Reduction In Gs Research Articles

Long-term night chilling of cotton (Gossypium hirsutum) does not result in reduced CO2 assimilation.

The aim of this study was to characterise the response of CO2 assimilation (A) of cotton (Gossypium hirsutum L.) to short- and long-term exposures to night chilling. We hypothesised that short-term exposures to night chilling would induce reductions in gs and, therefore, A during the following days, while growth of cotton plants for several weeks in cool night conditions would cause elevated leaf carbohydrate content, leading to the down-regulation of the capacity for A. Transferring warm-grown seedlings of wild type cotton, transgenic cotton with elevated sucrose-phosphate synthase activity (SPS+) that might produce and export more sucrose from the leaf, and a segregating null to cool nights (9°C minimum) for 1 or 2 d caused a small reduction in A (12%) and gs (21-50%) measured at 28°C. Internal CO2 did not change, suggesting some biochemical restriction of A along with a gs restriction. After 30 d, new leaves that developed in cool nights exhibited acclimation of A and partial acclimation of gs. Despite the elevated leaf carbohydrate content when plants were grown to maturity with night chilling, no reduction in A, gs, carboxylation capacity, electron transport capacity, or triose-phosphate utilisation capacity occurred. Instead, growth in cool nights tended to retard the diminishing of photosynthetic parameters and gs for aging stem and subtending leaves. However, elevated SPS activity did not affect any photosynthetic parameters. Therefore, when cotton that is well fertilised with nitrogen is grown with continuous night chilling, photosynthesis should not be negatively affected. However, an occasional exposure to cool nights could result in a small reduction in A and gs for leaves that have developed in warm night conditions.

Adaptive Responses of Soybean and Cotton to Water Stress: I. Transpiration Changes in Relation to Stomatal Area and Stomatal Conductance

The adaptive responses of soybean and cotton to various irrigation levels were explored in terms of transpiration, stomatal role in transpiration, leaf temperature (Tl) and CO2 assimilation rate (An). Compared with cotton, soybean showed a lower flow rate of stem sap (FRSS), transpiration rate (E), stomatal conductance (gs), stomatal density and An and had a smaller stomatal area but larger leaf area, heavier root dry matter and higher Tl at all irrigation levels. Under water stress conditions, FRSS, E, gs, and An decreased and Tl increased more in soybean than in cotton. Stomatal area decreased in response to water stress though nonsignificantly but stomatal density was not affected by water stress in soybean. Stomatal area decreased significantly in response to water stress in cotton. We concluded that soybean and cotton adapted to water stress differently. Soybean adapted to water stress by reducing transpiration while cotton adapted to water stress by maintaining higher transpiration as compared with soybean. Soybean reduced the transpiration rate by reducing gs. Reduction of gs in soybean was due to reduced FRSS, which might have resulted from the lower root moisture absorption efficiency. The higher transpiration in cotton was due to a higher gs, which was supported by a higher FRSS, larger stomatal area, and probably the diaheliotropism. The higher gs and transpiration rate suppressed the increase in Tl thus preventing the decrease of An in response to water stress.

Inhibition of photosynthesis by high temperature in oak (Quercus pubescens L.) leaves grown under natural conditions closely correlates with a reversible heat‐dependent reduction of the activation state of ribulose‐1,5‐bisphosphate carboxylase/oxygenase

ABSTRACTInhibition of the net photosynthetic CO2 assimilation rate (Pn) by high temperature was examined in oak (Quercus pubescens L.) leaves grown under natural conditions. Combined measurements of gas exchange and chlorophyll (Chl) a fluorescence were employed to differentiate between inhibition originating from heat effects on components of the thylakoid membranes and that resulting from effects on photosynthetic carbon metabolism. Regardless of whether temperature was increased rapidly or gradually, Pn decreased with increasing leaf temperature and was more than 90% reduced at 45 °C as compared to 25 °C. Inhibition of Pn by heat stress did not result from reduced stomatal conductance (gs), as heat‐induced reduction of gs was accompanied by an increase of the intercellular CO2 concentration (Ci). Chl a fluorescence measurements revealed that between 25 and 45 °C heat‐dependent alterations of thylakoid‐associated processes contributed only marginally, if at all, to the inhibition of Pn by heat stress, with photosystem II being remarkably well protected against thermal inactivation. The activation state of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) decreased from about 90% at 25 °C to less than 30% at 45 °C. Heat stress did not affect Rubisco per se, since full activity could be restored by incubation with CO2 and Mg2+. Western‐blot analysis of leaf extracts disclosed the presence of two Rubisco activase polypeptides, but heat stress did not alter the profile of the activase bands. Inhibition of Pn at high leaf temperature could be markedly reduced by artificially increasing Ci. A high Ci also stimulated photosynthetic electron transport and resulted in reduced non‐photochemical fluorescence quenching. Recovery experiments showed that heat‐dependent inhibition of Pn was largely, if not fully, reversible. The present results demonstrate that in Q. pubescens leaves the thylakoid membranes in general and photosynthetic electron transport in particular were well protected against heat‐induced perturbations and that inhibition of Pn by high temperature closely correlated with a reversible heat‐dependent reduction of the Rubisco activation state.

