对植物叶片水分蒸腾散失和CO_2同化速率影响的关键环节,是探讨采煤塌陷影响下植物能量与水分交换动态的基础,而采煤矿区植物叶片气孔导度、蒸腾与光合作用速率对采煤塌陷影响下土壤含水量变化的响应如何尚不清楚。选取神东煤田大柳塔矿区52302工作面为实验场地,以生态修复物种柠条为研究对象,对采煤塌陷区和对照区柠条叶片气孔导度、蒸腾和光合作用速率以及土壤体积含水量进行监测,分析了采煤塌陷条件下土壤含水量的变化以及其对柠条叶片气孔导度、蒸腾与光合作用速率的影响。结果显示:(1)煤炭井工开采在地表形成大量裂缝,破坏了土体结构,潜水位埋深降低,土壤含水量均低于沉陷初期,相对于对照区,硬梁和风沙塌陷区土壤含水量分别降低了18.61%、21.12%;(2)柠条叶片气孔导度、蒸腾和光合作用速率均与土壤含水量呈正相关关系;煤炭开采沉陷增加了地表水分散失,加剧了土壤水分胁迫程度,为了减少蒸腾导致的水分散失,柠条叶片气孔阻力增加,从而气孔导度降低,阻碍了光合作用CO_2的供应,从而导致柠条叶片光合作用速率的降低,蒸腾速率也显著降低。     

Photosynthesis during leaf ontogeny in field-grown Nicotiana tabacum L. lines selected by survival at low CO2 concentrations

Leaf photosynthesis, stomatal conductance, internal CO₂ concentrationand leaf composition (photosynthetic pigments, total solubleprotein and Rubisco content) during leaf ontogeny of field grownNicotiana tabacum L. lines selected for survival at low atmosphericCO₂ concentrations are described. Selection at low CO₂ concentrations resutted in lines with highertotal dry matter production than their parent cultivar, butthis could not be related to improved photosynthesis which theselection method was designed to achieve (shown in previouswork). In the present work we report higher rates of photosynthesisin the selected lines in mature and old leaves, not found inyoung leaves when the capacity for photosynthesis was maximum.Differences in the regulation of the photosynthetic carbon reductioncycle as well as differences in the diffusive characteristicsof the mesophyll, due to changes in the size of the cells, maybe the cause for the higher rates of photosynthesis during leafsenescence in the selected lines. Key words: Photosynthesis, leaf ontogeny, tobacco, lines     

Photosynthesis by Flag Leaves of Wheat in Relation to Protein, Ribulose Bisphosphate Carboxylase Activity and Nitrogen Supply

The effects of nitrate supply on the composition (cell numbers,protein and chlorophyll contents) of flag leaves of winter wheatgrown with two amounts of N fertilizer and of spring wheat grownin the glasshouse under controlled nitrate supply are describedand related to photosynthesis. Nitrogen deficiency decreasedthe size of leaves, mainly by reducing cell number and, to asmaller extent, by decreasing cell volume. Protein content perunit leaf area, per cell and per unit cell volume was largerwith abundant N. Total soluble protein, ribulose bisphosphatecarboxylase-oxygenase (RuBPc-o) protein and chlorophyll changedin proportion irrespective of nitrogen supply and leaf age.Photosynthesis per unit area of flag leaf and carboxylationefficiency in both winter and spring wheat were proportionalto the amount of total soluble protein up to 7.0 g m^–2and to the amount of RuBPc-o protein up to 4.0 g m^–2.However, photosynthesis did not increase in proportion to theamount of total soluble or RuBPc-o protein above these amounts.In young leaves with a high protein content the measured ratesof photosynthesis were lower than expected from the amount andactivity of RuBPc-o. Carboxylation per unit of RuBPc-o protein,measured in vitro, was slightly greater in N-deficient leavesof winter wheat but not of spring wheat. RuBPc-o activity perunit of RuBPc-o protein was similar in winter and spring wheatleaves and remained approximately constant with age, but increasedin leaves showing advanced senescence. RuBPc-o protein fromN-deficient leaves migrated faster on polyacrylamide gels thanprotein from leaves with high N content. Regulation of the rateof photosynthesis in leaves and chloroplasts with a high proteincontent is discussed. The conductance of the cell to the fluxof CO₂ from intercellular spaces to RuBPc-o active sites iscalculated, from cell surface areas and CO₂ fluxes, to decreasethe CO₂ partial pressure at the active site by less than 0.8Pa at an internal CO₂ partial pressure of 34 Pa. Thus the decreasein partial pressure of CO₂ is insufficient to account for theinefficiency of RuBPc-o in vivo at high protein contents. Otherlimitations to the rate of photosynthesis are considered. Key words: Wheat, photosynthesis, nitrogen, ribulose, bisphosphate carboxylase     

