Leaf Conductance in Relation to Rate of CO(2) Assimilation: II. Effects of Short-Term Exposures to Different Photon Flux Densities

When photon flux density incident on attached leaves of Zea mays L. was varied from the equivalent of 0.12 of full sunlight to full sunlight, leaf conductance to CO₂ transfer, g, changed in proportion to the change in rate of CO₂, assimilation, A, with the result that intercellular partial pressure of CO₂ remained almost constant. The proportionality was the same as that previously found in g and A measured at one photon flux density in plants of Zea mays L. grown at different levels of mineral nutrition, light intensities, and ambient partial pressures of CO₂. In shade-grown Phaseolus vulgaris L. plants, A as photon flux density was increased from about 0.12 up to about 0.5 full sunlight, the proportionality being almost the same in plants grown at low and at high light intensity.

When photon flux density incident on the adaxial and abaxial surfaces of the isolateral leaves of Eucalyptus pauciflora Sieb. ex Spreng was varied, g and A also varied proportionally. The leaf conductance in a particular surface was affected by the photon flux density at the opposite surface to a greater extent than was expected on the basis of transmittance. The results indicated that stomata may, in some way, be sensitive to the photon flux absorbed within the leaf as a whole.

     

Inhibition of photosynthesis by carbohydrates in wheat leaves

The rate of net CO₂ assimilation of mature wheat (Triticum aestivum L.) leaves in ambient air (21% O₂, 340 microbars CO₂) declined with time of illumination at temperatures lower than 25°C, but not at higher temperatures, and the rate of decline increased when maintained in air with higher CO₂ concentration (700-825 microbars). In this latter case, the decline in the rate of net CO₂ assimilation also occurred at high temperatures. Stomatal conductance also declined with time in some cases and stomata became more sensitive to CO₂, but this was not the primary cause of the decrease in CO₂ assimilation because internal partial pressure of CO₂ remained constant. Treatments which reduced the rate of translocation (e.g. lower temperatures, chilling the base of the leaf) produced a marked decline in CO₂ assimilation of leaves in atmospheric and high CO₂ concentrations. The decreased net CO₂ assimilation was correlated with carbohydrate accumulation in each case, suggesting end product inhibition of photosynthesis. Analysis of CO₂ assimilation in high carbohydrate leaves as a function of intercellular CO₂ partial pressure showed reduction in the upper part of the curve. The initial slope of this curve, however, was not affected. Photosynthetic rates in the upper part of this curve generally recovered after a short period in darkness in which carbohydrates were removed from the leaf. The stimulation of net CO₂ assimilation by 2% O₂ (Warburg effect), and the apparent quantum yield, decreased after several hours of light.     

Zeaxanthin Synthesis, Energy Dissipation, and Photoprotection of Photosystem II at Chilling Temperatures

When leaves of a mangrove, Rhizophora mangle, were exposed to an excess of light at chilling temperatures, synthesis of zeaxanthin through violaxanthin de-epoxidation as well as nonphotochemical fluorescence quenching were markedly reduced. The results suggest a protective role of energy dissipation against the adverse effects of high light and chilling temperatures: leaves of R. mangle that had been preilluminated in 2% O₂, 0% CO₂ at low photon flux density and showed a high level of zeaxanthin, and leaves that had been kept in the dark and contained no zeaxanthin, were both exposed to high light and chilling temperatures (5°C leaf temperature) in air and then held under control conditions in low light in air at 25°C. Measurements of chlorophyll a fluorescence at room temperature showed that the photochemical efficiency of PSII and the yield of maximum fluorescence of the preilluminated leaf recovered completely within 1 to 3 hours under the control conditions. In contrast, the fluorescence responses of the predarkened leaf in high light at 5°C did not recover at all. During a dark/light transient in 2% O₂, 0% CO₂ in low light at 5°C, nonphotochemical fluorescence quenching increased linearly with an increase in the zeaxanthin content in leaves of R. mangle. In soybean (Glycine max) leaves, which contained a background level of zeaxanthin in the dark, a similar treatment with excess light induced a level of nonphotochemical fluorescence quenching that was not paralleled by an increase in the zeaxanthin content.     