Genotypic variation in drought response of silver birch (Betula pendula): leaf water status and carbon gain.

To assess genotypic variation in drought response of silver birch (Betula pendula Roth), we studied the plasticity of 16 physiological traits in response to a 12-14-week summer drought imposed on four clones in two consecutive years. In a common garden experiment, 1-year-old clonal trees from regions with low (550 mm year(-1)) to high rainfall (1270 mm year(-1)) were grown in 45-l pots, and leaf gas exchange parameters, leaf water potentials, leaf osmotic potentials and leaf carbon isotope signatures were repeatedly measured. There were no clonal differences in leaf water potential, but stomatal conductance (gs), net photosynthesis at ambient carbon dioxide concentration, photosynthetic water-use efficiency, leaf carbon isotope composition (delta13C) and leaf osmotic potentials at saturation (Pi0) and at incipient plasmolysis (Pip) were markedly influenced by genotype, especially gs and osmotic adjustment. Genotypes of low-rainfall origin displayed larger osmotic adjustment than genotypes of high-rainfall origin, although their Pi0 and Pip values were similar or higher with ample water supply. Genotypes of low-rainfall origin had higher gs than genotypes of high-rainfall origin under both ample and limited water supply, indicating a higher water consumption that might increase competitiveness in drought-prone habitats. Although most parameters tested were significantly influenced by genotype and treatment, the genotype x treatment interactions were not significant. The genotypes differed in plasticity of the tested parameters and in their apparent adaptation to drought; however, among genotypes, physiological plasticity and drought adaptation were not related to each other. Reduction of gs was the first and most plastic response to drought in all genotypes, and allowed the maintenance of high predawn leaf water potentials during the drought. None of the clones exhibited non-stomatal limitation of photosynthesis. Leaf gs, photosynthetic capacity, magnitude of osmotic adjustment and delta13C were all markedly lower in 2000 than in 1999, indicating root limitation in the containers in the second year.

Seasonal leaf gas exchange and water potential in a woody cerrado species community

Predawn leaf water potential (psipd) and morning values of leaf gas exchange, as net photosynthesis (A), stomatal conductance (gs), transpiration (E), and morning leaf water potential (psimn) were determined seasonally in 22 woody cerrado species growing under natural conditions. Despite the lower mean values of psipd in the dry season (-0.35 ± 0.23 MPa) compared to the wet season (-0.08 ± 0.03 MPa), the lowest psipd in the dry season (-0.90 ± 0.00 MPa) still showed a good nocturnal leaf water status recovery for all species studied through out the year. Mean gs values dropped 78 % in the dry season, when the vapor pressure of the air was 80% greater than in the wet season. This reduction in gs led to an average reduction of 33% in both A and E, enabling the maintainance of water use efficiency (WUE) during the dry season. Network connectance analysis detected a change in the relationship between leaf gas exchange and psimn in the dry season, mainly between gs-E and E-WUE. A slight global connectance value increase (7.25 %) suggested there was no severe water stress during the dry season. Multivariate analysis showed no link between seasonal response and species deciduousness, suggesting similar behavior in remaining leaves for most of the studied species concerning leaf gas exchange and psimn under natural drought.

