Effects of winter cold stress on photosynthesis and photochemical efficiency of PSII of the Mediterranean <Emphasis Type="Italic">Cistus albidus</Emphasis> L. and <Emphasis Type="Italic">Quercus ilex</Emphasis> L.

This study examined the photosynthetic and growth performances of potted plants of Cistus albidus L. and Quercus ilex L. submitted either to natural Mediterranean winter conditions or to mild greenhouse conditions. Plants grown outdoors exhibited lower light and CO₂-saturated CO₂ assimilation rates (A_sat) and apparent quantum yield (_i) than those indoors. Until mid-winter, C. albidus had higher A_sat than Q. ilex, but differences disappeared after a period of severe cold. Maximal photochemical efficiency of PSII (Fv/Fm) measured predawn was higher in C. albidus than in Q. ilex, and decreased throughout the season in outdoor plants. Fv/Fm also decreased at light saturation (A_sat) in both species. Fv/Fm was correlated with photosynthetic capacity and efficiency (quantum yield), but the resulting regression slopes were different between the two species. At the physiological level, C. albidus seemed to cope better with cold stress than Q. ilex. However, winter stress induced reduction of leaf absorptance, increased leaf mass per area, extensive leaf damage and high plant mortality in C. albidus. This suggests that the high performance of C. albidus leaves is not likely to be maintained for long periods of cold stress, and may therefore depend on continuous leaf replacement. Quercus ilex showed a conservative behaviour, with low net assimilation rates but greater leaf and plant survival than C. albidus.     

Water stress and photoinhibition in acclimatization ofRosa hybrida plantlets

Summary MicropropagatedRosa hybrida plantlets were simultaneously rooted and acclimatized under 100 and 200 μmol m⁻² s⁻¹ light for 2 wk. At the end of the first week of acclimatization, the plantlets were transferred onto a low water potential medium (from −0.06 MPa to −0.3 MPa). Dry weight was decreased by increased hight and low water potential. Photoinhibition of photosynthesis, expressed as a decrease in Fv/Fm ratio and ΦPSII and an increase in 1 −qp, occurred in plants grown under 200 μmol m⁻² s⁻¹. When high light (200 μmol m⁻² s⁻¹) and water stress were applied simultaneously, their effects on chlorophyll fluorescence parameters depended on stress duration; after 1 d of water stress, photoinhibition was more pronounced; after 7 d of stress, Fv/Fm ratio and ΦPSII were higher than after 1 d of stress; photoinhibition was reduced. This suggests that after a 1-d stress, the effect of water stress alone included a superimposed effect of photoinhibition to which the water-stressed plants were sensitized; after 7 d, plantlets had adapted to water stress. The photoprotective effects under high light might result in energy dissipative mechanisms linked to photochemical and nonphotochemical quenching other than CO₂ fixation.     

Diffusion limitations and metabolic factors associated with inhibition and recovery of photosynthesis from drought stress in a C3 perennial grass species