The effects of development at sub-optimal growth temperatures on photosynthetic capacity and susceptibility to chilling-dependent photoinhibition in Zea mays

When plants of Zea mays L. cv. LG11 that have been grown at optimal temperatures are transferred to chilling temperatures (0–12°C) photoinhibition of photosynthetic CO₂ assimilation can occur. This study examines how growth at sub-optimal temperatures alters both photosynthetic capacity and resistance to chilling-dependent photoinhibition. Plants of Z. mays cv. LG11 were grown in controlled environments at 14, 17, 20 and 25°C. As a measure of the capacity for photosynthesis under light limiting conditions, the maximum quantum yields of CO₂ assimilation (φ^a.c) and O₂ evolution (φ^a.o) were determined for the laminae of the second leaves at photon fluxes of 50–150 μmol m^-2s^-1. To determine photosynthetic capacity at photon fluxes approaching light saturation, rates of CO₂ uptake (A₁₅₀₀) and O₂ evolution (A₁₅₀₀) were determined in a photon flux of 1500 μmol m^-2s^-1. In leaves developed at 14°C, φ and φ were 26 and 43%, respectively, of the values for leaves grown at 25°C. Leaves grown at 17°C showed intermediate reductions in φ and φ, whilst leaves developed at 20°C showed no significant differences from those grown at 25°C. Similar patterns of decrease were observed for A₁₅₀₀ and A_1500.0 with decreasing growth temperature. Leaves developed at 25°C showed higher rates of CO₂ assimilation at all light levels and measurement temperatures in comparison to leaves developed at 17 and 14°C. A greater reduction in A₁₅₀₀ relative to A_1500.0 with decreasing growth temperature was attributed to increased stomatal limitation. Exposure of leaves to 800–1000 μmol m^-2 s^-1 when plant temperature was depressed to ca 6.5°C produced a photoinhibition of photosynthetic CO₂ assimilation in all leaves. However, in leaves developed at 17°C the decrease in A₁₅₀₀ following this chilling treatment was only 25% compared to 90% in leaves developed at 25°C. Recovery following chilling was completed earlier in leaves developed at 17°C. The results suggest that growth at sub-optimal temperatures induces increased tolerance to exposure to high light at chilling temperatures. This is offset by the large loss in photosynthetic capacity imposed by leaf development at sub-optimal temperatures.     

Photorespiration is Essential for the Protection of the Photosynthetic Apparatus of C3 Plants Against Photoinactivation Under Sunlight

CO₂ assimilation, transpiration and modulated chlorophyll fluorescence of leaves of Chenopodium bonus-henricus (L.) were measured in the laboratory and, at a high altitude location, in the field. Direct calibration of chlorophyll fluorescence parameters against carbon assimilation in the presence of 1 or 0.5% oxygen (plus CO₂) proved necessary to calculate electron transport under photorespiratory conditions in individual experiments. Even when stomata were open in the field, total electron transport was two to three times higher in sunlight than indicated by net carbon gain. It decreased when stomata were blocked by submerging leaves under water or by forcing them to close in air by cutting the petiole. Even under these conditions, electron transport behind closed stomata approached 10 nmol electrons m^?2 leaf area s^?1 at temperatures between 25 and 30 °C. No photoinactivation of photosystem II was indicated by fluorescence analysis after a day's exposure to full sunlight. Only when leaves were submerged in ice was appreciable photoinactivation noticeable after 4 h exposure to sunlight. Even then almost full recovery occurred overnight. Electron transport behind blocked stomata was much decreased when leaves were darkened for 70 min (in order to deactivate light-regulated enzymes of the Calvin cycle) before exposure to full sunlight. Brief exposure of leaves to HCN (to inhibit photoassimilation and photorespiration) also decreased electron transport drastically compared to electron transport in unpoisoned leaves with blocked stomata. Non-photochemical fluorescence quenching and reduction of Q_A, the primary electron acceptor of photosystem II was increased by HCN-poisoning. Very similar observations were made when glyceraldehyde was used instead of HCN to inhibit photosynthesis and photorespiration. In HCN-poisoned leaves, residual electron transport increased linearly with temperature and showed early light saturation revealing characteristics of the Mehler reaction. During short exposure of these leaves to photon flux densities equivalent to 25% of sunlight, no or only little photoinactivation of photosystem II was observed. However, prolonged exposure to sunlight caused inactivation even though non-photochemical quenching of chlorophyll fluorescence was extensive. Simultaneously, oxidation of cellular ascorbate and glutathione increased. Inactivation of photosystem II was reversible in dim light and in the dark only after short times of exposure to sunlight. Glyceraldehyde was very similar to HCN in increasing the sensitivity of photosystem II in leaves to sunlight. We conclude from the observations that the electron transport permitted by the interplay of photoassimilatory and photorespiratory electron transport is essential to prevent the photoinactivation of photosynthetic electron transport. The Mehler and Asada reactions, which give rise to strong nonphotochemical fluorescence quenching, are insufficient to protect the chloroplast electron transport chain against photoinactivation.     