Reduced stomatal conductance in sweetgum (Liquidambar styraciflua) sustained over long‐term CO2 enrichment

• Over 4 yr (1998–2001) we examined the effects of CO2 enrichment on stomatal conductance (gs) of sun and shade leaves of overstory sweetgum (Liquidambar styraciflua) grown at the Duke Forest Free Air Carbon CO2 Enrichment (FACE) experiment. • Gas-exchange measurements were taken in June and September of each year and relationships between water stress and stomatal conductance were examined. Stomatal density was measured in June 2000. Relative stomatal limitation (lg) was calculated from gas-exchange measurements. • We found a 28% reduction of gs in elevated CO2 that was sustained over the study period and was similar in sun and shade leaves. Elevated CO2 reduced lg by 26%. Stomatal density was not affected by CO2 enrichment. Elevated CO2 did not change the sensitivity of gs to soil moisture or vapor pressure deficit. • The data illustrate that decreased gs of sweetgum leaves in CO2 enrichment is consistent over long periods of time and under varying environmental conditions.

Stomatal regulation in Douglas fir following a fungal-mediated chronic reduction in leaf area

Pathogens can cause chronic premature needle abscission in coniferous species. To assess the potential impacts on tree productivity, stomatal regulation was investigated in Douglas fir with chronic stomatal occlusion and defoliation from varying levels of the Swiss needle cast (SNC) fungus, Phaeocryptopus gaeumannii. Levels of SNC disease and subsequent defoliation were manipulated by choosing six sites with varying levels of disease and by foliar applications of fungicides on six trees per site. Diurnal measurements of leaf water potential (Ψleaf), stomatal conductance (gs) and vapor pressure deficit (D) were made on six fungicide treated and six control trees per site. In addition, leaf specific hydraulic conductance was calculated on a single branch (KL_B) from three trees per treatment per site. Stomatal conductance at D=1 kPa (gsref) was negatively correlated with fungal colonization (number of fruiting bodies present in needle stomata) and positively correlated with KL_B. Despite reduced needle retention in diseased trees, KL declined due to a reduction in sapwood area and permeability (i.e., increasing presence of latewood in functional sapwood). In general, stomatal sensitivity to D for all foliage was consistent with stomatal regulation based on a simple hydraulic model [gs=KL(Ψsoil−Ψleaf)/ D], which assumes strict stomatal regulation of Ψleaf. However, when fungal presence reduced maximum gs below the potential maximum supported by hydraulic architecture, stomatal sensitivity was lower than expected based on the theoretical relationship: dgs/dlnD=0.6·gsref. The results indicate that losses in productivity associated with physical blockage of stomata and defoliation are compounded by additional losses in KL and a reduction in gs in remaining functional stomata.

Modelling stomatal conductanceof field‐grown sunflower under varying soil water content and leafenvironment: comparison of three models of stomatal response toleaf environment and coupling with an abscisic acid‐based modelof stomatal response to soil drying

AbstractStomatal conductance, gs, responds both tothe immediate or local environment of the leaf, such as CO2 partialpressure and irradiance, and to root‐sourced signals of water stress,particularly abscisic acid (ABA). Two models for the combined controlof gs were formulated and tested in sunflower(Helianthus annuus). First, several empirical models weretested for the local control, demonstrating that the Ball–Berrymodel [Ball, Woodrow & Berry (in Progress in PhotosynthesisResearch Vol. 4, pp. 5.221–5.224: M. Nijhoff,Dordrecht, The Netherlands) 1987] is consistently amongthe most accurate. A problem of statistical non‐independence inthis model is shown to be minor. The model offers regularity ofparameter values among most species and, despite an oversimplicationin representing known humidity‐response mechanisms, it incorporates othersignalling loops from CO2 and assimilation. In the firstcombined model, ABA as its concentration in xylem sap, [ABA]xy,down‐regulates the slope, m, in the Ball–Berry modelby the factor gfac = exp(– β[ABA]xy).The ABA‐induced reduction in gs decreases CO2 assimilation andsurface humidity, thus appearing to induce the local‐control mechanismto amplify the ABA‐induced stomatal closure. In the second combinedmodel, gs is estimated as the minimum of the local(Ball–Berry) response and the product gfac gs,max,with gs,max as a maximal unstressed conductance.Both models can predict gs from the external environmentalvariables with good accuracy (r2 near 0·8 over20‐fold variations in gs). Further analyses showthat gs responds to humidity almost quadraticallyrather than linearly. It also responds to assimilation as a powerlaw with an exponent that is significantly less than 1. These limitations,shared by other models, suggest more research into biochemical signalling.