Stomatal closure and metabolic impairment under drought stress limits photosynthesis. The objective of this study was to determine major stomatal and metabolic factors involved in photosynthetic responses to drought and recovery upon re‐watering in a C₃ perennial grass species, Kentucky bluegrass (Poa pratensis L.). Two genotypes differing in drought resistance, ‘Midnight’ (tolerant) and ‘Brilliant’ (sensitive), were subjected to drought stress for 15 days and then re‐watered for 10 days in growth chambers. Single‐leaf net photosynthetic rate (A), stomatal conductance (g_s) and transpiration rate (Tr) decreased during drought, with a less rapid decline in ‘Midnight’ than in ‘Brilliant’. Photochemical efficiency, Rubisco activity and activation state declined during drought, but were significantly higher in ‘Midnight’ than in ‘Brilliant’. The relationship between A and internal leaf CO₂ concentration (A/Ci curve) during drought and re‐watering was analyzed to estimate the relative influence of stomatal and non‐stomatal components on photosynthesis. Stomatal limitation (Ls %), non‐stomatal limitation (Lns %), CO₂ compensation point (CP) and dark respiration (Rd) increased with stress duration in both genotypes, but to a lesser extent in ‘Midnight’. Maximum CO₂ assimilation rate (A_max), carboxylation efficiency (CE) and mesophyll conductance (g_m) declined, but ‘Midnight’ had significantly higher levels of A_max, CE and g_m than ‘Brilliant’. Maximum carboxylation rate of Rubisco (V_cmax) and ribulose‐1,5‐bisphospate (RuBP) regeneration capacity mediated by maximum electron transport rate (J_max) decreased from moderate to severe drought stress in both genotypes, but to a greater extent in ‘Brilliant’ than in ‘Midnight’. After re‐watering, RWC restored to about 90% of the control levels in both genotypes, whereas A, g_s, Tr and Fv/Fm was only partially recovered, with a higher recovery level in ‘Midnight’ than in ‘Brilliant’. Rubisco activity and activation state restored to the control level after re‐watering, with more rapid increase in ‘Midnight’ than in ‘Brilliant’. The values of Ls, Lns, CP and Rd declined, and A_max, CE, V_cmax, J_max and g_m increased after re‐watering, with more rapid change in all parameters in ‘Midnight’ than in ‘Brilliant’. These results indicated that the maintenance of higher A and A_max under drought stress in drought‐tolerant Kentucky bluegrass could be attributed to higher Rubico activation state, higher CE and less stomatal limitation. The ability to resume metabolic activity (A_max, CE, Fv/Fm and Rubisco) was observed in the drought‐tolerant genotype and is the most likely cause for the increased recuperative ability of photosynthesis. Incomplete recovery of photosynthesis upon re‐watering could be attributable to lasting stomatal limitations caused by severe drought damage in both genotypes. Promoting rapid stomatal recovery from drought stress may be critical for plants to resume full photosynthetic capacity in C₃ perennial grass species.     

Characterization of photosynthesis, photoinhibition and the activities of C4 pathway enzymes in a superhigh-yield rice, Liangyoupeijiu

Characteristics of photosynthetic gas exchange, photoinhibition and C₄ pathway enzyme activities in both flag leaves and lemma were compared between a superhigh-yield rice (Oryza sativa L.) hybrid, Liangyoupeijiu and a traditional rice hybrid, Shanyou63. Liangyoupeijiu had a similar light saturated assimilation rate (Asat) to Shanyou63, but a much higher apparent quantum yield (AQY), carboxylation efficiency (CE) and quantum yield of CO₂ fixation (ΦCO₂). Liangyoupeijiu also showed a higher resistance to photoinhibition and higher non-radiative energy dissipation associated with the xanthophyll cycle than Shanyou63 when subjected to strong light. In addition, Liangyoupeijiu had higher activities of the C₄ pathway enzymes in both flag leaves and lemmas than Shanyou63. These results indicate that higher light and CO₂ use efficiency, higher resistance to photoinhibition and C₄ pathway in both flag leaf and lemma may contribute to the higher yield of the superhigh-yield rice hybrid, Liangyoupeijiu.     

Photorespiration and photoprotection of grapevine (<Emphasis Type="Italic">Vitis vinifera</Emphasis> L. cv. Cabernet Sauvignon) under water stress

In order to investigate the photoprotective function of photorespiration in grapevine under water stress, potted grapevines (Vitis vinifera L. cv. Cabernet Sauvignon) were randomly divided into three uniform groups for well-watered [watered every morning to keep the relative water content (RWC) of soil over 70 %], water-stress adapted (drought-adapted at 30 % relative soil water content for 30 days), and water stress without adaptation treatment (water-stressed to 30 % relative soil water content for 3 days). Net assimilation rate (A _N), stomatal conductance (g _s), substomatal CO₂ concentration (C _i), transpiration rate (E), actual photochemical efficiency of PSII (Φ_PSII), and maximum photochemical efficiency of PSII (F_v/F_m) were recorded by combining measurements of gas exchange and chlorophyll fluorescence. Gross photorespiration (P_r), photosynthetic electron partitioning (J_C/J_T), photochemical quenching coefficient (qP), and non-photochemical quenching (NPQ) were also calculated. The ratio of net assimilation rate to transpiration rate (A _N/E) was used as an indicator of water use efficiency (WUE). A _N, apparent P_r, Φ_PSII, F_v/F_m, qp, and g _s decreased, NPQ increased, and gross P_r sustained at a high level under water stress. This suggests that both photorespiration and energy dissipation play important roles in protecting photosynthetic apparatus against photoinhibition. C _i in water-stressed plants without adaptation treatment increased, which indicates the leaves suffered a non-stomatal limitation, while the water-stress adaped plants only suffered a stomatal limitation indicated by low C _i.     