Very high CO2 partially restores photosynthesis in sunflower at low water potentials

We re-examined the question of whether the stomata limit photosynthesis in dehydrated sunflower (Helianthus annuus L.) plants having low leaf water potentials. A gas-exchange apparatus was modified to operate at external CO₂ partial pressures as high as 3000 Pa (3%), which were much higher than previously achieved. This allowed photosynthesis and stomatal behavior to be monitored simultaneously at very high CO₂ in the same leaf. The data were compared with those from leaves treated with abscisic acid (ABA) where effects on photosynthesis are entirely stomatal. Photosynthesis was inhibited at low water potential and was only slightly enhanced by increasing the external CO₂ partial pressure from 34 Pa (normal air) to 300 Pa. Photosynthesis in ABA-treated leaves was similarly inhibited but recovered fully at 300 Pa. In both cases, the stomata closed to the same extent as judged from the average conductance of the leaves. Because the ABA effect resulted from diffusion limitation for CO₂ caused by stomatal closure, the contrasting data show that most of the dehydration effect was nonstomatal at low water potentials. When CO₂ partial pressures were raised further to 3000 Pa, photosynthesis increased somewhat at low water potentials but not in ABA-treated leaves. This indicates that some nonstomatal component of photosynthesis responded differently in leaves at low water potential and leaves treated with ABA. Because this component was only partially restored by very high CO₂, it was likely to be metabolic and was an important source of photosynthetic inhibition.Abbreviations and Symbol ABA abscisic acid - Chl chlorophyll - p_a external partial pressure of CO₂ - P_i intercellular partial pressure of CO₂ - _w water potential This work was supported by grant DE-FG02-87ER13776 from the Department of Energy and a grant from E.I. DuPont de Nemours and Company.     

Combined low temperature-high light effects on gas exchange properties of jojoba leaves

Jojoba (Simmondsia chinensis [Link] Schneider) is an important crop in desert climates. A relatively high frequency of periods of chilling and high photon flux density (PFD) in this environment makes photoinhibition likely, resulting in a reduction of assimilation capacity in overwintering leaves. This could explain the low net photosynthesis found in shoots from the field (4-6 micromoles per square meter per second) when compared to greenhouse grown plants (12-15 micromoles per square meter per second). The responses of photosynthesis and stomatal conductance to changes in absorbed PFD and in substomatal partial pressure of CO₂ were measured on jojoba leaves recovering from chilling temperature (4°C) in high or low PFD. No measurable gas exchange was found immediately after chilling in either high or low PFD. For leaves chilled in low PFD, the original quantum yield was restored after 24 hours. The time course of recovery from chilling in high PFD was much longer. Quantum yield recovered to 60% of its original value in 72 hours but failed to recover fully after 1 week. Measurements of PSII chlorophyll fluorescence at 77 K showed that the reduced quantum yield was caused by photoinhibition. The ratio of variable to maximal fluorescence fell from a control level of 0.82 to 0.41 after the photoinhibitory treatment and recovery was slow. We also found a large increase in net assimilation rate and little closure of stomata as CO₂ was increased from ambient partial pressure of 35 to 85 pascals. For plants grown in full light, the increase in net assimilation rate was 100%. The photosynthetic response at high CO₂ concentration may constitute an ecological advantage of jojoba as a crop in the future.     