Environmental and stomatal control of photosynthetic enhancement in the canopy of a sweetgum (Liquidambar styraciflua L.) plantation during 3 years of CO2 enrichment

AbstractLight‐saturated photosynthetic and stomatal responses to elevated CO2 were measured in upper and mid‐canopy foliage of a sweetgum (Liquidambar styraciflua L) plantation exposed to free‐air CO2 enrichment (FACE) for 3 years, to characterize environmental interactions with the sustained CO2 effects in an intact deciduous forest stand. Responses were evaluated in relation to one another, and to seasonal patterns and natural environmental stresses, including high temperatures, vapour pressure deficits (VPD), and drought. Photosynthetic CO2 assimilation (A) averaged 46% higher in the +200 µmol mol−1 CO2 treatment, in mid‐ and upper canopy foliage. Stomatal conductance (gs) averaged 14% (mid‐canopy) and 24% (upper canopy) lower under CO2 enrichment. Variations in the relative responses of A and gs were linked, such that greater relative stimulation of A was observed on dates when relative reductions in gs were slight. Dry soils and high VPD reduced gs and A in both treatments, and tended to diminish treatment differences. The absolute effects of CO2 on A and gs were minimized whenever gs was low (<0·15 mol m−2 s−1), but relative effects, as the ratio of elevated to ambient rates, varied greatly under those conditions. Both stomatal and non‐stomatal limitations of A were involved during late season droughts. Leaf temperature had a limited influence on A and gs, and there was no detectable relationship between prevailing temperature and CO2 effects on A or gs. The responsiveness of A and gs to elevated CO2, both absolute and relative, was maintained through time and within the canopy of this forest stand, subject to seasonal constraints and variability associated with limiting air and soil moisture.

Nitrogen-limited growth of lettuce is associated with lower stomatal conductance.

• C assimilation (A) has been shown to limit the growth of young Lactuca sativa (lettuce) plants following an interruption in their external N supply. Further data from these plants were used to test two hypotheses: that N-limited growth of lettuce is associated with lower stomatal conductance (gs ); and that reductions in gs result from adjustments to stomatal frequency or distribution. • The photosynthetic characteristics, nitrate and organic N-concentrations, as well as epidermal and stomatal distributions, were determined in leaves of hydroponically grown lettuce plants supplied continuously with N or with N removed for up to 14d. • Although N-limited plants had lower maximum rates of A, comparisons at equivalent values of gs showed that A was not directly limited by organic-N but by gs . Reductions in gs under N-limiting conditions did not associate with adjustments to stomatal frequency or distribution. • Associations between plant N and A could arise either through stomata responding directly to signals induced by N deprivation or to increased CO2 partial pressure at the sites of carboxylation.

Mean canopy stomatal conductance responses to water and nutrient availabilities in Picea abies and Pinus taeda.

We compared sap-flux-scaled, mean, canopy stomatal conductance (GS) between Picea abies (L.) Karst. in Sweden and Pinus taeda (L.) in North Carolina, both growing on nutritionally poor soils. Stomatal conductance of Picea abies was approximately half that of Pinus taeda and the sensitivity of GS in Picea abies to vapor pressure deficit (D) was lower than in Pinus taeda. Optimal fertilization increased leaf area index (L) two- and threefold in Pinus taeda and Picea abies, respectively, regardless of whether irrigation was increased. Although it increased L, fertilization did not increase GS in Picea abies unless irrigation was also provided. In Pinus taeda growing on coarse, sandy soils, the doubling of L in response to fertilization reduced GS sharply unless irrigation was also provided. The reduction in GS with fertilization in the absence of irrigation resulted from the production of fine roots with low saturated hydraulic conductivity. When Pinus taeda received both fertilization and irrigation, the increase in L was accompanied by a large increase in GS. In Pinus taeda, a reference GS (defined as GS at D = 1 kPa; GSR) decreased in all treatments with decreasing volumetric soil water content (theta). In Picea abies, theta varied little within a treatment, but overall, GSR declined with theta, reaching lowest values when drought was imposed by the interception of precipitation. Despite the large difference in GS both between Picea abies and Pinus taeda and among treatments, stem growth was related to absorbed radiation, and stem growth response to treatment reflected mostly the changes in L.

Leaf gas exchange characteristics differ among Sonoran Desert riparian tree species.