Apparatus for the Simultaneous Measurement of Water Vapour and Carbon Dioxide Exchanges of Single Leaves

Equipment is described which delivers air with concentrationsof CO₂ and water vapour closely controlled in the ranges 0 to2500 ppm and 5 to 15 mb respectively, at flow rates of up to10 1 min^-1, to each four leaf chambers. The leaf temperatureis controlled to ±0.5 °C and, with a light intensityof 0.3 cal cm^-2 min^-1 visible radiation (0.4 to 0.7 µm)leaf temperature can be maintained at 17.5 °C.The apparatusused to measure the concentration differences between the watervapour and CO₂ entering and leaving the leaf chamber (used tocalculate transpiration, photosynthetic, and respiration rates)is described in detail.Results of tests, which show the necessityfor mounting a fan within the leaf chamber, are reported.Typicallight- and CO₂-response curves are given for kale leaves (Brassicaoleracca var. acephala) and an attempt is made to quantify theerrors in the measurement of photosynthesis and transpiration.     

Patterns of Stomatal Behaviour, Transpiration, and CO2 Exchange in Pea Following Infection by Powdery Mildew (Erysiphe pisi)

A comparison was made of stomatal behaviour, and related phenomena,between leaves of garden pea (Pisum sativum cv. Feltham First)inoculated with powdery mildew fungus (Erysiphe pisi) and uninfectedleaves on healthy plants. Twenty four hours after inoculation,stomata opened more widely in the light in infected leaves thanin healthy leaves. Thereafter, stomatal opening was progressivelyreduced by infection and stomata failed to close completelyin the dark until, 7 d after inoculation, all movements ceasedand stomata remained partly open. Transpiration in the lightfollowed closely the pattem of stomatal opening and, after anearly increase compared with healthy controls, was progressivelyreduced by infection. Evidence is presented that transpirationfrom the fungus was less than the reduction in transpiraationfrom the leaf which was caused when development of the myceliumincreased the boundary layer resistance of the leaf. Seven daysafter inoculation, transpiration in the dark was greater frominfected leaves than from healthy leaves because of partly openstomata in the dark. Net photosynthesis in infected leaves was reduced within 24h of inoculation to a level below that found in healthy leavesand thereafter it declined progressively. The initial reductionwas due to a transient increase in photorespiration, for whenthe glycolate pathway was inhibited by a 2% O₂ concentrationthere was no difference between the (gross) photosynthetic ratesof healthy and infected leaves. Changes in photorespirationrate were confirmed from the interpretation of the CO₂ burston darkening. Reduced stomatal opening was a contributory causeof the reduction in net photosynthesis in the later stages ofinfection. Since the rate of gross photosynthesis, but not therate of photorespiration, of infected plants fell below thatof healthy plants, and infected plants had a higher rate ofrelease of CO₂ in the dark than healthy plants from the thirdday after inoculation onwards, infected plants consume an increasinglygreater proportion of their photosynthate in respiratory processesthan do healthy plants. The CO₂ compensation point of infectedplants increased at every time of sampling after inoculation.     

An Evaluation of the Effects of Ethylene on Carbon Assimilation in Lycopersicon esculentum Mill

Leaf and whole plant gas exchange rates of Lycopersicon esculentumMill, were studied during several days of continuous exposureto ethylene. Steady-state photosynthesis and transpiration ratesof control and ethylene-treated individual leaves were equivalent.However, the photosynthesis and transpiration rates of treatedleaves required at least five times longer to reach 50% of thesteady-state rate. This induction lag was attributed to ethylene—inducedleaf epinasty and temporary acclimation to lower incident lightlevels immediately prior to measurement of gas exchange. Thewhole plant net carbon exchange rate (NCER) of a representativetreated plant was also reduced by 51% after 24 h exposure toethylene relative to both its pre-treatment rate and that ofthe control. Ethylene exposure reduced the growth rate of thetreated plant by 50% when expressed as carbon (C) gain. Theinhibition of NCER and growth rate associated with epinastywas completely reversed when the epinastic leaves were returnedto their original positions and light interception was re-established.The results demonstrate that the inhibition of whole plant CO₂assimilation is indirect and due to reduced light interceptionby epinastic leaves. Morphological changes caused by environmentalethylene are thus shown to reduce plant C accumulation withoutinhibiting leaf photosynthesis processes per se. Key words: Ethylene, carbon assimilation, growth     