Relative contributions of zeaxanthin-related and zeaxanthin-unrelated types of ;high-energy-state' quenching of chlorophyll fluorescence in spinach leaves exposed to various environmental conditions

We have identified two rapidly relaxing components of non-photochemical fluorescence quenching which suggests that dissipative processes occur in two different sites in the photochemical system of leaves. Under a variety of treatment conditions involving different leaf temperatures, photon flux densities (PFD), exposure times, and in the presence of 5% CO₂ or 2% O₂, no CO₂, the components of nonphotochemical fluorescence quenching were characterized with respect to their sensitivity to dithiothreitol (DTT, which completely inhibits zeaxanthin formation), the effect on instantaneous fluorescence, and the rapidity of relaxation upon darkening. Under most circumstances the DTT-sensitive component (associated with a quenching of instantaneous fluorescence and correlated with zeaxanthin) represented the majority of the rapidly relaxing portion of fluorescence quenching. A DTT-insensitive (zeaxanthin-independent) component, which also relaxed rapidly upon darkening but was not associated with a quenching of instantaneous fluorescence, became proportionally greater in an atmosphere of 2% O₂ and no CO₂, at elevated leaf temperatures, and to some degree during the induction of photosynthesis (1 minute after the onset of illumination). A third component which was also DTT-insensitive and was sustained upon darkening, was largely suppressed in 2% O₂, O% CO₂. We conclude that, under conditions favorable for photosynthesis, energy dissipation occurred mainly in the chlorophyll antennae whereas, under conditions less favorable for photosynthesis, a second dissipation process, probably in or around the reaction center of photosystem II, also developed. Furthermore, evidence is presented that the zeaxanthin-associated dissipation process prevents sustained inactivation of photochemistry by excessive light.     

Insensitivity of Water-Oxidation and Photosystem II Activity in Tomato to Chilling Temperatures

Chilling tomato plants (Lycopersicon esculentum Mill. cv. Rutgers and cv. Floramerica) in the dark resulted in a sizable inhibition in the rate of light- and CO₂-saturated photosynthesis. However, at low light intensity, the inhibition disappeared and the absolute quantum yield of CO₂ reduction was diminished only slightly. The quantum yield of photosystem II (PSII) electron flow was 18% lower when measured in chloroplasts isolated from chilled leaves than in chloroplasts isolated from unchilled leaves. Even though the maximum rate of PSII turnover in these chloroplasts was 12% lower subsequent to chilling, it was in all cases two or more times that required to support the light- and CO₂-saturated rate of photosynthesis measured in the attached leaf. The concentration of active PSII centers in chloroplasts isolated from leaves either before or after chilling was determined by measurement of the products of water oxidation from a series of saturating flashes short enough to turnover the electron transport carriers only a single time. There was no significant change in the concentration of active PSII centers due to dark chilling.

It was concluded that PSII activity and water oxidation capacity are not significantly impaired in tomato by chilling in the dark and therefore are not primary aspects of the inhibition of CO₂ reduction observed in attached leaves.

     

Impairment of photosynthesis by chilling-temperatures in tomato

Chilling of attached tomato leaves (cv. Rutgers) in the dark for 16 hours at 1 C decreased both photosynthesis and transpiration. To separate the effects of chilling on stomatal CO₂ conductance from more direct effects of chilling on the chloroplasts' activities, measurements of photosynthesis and transpiration were made at atmospheric and saturating CO₂ levels. At atmospheric CO₂, the inhibition of photosynthesis was approximately 60%, of which about 35% was attributable to the impairment of chloroplast function and about 25% was attributable to decreased stomatal conductance. However, the affinity of the photosynthetic apparatus for CO₂ was not changed by chilling, since the dependence of the relative rate of photosynthesis on the intercellular CO₂ concentration was unaltered. The apparent quantum requirement for CO₂ reduction also was identical in chilled and unchilled plants. This observation contradicts the widely held notion that the chilling-induced inhibition of photosynthesis is caused by an impairment of the water oxidation mechanism. The impairment of chloroplast activity was not a consequence of an unfavorable water status within the leaf, since chilling caused only a small drop (1 bar) in water potential. A small loss of chlorophyll resulted as a secondary effect of chilling, but this loss of chlorophyll was eliminated as a cause of the inhibition of photosynthesis.     