We investigated leaf gas exchange responses to leaf temperature, leaf-to-air vapor pressure deficit (VPD), and predawn and midday shoot water potential (psipd and psimd, respectively) of two native Sonoran Desert riparian tree species, Fremont cottonwood (Populus fremontii S. Wats.) and Goodding willow (Salix gooddingii Ball), and one exotic riparian tree species, saltcedar (Tamarix chinensis Lour. and related species). Measurements were made at two sites over 2 years that differed climatically. Because multiple linear regression models explained less than 29% of the variation in stomatal conductance (gs) and less than 48% of the variation in net photosynthetic rate (Pn) of all species, we used boundary-line analysis to compare gas exchange responses among species. Gas exchange rates were high in all species. The hyperbolic relationship between Pn and gs suggested that initial reductions in gs at high gs did not inhibit Pn. Reductions in gs of cottonwood and willow occurred at psimd values at or below previously reported xylem cavitation thresholds (-1.6 and -1.4 MPa, respectively), indicating tight stomatal regulation of water loss and a narrow cavitation safety margin. In contrast, reductions in gs of saltcedar occurred at psimd values well above the cavitation threshold (-7.0 MPa), but at much lower psimd values than in cottonwood and willow, suggesting a wider cavitation safety margin and less tight regulation of water loss in saltcedar. High VPD had a smaller effect on leaf gas exchange in willow than in cottonwood. In contrast, willow had a less negative psipd threshold for stomatal closure than cottonwood. Compared with cottonwood and willow, leaf gas exchange of saltcedar was more tolerant of high VPD and low psipd. These physiological characteristics of saltcedar explain its widespread success as an invader of riparian ecosystems containing native Fremont cottonwood and Goodding willow in the Sonoran Desert.

Direct and acclimatory responses of stomatal conductance to elevated carbon dioxide in four herbaceous crop species in the field

SummaryIn order to separate the net effect of growth at elevated [CO2] on stomatal conductance (gs) into direct and acclimatory responses, mid‐day values of gs were measured for plants grown in field plots in open‐topped chambers at the current ambient [CO2], which averaged 350 μmol mol−1 in the daytime, and at ambient + 350 μmol mol−1[CO2] for winter wheat, winter barley, potato and sorghum. The acclimatory response was determined by comparing gs measured at 700 μmol mol−1[CO2] for plants grown at the two [CO2]. The direct effect of increasing [CO2] from 350 to 700 μmol mol−1 was determined for plants grown at the lower concentration. Photosynthetic rates were measured concurrently with gs. For all species, growth at the higher [CO2] significantly reduced gs measured at 700 μmol mol−1[CO2]. The reduction in gs caused by growth at the higher [CO2] was larger for all species on days with low leaf to air water vapour pressure difference for a given temperature, which coincided with highest conductances and also the smallest direct effects of increased [CO2] on conductance. For barley, there was no other evidence for stomatal acclimation, despite consistent down‐regulation of photosynthetic rate in plants grown at the higher [CO2]. In wheat and potato, in addition to the vapour pressure difference interaction, the magnitude of stomatal acclimation varied directly in proportion to the magnitude of down‐regulation of photosynthetic rate through the season. In sorghum, gs consistently exhibited acclimation, but there was no down‐regulation of photosynthetic rate. In none of the species except barley was the direct effect the larger component of the net reduction in gs when averaged over measurement dates. The net effect of growth at elevated [CO2] on mid‐day gs resulted from unique combinations of direct and acclimatory responses in the various species.

Decreased root hydraulic conductivity reduces leaf water potential, initiates stomatal closure and slows leaf expansion in flooded plants of castor oil (Ricinus communis) despite diminished delivery of ABA from the roots to shoots in xylem sap

Soil flooding reduced stomatal conductance (gs) and slowed transpiration, CO2uptake and leaf elongation inRicinus communiswithin 2–6 h. These flood‐induced responses developed further over the next 21 h. They were not associated with increased delivery of abscisic acid (ABA) in xylem sap. Instead, ABA delivery from flooded roots decreased 6‐fold within 3 h, and remained low thereafter. Root hydraulic conductance (Lp) was depressed 47% below control values within 2 h of soil flooding, and declined further during the next 21 h. The smaller Lp temporarily decreased leaf water potentials (ΨL) by up to −0.4 MPa, and caused visible wilting 3 h into the flooding treatment at 80% relative humidity. Consequently, ABA concentrations in the shoot were increased, as indicated by analyses of phloem sap. Wilting, fall in ΨL and a reduction in gs were delayed for 6 h when 0.6 MPa pneumatic pressure (technical maximum) was applied to the roots. In flooded plants, phloem sap ABA concentrations returned to normal after 24 h. The initial stomatal closure, caused by soil flooding inR. communis, is attributed to decreased leaf hydration arising from the reduced LP of oxygen‐deficient roots. Continued stomatal closure and slow leaf expansion beyond 24 h were presumably achieved by non‐hydraulic means.