In-Phase Cycling of Photosynthesis and Conductance at Saturating Carbon Dioxide Pressure Induced by Increases in Water Vapour Pressure Deficit

Bunce, J. A. 1987. In-phase cycling of photosynthesis and conductanceat saturating carbon dioxide pressure induced by increases inwater vapour pressure deficit.—J. exp. Bot. 38: 1413–1420. The leaf to air water vapour deficit was increased suddenlyfrom about 1·0 to 2·5 IcPa for single leaves ofsoybean (Glycine max L. Merr.) plants held at 30 °C, 2·0mmol m ^–2 s^–1 photosynthetic photon flux density(PPFD) and carbon dioxide pressures saturating to photosynthesis.After a lag of about 10 min, photosynthetic rate and stomatalconductance to water vapour began to decrease, and then cycledin phase with each other. The period of the cydes was about20 min. During these cycles the substomatal carbon dioxide pressurewas constant in the majority of leaves examined, and was alwaysabove saturation for photosynthesis. Epidermal impressions showedthat most stomata changed in aperture during the cycles, andthat very few were ever fully closed. Water potential measuredon excised discs changed by at most 0·1 MPa from theminima to the maxima in transpiration rate. In contrast, forleaves of sunflower (Helianthus animus L.) grown at low PPFD,the increase in VPD led to leaf wilting and decreased photosynthesis,followed by recovery of turgor and photosynthesis as stomatalconductance began to decrease. In these leaves photosynthesisand conductance then cycled approximately 180° out of phase.It is suggested that in soybeans decreased leaf conductanceinduced by high VPD provided a signal which decreased the rateof photosynthesis at carbon dioxide saturation by a mechanismthat was not related to a water deficit in the mesophyll. Key words: Photosynthesis, stomatal conductance, cycling, vapour pressure deficit     

The Relation Between the Nitrogen Concentration and Photosynthetic Capacity of Potato (Solanum tuberosum L.) Leaves

Measurements of the rate of light-saturated photosynthesis (P_max)were made on terminal leaflets of potato plants growing in cropssupplied with 0, 3, 6, 12, 24 and 36 g N m^–2. Measurementswere made between 100 and 154 d after planting. Two types ofleaf were selected—the fourth leaf on the second-levelbranch (L₄, _B1) and the youngest terminal leaflet that was measurable(L_YM). Later, the total nitrogen concentration of each leaflet(N_L) was measured. A linear regression between P_max and N_L,common to both leaf positions, explained 68.5% of the totalvariation. With L₄, _B1 leaves there was a significant improvementin the proportion of variation explained when regressions withseparate intercepts and a common slope were fitted to individualfertilizer treatments. These results suggest that an increasingproportion of leaf nitrogen was not associated with the performanceof the photosynthetic system with increasing nitrogen supply.This separation between nitrogen treatments was not as clearfor L_YM leaves. Stomatal conductance to transfer of water vapourwas neither influenced by leaf position nor directly by nitrogensupply. Rather conductance declined in parallel with the declinein photosynthetic capacity. Solanum tuberosum, potato, nitrogen, photosynthesis, stomatal conductance, leaf     

Evidence for the Occurrence of Feedback Inhibition of Photosynthesis in Rice

The response of photosynthesis in the flag leaf of rice (Oryzasativa) to elevated CO₂ or reduced O₂ was investigated relativeto other environmental factors using steady-state gas exchangetechniques. We found under moderate conditions of temperatureand photosynthetic flux density (PFD) (26°C and 700µmolquanta m^–2s^–1 similar to growth conditions) photosynthesisin the flag leaf of rice during heading and grain filling saturatedat near ambient levels of CO₂, with a concomitant loss of O₂sensitivity, when a high stomatal conductance was maintainedby high humidity (low vapor pressure deficit). Under 18°Cthere was near complete loss of O₂ sensitivity of photosynthesisat normal ambient levels of CO₂. This is in contrast to thelarge enhancement of photosynthesis by supra-atmospheric levelsof CO₂ and sub-atmospheric levels of O₂ by suppression of photorespirationwhen there is no limitation on utilizing the initial productof CO₂ assimilation (triose-P) as predicted from Ribulose-l,5-bisphosphatecarboxylase/oxygenase (Rubisco) kinetic properties. Thus, lossof sensitivity to CO₂ and O₂ has been previously explained asa limitation on utilization of triose-P to synthesize carbohydrates.Under high PFD at 25°C, the rate of photosynthesis in ricedeclined over a period of hours around midday, while the intercellularlevels of CO₂ remained constant suggesting a limitation on utilizationof photosynthate. Short-term fluctuations in climatic factorsincluding temperature, light and humidity could result in afeedback limitation on photosynthesis in rice which may be exacerbatedby rising CO₂. (Received March 12, 1998; Accepted May 14, 1998)     