Leaf photosynthetic parameters of two maize (Zea mays) cultivars in response to various patterns of chilling temperatures and photon flux densities

Chilling‐induced photosynthetic impairment was examined in leaves of maize (Zea mays L.) seedlings of two cultivars, one adapted to western Europe and one adapted to Mexican highlands. Three experiments were performed in a controlled environment. The effects of chilling night temperatures, of chilling at high light intensity and of variable chilling day temperatures on photosynthetic parameters, were evaluated. Chilling in the dark period resulted in stomatal limitation of net photosynthesis. Chilling at moderate to high light intensities caused chilling‐dependent photoinhibition of CO₂ uptake. Photobleached maize leaves did not resume normal photosynthetic function. Maize cv. Batan 8686 from the highlands of Mexico was less susceptible to photosynthetic damage than maize cv. Bastion adapted for cultivation in W. Europe, when exposed to chilling night temperatures, or to mild chilling photoinhibitory conditions.     

Light and the maintenance of photosynthetic competence in leaves of Populus balsamifera L. during short-term exposures to high concentrations of sulfur dioxide

Leaves of Populus balsamifera grown under full natural sunlight were treated with 0, 1, or 2 l SO₂·1^-1 air under one of four different photon flux densities (PFD). When the SO₂ exposures took place in darkness or at 300 mol photons·m^-2·s^-1, sulfate accumulated to the levels predicted by measurements of stomatal conductance during SO₂ exposure. Under conditions of higher PFD (750 and 1550 mol·m^-2·s^-1), however, the predicted levels of accumulated sulfate were substantially higher than those obtained from anion chromatography of the leaf extracts. Light-and CO₂-saturated capacity as well as the photon yield of photosynthetic O₂ evolution were reduced with increasing concentration of SO₂. At 2 l SO₂·1^-1 air, the greatest reductions in both photosynthetic, capacity and photon yield occurred when the leaves were exposed to SO₂ in the dark, and increasingly smaller reductions in each occurred with increasing PFD during SO₂ exposure. This indicates that the inhibition of photosynthesis resulting from SO₂ exposure was reduced when the exposure occurred under conditions of higher light. The ratio F _v/F _M (variable/maximum fluorescence emission) for photosyntem II (PSII), a measure of the photochemical efficiency of PSII, remained unaffected by exposure of leaves to SO₂ in the dark and exhibited only moderate reductions with increasing PFD during the exposure, indicating that PSII was not a primary site of damage by SO₂. Pretreatment of leaves with SO₂ in the dark, however, increased the susceptibility of PSII to photoinhibition, as such pretreated leaves exhibited much greater reductions inF _V/F _M when transferred to moderate or high light in air than comparable control leaves.Abbreviations and symbols A₁₂₀₀ photosynthetic capacity (CO₂-saturated rate of O₂ evolution at 1200 mol photons·m^-2·s^-1) - F_o instantaneous fluorescence emission - F_M maximum fluorescence emission - F_V variable fluorescence emission - PFD photon flux density (400–700 nm) - PSII photosystem II     

Regulation of photosynthesis and antioxidant metabolism in maize leaves at optimal and chilling temperatures: review

Maize (Zea mays L.) is a chilling (below 15 °C) sensitive plant that shows little capacity to acclimate to low growth temperatures. Maize leaves are extremely sensitive to chilling injury, which usually results in premature leaf senescence. Leaves exposed to temperatures below 10 °C in the light show substantial inhibition of CO₂ assimilation and down-regulation of photosynthetic electron transport. However, the intrinsic relationships between the quantum efficiencies of photosystems I and II are not modified by chilling. Moreover, the integral relationships between non-cyclic electron transport and CO₂ fixation are similar in chilled and unchilled leaves. In this review we examine the roles and importance of photosynthetic regulation, carbon metabolism and antioxidant metabolism in determining the sensitivity of maize leaf photosynthesis to chilling. The distinct cellular localisation patterns of antioxidant enzymes such as glutathione reductase (EC 1.6.4.2) and dehydroascorbate reductase (EC 1.8.5.1) can restrict the recycling of antioxidants associated with photosynthesis during chilling. Disruption of circadian regulation of metabolism and insufficient antioxidant defence are postulated to cause chilling sensitivity.     