Stomatal acclimation to increased CO2 concentration in a Florida scrub oak species Quercus myrtifolia Willd

ABSTRACTNative scrub‐oak communities in Florida were exposed for three seasons in open top chambers to present atmospheric [CO2] (approx. 350 μmol mol−1) and to high [CO2] (increased by 350 μmol mol−1). Stomatal and photosynthetic acclimation to high [CO2] of the dominant species Quercus myrtifolia was examined by leaf gas exchange of excised shoots. Stomatal conductance (gs) was approximately 40% lower in the high‐ compared to low‐[CO2]‐grown plants when measured at their respective growth concentrations. Reciprocal measurements of gs in both high‐ and low‐[CO2]‐grown plants showed that there was negative acclimation in the high‐[CO2]‐grown plants (9–16% reduction in gs when measured at 700 μmol mol−1), but these were small compared to those for net CO2 assimilation rate (A, 21–36%). Stomatal acclimation was more clearly evident in the curve of stomatal response to intercellular [CO2] (ci) which showed a reduction in stomatal sensitivity at low ci in the high‐[CO2]‐grown plants. Stomatal density showed no change in response to growth in high growth [CO2]. Long‐term stomatal and photosynthetic acclimation to growth in high [CO2] did not markedly change the 2·5‐ to 3‐fold increase in gas‐exchange‐derived water use efficiency caused by high [CO2].

Tolerance to Water Stress in Tomato Cultivars

The effects of plant water stress imposed at vegetative, flowering, and fruiting stages of four cultivars of tomato (Lycopersicon esculentum Mill.) on net photosynthetic rate (PN), stomatal conductance (gs), transpiration rate (E), osmotic adjustment, and crop water stress index (CWSI) were investigated. Osmotic adjustment was the highest in cv. Arka Meghali, followed by cv. RFS-1. CWSI was lowest in cv. Arka Meghali and highest in cv. Pusa Ruby. Significant reduction in gs, E, and PN was observed in all the cultivars. The maximum reduction in E was observed in cv. Arka Saurabh during the fruiting stage (62.4 %) and maximum reduction in PN at the flowering stage in Pusa Ruby (53.1 %). Maximum PN was observed in Arka Meghali under water stress. The values of internal CO2 concentration (Ci) did not follow the decrease in gs which might be taken as an indication of mesophyll (non-stomatal) limitation to PN. Magnitude of PN decrease accompanying gs reductions varied in the four cultivars. Arka Meghali which had highest rate of gas exchange efficiency (PN/gs) under water deficits can be recommended for rainfed cultivation.

Influence of Manganese Toxicity on Photosynthesis in Ricebean (Vigna Umbellata) Seedlings

Influence of manganese (Mn) toxicity on photosynthesis in ricebean (Vigna umbellata) was studied by the measurement of gas exchange characteristics and chlorophyll fluorescence parameters. The net photosynthetic rate (PN), transpiration rate (E), and stomatal conductance (gs) were reduced with increasing Mn concentration in nutrient solution. The reduction in gs and E was more pronounced at 6 d of Mn treatment. However, PN declined at 2 d of Mn treatment implying that the reduction in photosynthesis was not due to the direct effect of Mn on stomatal regulation. Mn did not affect the maximum efficiency of photosystem 2 (PS2) photochemistry (Fv/Fm). A reduction in photochemical quenching (qP) and excitation capture efficiency of open PS2 (Fv′/Fm′) with a concomitant increase in qN was observed. This implies that reduced demand for ATP and NADPH due to the reduction in photosynthesis causes a down-regulation of PS2 photochemistry and thus a high pH gradient (increase in qN) and limited electron transport (decreased qP).

Differential sensitivity of stomatal and non-stomatal components to NaCl or Na2SO4 salinity in horsegram, Macrotyloma uniflorum (Lam.)