Stomatal and Mesophyll Limitations of Photosynthesis in Phosphate Deficient Sunflower, Maize and Wheat Plants

The effects of phosphate deficiency on the composition and photosyntheticCO₂ assimilation rates of fully expanded leaves of sunflower,maize and wheat plants are described. The regulation of photosynthesisby stomatal and mesophyll characteristics of leaves of differentphosphate status is analysed and related to structure. Phosphatedeficient leaves had small concentrations of inorganic phosphate,Pi, in the tissue water. Rate of photosynthesis in leaves andstomatal conductance were smaller in plants grown with inadequatephosphate when measured under any given light intensity or CO₂partial pressure. Despite the decrease in stomatal conductance(and without evidence of patchy stomatal closure), the relativestomatal limitation of photosynthesis was similar in the plantsgrown with deficient or abundant phosphate. However, the mesophyllcapacity for photosynthesis was greatly limited by phosphatedeficiency. Leaves deficient in phosphate had larger numbersof small size cells per unit leaf area than leaves with adequatephosphate. The total soluble protein content of leaves decreasedwith phosphate deficiency in all three species; however, theleaf chlorophyll content was decreased only in sunflower andmaize and not in wheat. These results suggest that stomatalconductance did not restrict the CO₂ diffusion rate, ratherthe metabolism of the mesophyll was the limiting factor. Thisis shown by poor carboxylation efficiency and decreased apparentquantum yield for CO₂ assimilation, both of which contributedto the increase in relative mesophyll limitation of photosynthesisin phosphate deficient plants. Key words: Apparent quantum yield, carboxylation efficiency, phosphate nutrition, photosynthesis, stomatal and mesophyll limitation     

Phosphoglycerate dehydrogenase from soybean nodules : partial purification and some kinetic properties

The relationship between single leaf photosynthesis and conductance was examined in cotton (Gossypium hirsutum L.) across a range of environmental conditions. The purpose of this research was to separate and define the degree of stomatal and nonstomatal limitations in the photosynthetic process of field-grown cotton.

Photosynthetic rates were related to leaf conductance of upper canopy leaves in a curvilinear manner. Increases in leaf conductance of CO₂ in excess of 0.3 to 0.4 mole per square meter per second did not result in significant increases in gross or net photosynthetic rates. No tight coupling between environmental influences on photosynthetic rates and those affecting conductance levels was evident, since photosynthesis per unit leaf conductance did not remain constant. Slowly developing water stress caused greater reductions in photosynthesis than in leaf conductance, indicating nonstomatal limitations of photosynthesis.

Increases in external CO₂ concentration to levels above ambient did not produce proportional increases in photosynthesis even though substomatal or intercellular CO₂ concentration increased. The lack of a linear increase in photosynthetic rate in response to increases in leaf conductance and in response to increases in external CO₂ concentration demonstrated that nonstomatal factors are major photosynthetic rate determinants of cotton under field conditions.

     

Regulation of Photosynthesis of C3 Plants in Response to Progressive Drought: Stomatal Conductance as a Reference Parameter

We review the photosynthetic responses to drought in field-growngrapevines and other species. As in other plant species, therelationship between photosynthesis and leaf water potentialand/or relative water content in field-grown grapevines dependson conditions during plant growth and measurements. However,when light-saturated stomatal conductance was used as the referenceparameter to reflect drought intensity, a common response patternwas observed that was much less dependent on the species andconditions. Many photosynthetic parameters (e.g. electron transportrate, carboxylation efficiency, intrinsic water-use efficiency,respiration rate in the light, etc.) were also more stronglycorrelated with stomatal conductance than with water statusitself. Moreover, steady-state chlorophyll fluorescence alsoshowed a high dependency on stomatal conductance. This is discussedin terms of an integrated down-regulation of the whole photosyntheticprocess by CO₂ availability in the mesophyll. A study with sixMediterranean shrubs revealed that, in spite of some markedinterspecific differences, all followed the same pattern ofdependence of photosynthetic processes on stomatal conductance,and this pattern was quite similar to that of grapevines. Furtheranalysis of the available literature suggests that the above-mentionedpattern is general for C₃ plants. Even though the patterns describeddo not necessarily imply a cause and effect relationship, theycan help our understanding of the apparent contradictions concerningstomatal vs. non-stomatal limitations to photosynthesis underdrought. The significance of these findings for the improvementof water-use efficiency of crops is discussed.     