Photoinhibition of intact attached leaves of c(3) plants illuminated in the absence of both carbon dioxide and of photorespiration

When leaflets of bean and leaves of other species of C₃ plants are illuminated in the absence of CO₂ and at low O₂ partial pressure, the capacity for CO₂ assimilation at saturating light and its efficiency at low light intensities are inhibited. This photoinhibition is dependent on leaflet age and period of illumination. In young leaflets and following short exposure to these photoinhibitory conditions, some recovery of CO₂ assimilation capacity is observed immediately after treatment. Following substantial (70 to 80%) photoinhibition of CO₂ assimilation, recovery in fully expanded leaflets is observed only after 48 hours in normal air. The photoinhibition is largely prevented by providing CO₂ at partial pressures equivalent to the CO₂ compensation point, or by >210 millibars O₂ which permits internal CO₂ production by photorespiration. If leaflets are illuminated in 60 microbars CO₂ and 210 millibars O₂ (the CO₂ compensation point in air), no photoinhibition is observed. Electron transport processes and fluorescence emission associated with photosystem II are inhibited in chloroplast thylakoids isolated from leaflets after illumination in zero CO₂ and 10 millibars O₂. These studies support the hypothesis that CO₂ recycling through photorespiration is one means of effectively dissipating excess photochemical energy when CO₂ supply to illuminated leaves is limited.     

Estimation of the effect of photoinhibition on the carbon gain in leaves of a willow canopy

The occurrence of photoinhibition of photosynthesis in leaves of a willow canopy was examined by measuring the chlorophyll-a fluorescence ratio of F _V/F _M (F_M is the maximum fluorescence level of the induction curve, and F_V is the variable fluorescence, F _V=F _M–F ₀, where F₀ is the minimal fluorescence). The majority of the leaves situated on the upper parts of peripheral shoots showed an afternoon inhibition of this ratio on clear days. This was the consequence of both a decrease in F _M and a rise in F _O. In the same leaves the diurnal variation in intercepted photosynthetic photon flux density (PPFD) was monitored using leaf-mounted sensors. Using the multivariate method, partial least squares in latent variables, it is shown that the dose of PPFD, integrated and linearly weighted over the last 6-h period, best predicts photoinhibition. Photoinhibition occurred even among leaves that did not intercept PPFDs above 1000 mol·m^–2·s^–1. Exposure of leaves to a standard photoinhibitory treatment demonstrated that the depression in the F _V/F _M ratio was paralleled by an equal depression in the maximal quantum yield of CO₂ uptake and a nearly equal depression in the rate of bending (convexity) of the light-response curve of CO₂ uptake. As a result, the rate of net photosynthesis is depressed over the whole natural range of PPFD. By simulating the daily course in the rate of net photosynthesis, it is estimated that in the order of one-tenth of the potential carbon gain of peripheral willow shoots is lost on clear days as a result of photoinhibition. This applies to conditions of optimal temperatures. Photoinhibition is even more pronounced at air temperatures below 23° C, as judged from measurements of the F_V/F_M ratio on clear days: the afternoon inhibition of this ratio increased in a curvilinear manner from 15% to 25% with a temperature decrease from 23° to 14° C.Abbreviations and Symbols F_O minimum fluorescence - F_V variable fluorescence - F_M maximum fluorescence - PLS partial least squares in latent variables - PPFD photosynthetic photon flux density - VPD water vapour-pressure deficit This study was supported by the Swedish Natural Science Research Council. We are indebted to Dr. Jerry Leverenz (Department of Plant Physiology, University of Umeå, Sweden) for guidance with the modelling of the photosynthesis data.     