14CO2 assimilation rate (P), leaf diffusive conductance (gs), photosynthetic electron flow, and activities of enzymes of Calvin cycle were studied in a horsegram [Macrotyloma uniflorum (Lam.)] in response to salinity induced by NaCl or Na2SO4. A significant reduction in P and gs by both salt treatments was registered. Na2SO4 caused a greater reduction in gs than the NaCl salinity. Studies with isolated chloroplasts confirmed a greater sensitivity to NaCl than to Na2SO4. Salinity inhibited the photosynthetic electron transport. The activity of ribulose-1,5-bisphosphate carboxylase (E.C.4.1.1.39) was under salinity inhibited more than the activities of other three enzymes of the Calvin cycle, ribulose-5-phosphate kinase (E.C.2.7.1.19), ribose-5-phosphate isomerase (E.C.5.3.16), and NADP-glyceraldehyde-3-phosphate dehydrogenase (E.C.1.2.13). These inhibitions lead to a reduced capacity for ribulose-1,5-bisphosphate regeneration. Isolated chloroplasts extracted from salt stressed plants and supplemented with the substrates of Calvin cycle could elevate P, but the P was always lower than in the controls. Decreased P in horsegram exposed to high salinity can be attributed to both stomatal and non-stomatal components, however, the sensitivity to the salt source, NaCl or Na2SO4, was different.

Effects of elevated CO2 on leaf gas exchange in beech and oak at two levels of nutrient supply: consequences for sensitivity to drought in beech

ABSTRACTBeech (Fagus sylvatica L.) and pedunculate oak (Quercus robur L.) were grown from seed for two whole seasons at two CO2 concentrations (ambient and ambient + 250 μmol mol−1) with two levels of soil nutrient supply. Measurements of net leaf photosynthetic rate (A) and stomatal conductance (gs) of well‐watered plants were taken over both seasons; a drought treatment was applied in the middle of the second growing season to a separate sample of beech drawn from the same population. The net leaf photosynthetic rate of well‐watered plants was stimulated in elevated CO2 by an average of 75% in beech and 33% in oak; the effect continued through both growing seasons at both nutrient levels. There were no interactive effects of CO2 concentration and nutrient level on A or gs in beech or oak. Stomatal conductance was reduced in elevated CO2 by an average of 34% in oak, but in beech there were no significant reductions in gs except under cloudy conditions (–22% in elevated CO2). During drought, there was no effect of CO2 concentration on gs in beech grown with high nutrients, but for beech grown with low nutrients, gs was significantly higher in elevated CO2, causing more rapid soil drying. With high nutrient supply, soil drying was more rapid at elevated CO2 due to increased leaf area. It appears that beech may substantially increase whole‐plant water consumption in elevated CO2, especially under conditions of high temperature and irradiance when damage due to high evaporative demand is most likely to occur, thereby putting itself at risk during periods of drought.

Gas exchange in leaves of the root hemiparasitemelampyrum arvense L. before and after attachment to the host plant

Gas exchange characteristics of a hemiparasiteMelampyrum arvense L. before and after attachment to the hostCapsella bursa pastoris (L.) Med. were compared. The net photosynthetic rates (PN) on a leaf area basis were extremely low and in comparison to the value obtained for the host were about 15 % and 23 % for the unattached and attached hemiparasite, respectively. Also the concentration of photosynthetic pigments was low (as compared with the host the content of chlorophylls was about 33 % and 49 % and of carotenoids about 38 % and 36 % in the unattached and attached hemiparasite, respectively). On the other hand the rates of respiration were high (about 1.8 and 2.6 times higher in the unattached and attached hemiparasite, respectively, than in the host). In darkness stomatal conductance (gS) of the host and the unattached hemiparasite was rapidly reduced to 10 % of the value obtained in light, gS of the attached hemiparasite was decreased only by about 30%. A total reduction of gS occurred at relative water content (RWC) of 85 %, 75% and 45 % for the unattached hemiparasite, the host, and the attached hemiparasite, respectively. The transpiration (E) rate in the preparasitic stage was very low, being 2.6 and 4.5 times smaller than in the host and the attached hemiparasite, respectively. In the attached hemiparasite WUE was 7.5 and 3 times poorer than in the host and in the preparasitic stage, respectively.