Separation of Direct and Indirect Responses of Stomata to Light: Results from a Leaf Inversion Experiment at Constant Intercellular CO2 Molar Fraction

Previous work has shown that stomata respond directly to light,but it was not clear whether the only additional response isthrough CO₂, or whether some other metabolite is involved inthis response. Gas exchange experiments were done with normallypositioned and inverted leaves of Hedera helix to investigatethis problem. The macroscopic optical properties of the leavesand their anatomical structure were also studied. These experimentssnowed that there is no need to postulate the existence of amessenger other than CO₂ to explain the indirect response ofstomata to light. The experiments also showed that leaf inversionaffects both stomatal conductance and photosynthesis, and highlightthe difficulties involved in the interpretation of the effectof leaf inversion on stomata when stomatal conductance measurementsare not done concurrently with measurements of CO₂ flux densityand intercellular CO₂ molar fraction Key words: Hedera helix, ivy, gas exchange, leaf inversion, stomatal conductance, light, CO₂ flux density, photosynthesis     

The effect of water stress on photosynthetic carbon metabolism in four species grown under field conditions

Abstract. The effect of gradually-developing water-stress has been studied in Lupinus albus L., Helianthus annuus L., Vitis vinifera cv. Rosaki and Eucalyptus globulus Labill. Water was withheld and diurnal rhythms were investigated 4–8d later, when the predawn water deficit was more negative than in watered plants, and the stomata closed almost completely early during the photoperiod. The contribution of ‘stomatal’ and ‘non-stomatal’ components to the decrease of photosynthetic rate was investigated by (1) comparing the changes of the rate of photosynthesis in air with the changes of stomatal conductance and (2) measuring photosynthetic capacity in saturating irradiance and 15% CO₂. Three species (lupin, eucalyptus and sunflower) showed larger changes of stomatal conductance than photosynthesis in air, and showed little or no decrease of photosynthetic capacity in saturating CO₂. Photosynthesis in air also recovered fully overnight after watering the plants in the evening. In grapevines, stomatal conductance and photosynthesis in air changed in parallel, there was a marked decrease of photosynthetic capacity, and photosynthesis and stomatal conductance did not recover overnight after watering water-stressed plants. Relative water content remained above 90% in grapevine. We conclude that non-stomatal components do not play a significant role in lupins, sunflower or eucalyptus, but could in grapevine. The effect of water-stress on partitioning of photosynthate was investigated by measuring the amounts of sucrose and starch in leaves during a diurnal rhythm, and by measuring the partitioning of ¹⁴C-carbon dioxide between sucrose and starch. In all four species, starch was depleted in water-stressed leaves but sucrose was maintained at amounts similar to, or higher than, those in watered plants. Partitioning into sucrose was increased in lupins and eucalyptus, and remained unchanged in grapevine and sunflower. It is concluded that water-stressed leaves in all four species maintain high levels of soluble sugars in their leaves, despite having lower rates of field photosynthesis, decreased rates of export, and low amounts of starch in their leaves.     

Characterization of the Type of Photosynthetic Carbon Dioxide Fixation in Jute (Corchorus olitorius L.)

Activities of photosynthetic and photorespiratory enzymes viz.,ribulose bisphosphate carboxylase, phosphoenol pyruvate carboxylaseand glycolate oxidase from jute (Corchorus olitorius L.; cv.JRO 632) leaves were compared with those from maize (C₄) andsunflower (C₃) leaves. The photosynthetic CO₂ fixation products,the release of ¹⁴CO₂ in light and dark following photosynthesisin ¹⁴CO₂, chlorophyll a: b ratio, gross leaf photosyntheticrate and dry matter production rate were also studied. The resultsshow that jute is a C₃ plant. Key words: Jute, Corchorus olitorius, C₃ photosynthesis