Determination of carbon-reduction-cycle intermediates in leaves of Arbutus unedo L. suffering depressions in photosynthesis after application of abscisic acid or exposure to dry air

Gas exchange and contents of photosynthetic intermediates of leaves of Arbutus unedo L. were determined with the aim of recognizing the mechanisms of inhibition that were responsible for the midday depression of photosynthesis following exposure to dry air, and the decline in photosynthetic capacity following application of abscisic acid (ABA). Rapidly killed (<0.1 s) leaf samples were taken when gas analysis showed reduced CO₂ assimilation. Determination of the contents of 3-phosphoglyceric acid (PGA), ribulose 1,5-bisphosphate (RuBP), triose phosphates, fructose 1,6-bisphosphate and hexose phosphates in the samples showed that significant variation occurred only in the level of PGA. As a result, the ratio PGA/RuBP decreased with increasing inhibition of photosynthesis, particularly when application of ABA had been the cause. A comparison of metabolite patterns did not bring out qualitative differences that would have indicated that effects of ABA and of dry air had been caused by separate mechanisms. Depression of photosynthesis occurred in the presence of sufficient RuBP which indicated that the carboxylation reaction of the carbon-reduction-cycle was inhibited after application of ABA or exposure to dry air.Abbreviations and symbols ABA abscisic acid - C _a partial pressure of CO₂ in the ambient air - C _i partial pressure of CO₂ in the intercellular spaces - I quantum flux - PGA 3-phosphoglyceric acid - RuBP ribulose 1,5-bisphosphate - I _L leaf temperature - w water-vapor pressure difference between leaf and air     

Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: effect of growth temperature on photoinhibition and recovery

Intact leaves of kiwifruit (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson) from plants grown in a range of controlled temperatures from 15/10 to 30/25°C were exposed to a photon flux density (PFD) of 1500 μmol·m⁻²·s⁻¹ at leaf temperatures between 10 and 25°C. Photoinhibition and recovery were followed at the same temperatures and at a PFD of 20 μmol·m⁻²·s⁻¹, by measuring chlorophyll fluorescence at 77 K and 692 nm, by measuring the photon yield of photosynthetic O₂ evolution and light-saturated net photosynthetic CO₂ uptake. The growth of plants at low temperatures resulted in chronic photoinhibition as evident from reduced fluorescence and photon yields. However, low-temperature-grown plants apparently had a higher capacity to dissipate excess excitation energy than leaves from plants grown at high temperatures. Induced photoinhibition, from exposure to a PFD above that during growth, was less severe in low-temperature-grown plants, particularly at high exposure temperatures. Net changes in the instantaneous fluorescence,F ₀, indicated that little or no photoinhibition occurred when low-temperature-grown plants were exposed to high-light at high temperatures. In contrast, high-temperature-grown plants were highly susceptible to photoinhibitory damage at all exposure temperatures. These data indicate acclimation in photosynthesis and changes in the capacity to dissipate excess excitation energy occurred in kiwifruit leaves with changes in growth temperature. Both processes contributed to changes in susceptibility to photoinhibition at the different growth temperatures. However, growth temperature also affected the capacity for recovery, with leaves from plants grown at low temperatures having moderate rates of recovery at low temperatures compared with leaves from plants grown at high temperatures which had negligible recovery. This also contributed to the reduced susceptibility to photoinhibition in low-temperature-grown plants. However, extreme photoinhibition resulted in severe reductions in the efficiency and capacity for photosynthesis.     

Photochemical efficiency of Photosystem II and xanthophyll cycle components in Zea mays leaves exposed to water stress and high light

The effects of two light treatments (photosynthetically active photon flux density of either 650 or 1950 µmol m^–2 s^–1) on the photochemical efficiency of Photosystem II (PS II) (measured as variable to maximum fluorescence ratio) and on the xanthophyll cycle components was studied in wilted Zea mays leaves. For comparison, these parameters were followed under the same light conditions in well-hydrated leaves maintained either in normal or CO₂-free air. The net CO₂ assimilation of dehydrated leaves declined rapidly as their relative water content (RWC) decreased from 100 to 60% while the PS II efficiency measured after a prolonged dark period of 16 h declined only when RWC leaves was lower than 60%. Furthermore, drought caused an increase in the pool size of the xanthophyll cycle pigments and the presence of a sustained elevated level of zeaxanthin and antheraxanthin at the end of the long dark period. The leaf water deficit enhanced the sensitivity of PS II efficiency to light exposure. During illumination, strong inhibition of PS II efficiency and large violaxanthin deepoxidation was observed in wilted leaves even under moderate photon flux density compared to control leaves in the same conditions. After 2 h of darkness following the light treatment, the PS II efficiency that is dependent on the previous PPFD, decreased with leaf water deficit. Moreover, zeaxanthin epoxidation led to an accumulation of antheraxanthin in dehydrated leaves. All these drought effects on PS II efficiency and xanthophyll cycle components were also obtained in well-hydrated leaves by short-term CO₂ deprivation during illumination. We conclude that the increased susceptibility of PS II efficiency to light in wilted maize leaves is mainly explained by the decrease of CO₂ availability and the resulting low net CO₂ assimilation.     

Crassulacean Acid Metabolism and Photochemical Efficiency of Photosystem II in the Adaxial and Abaxial Parts of the Succulent Leaves of Kalanchoë daigremontiana Grown at Four Photon Flux Densities

Kalanchoë daigremontiana, a species possessing crassulacean acid metabolism, was grown at four photon flux densities (1300, 400, 60, and 25 micromole photons per square meter per second). In leaves which had developed at 1300 and 400 micromole photons per square meter per second, CO₂ was mainly incorporated through the lower, shaded leaf surfaces, and the chlorenchyma adjacent to the lower surfaces showed a higher degree of nocturnal acid synthesis than the chlorenchyma adjacent to the upper surfaces. In leaves acclimated to 60 and 25 micromole photons per square meter per second, the gradient in CAM activity was reversed, i.e. more CO₂ was taken up through the upper than through the lower surfaces and nocturnal acidification was higher in the tissue next to the upper surfaces. Total net carbon gain and total nocturnal acid synthesis were highest in leaves which had developed at 400 micromole photons per square meter per second. Chlorophyll content was markedly reduced in leaves which had developed at 1300 micromole photons per square meter per second, especially in the exposed adaxial parts. There was also a sustained reduction in photosystem II photochemical efficiency as indicated by measurements of the ratio of variable over maximum chlorophyll a fluorescence. These findings suggest that, at high growth photon flux densities, the reduced activity of the exposed portions of these succulent leaves is caused by (a) the adverse effects of excess light, (b) together with a genotypic component which favors CO₂ uptake and acid synthesis in the abaxial (lower) leaf parts even when light is not or only marginally excessive. This latter component is predominant at medium photon flux densities, e.g. at 400 micromole photons per square meter per second. It becomes overridden, however, under conditions of deep shade when strongly reduced light levels in the abaxial parts of the leaf chlorenchyma severely limit photosynthesis.     

Water stress in Eucalyptus pauciflora: comparison of effects on stomatal conductance with effects on the mesophyll capacity for photosynthesis,and investigation of a possible involvement of photoinhibition

Seedlings of Eucalyptus pauciflora Sieb. ex Spreng., grown in 4-1 pots, were stressed by withholding water while relationships between net assimilation rate (A) and intercellular partial pressure of CO₂ (p_i) in selected leaves were obtained repeatedly throughout the stress cycle. Water stress at first caused stomatal closure without any decline in the A(p_i) relationship. As stress became more severe, the A(p_i) relationship was affected as well. This always affected assimilation rate at both high and low intercellular partial pressures of CO₂. It was then tested whether water-stressed leaves were more prone to photoinhibition than unstressed ones. Plants were water-stressed while at the same time subjected to strong photon flux area density (2000 mol quanta·m^-2·s^-1). A possible light-induced inhibition was assessed by comparing quantum yields of photosynthesis with light directed onto one or the other surface of the leaf. A decline in quantum yield was observed, and the decline on the previously irradiated side was more pronounced than on the previously shaded side, but the effect was small and disappeared entirely within 1 d of rewatering the plants. It is concluded that photoinhibition can play a role, but not an important one, in the effect of water stress on the A(p_i) relationship in leaves of E. pauciflora.Abbreviations and symbols RuBP ribulose-1,5-bisphosphate - A net assimilation rate - p_i intercellular partial pressure of CO₂ - quantum yield of photosynthesis (net assimilation or RuBP-regeneration rate) - w difference in water content between air saturated at leaf temperature and the actual vapor content of the air, expressed as mole fraction