Recovery of photosynthesis in winter-stressed Scots pine

Abstract. . Winter-induced inhibition of photosynthesis in Scots pine (Pinns sylvestris L.) is caused by the combined effects of light and freezing temperatures; light causes photoinhibition of photosystem II (Strand & Oquist, 1985b, Physiologic Plantarum, 65 , 117–123), whereas frost causes inhibition of enzymatic steps of photosynthesis (Strand & Öquist, 1988, Plant, Cell & Environment, 11 , 231–238). To reveal limiting steps during recovery from winter stress, the potential of photosynthesis to recover and the actual recovery outdoors during spring, were studied in Scots pine. Studies of light dependent O₂-evolution under saturating CO₂ and recordings of room temperature fluorescence induction kinetics were used. When branches of pine, in February and March, were brought into the laboratory and kept at 18°Cand 100μmol m^?2 s^?1, light saturated rates and apparent quantum yields of photo-synthetic O₂-evolution recovered fully within approximately 48h. The photochemical efficiency of photosystem II, as measured by Fv/Fm ratios, recovered fully within 24h after an initial lag-phase of 2-3 h. Under natural winter conditions, the Fv/Fm ratio decreased more in exposed than in shaded pine, whereas the efficiency of photosynthesis was similarly inhibited in exposed and shadedpine. However, when recovery from winter stress occurred during spring, the Fv/Fm ratios of both shaded and exposed pine recovered well before photosynthesis. It is concluded that the light-induced photoinhibition component of winter stress in photosynthesis of pine recovers well before the frost induced component(s) of winter stress. In this context, reversible photoinhibition of photosynthesis in evergreen conifers is considered as a dynamic down-regulation of photosystem II to prevent more severe photodynamic damage of the thylakoid membrane when photosynthesis is inhibited by frost.     

The Effect of Photoinhibition on Photosynthetic Oxygen Production in the Brown Alga Dictyota dichotoma

Photoinhibition of photosynthesis in the brown alga, Dictyota dichotoma, was studied with a PAM fluorometer (Walz, Effeltrich, Germany) and a homemade oxygen measuring device. As a measure of fluorescence, Fv/Fm, and for the photosynthetic yield, ΔF/Fm', were used. Oxygen measurements show clearly that the observed degree, as well as the time course, of photoinhibition depends on the fluence rate of the light used to measure changes of the production rate. After photoinhibition of photosynthesis the depression of oxygen production caused by non-saturating fluence rates was generally much more pronounced than that caused by saturating or nearly saturating fluence rates. At minimal photoinhibition the initial slope and the convexity of the fluence rate-response curve of oxygen evolution decrease, whereas the level of light saturation decreases only after strong photoinhibition. Nevertheless, at different degrees of photoinhibition, changes in the degree of the upper bending of the fluence rate-response curve of oxygen production are also linearly correlated to changes in the fluorescence ratios (Fv/Fm and ΔF/Fm'). The action spectrum of photoinhibition, calculated on the basis of changes of Fv/Fm, indicates that the reaction center of PS I is not involved in photoinhibition. The lower effectiveness of blue light in comparison to effects of green and red light may be due to chloroplast displacement, as in the so-called strong light position, the light absorbed by the thalli in vivo is decreased.     

Estimating photosynthetic electron transport via chlorophyll fluorometry without Photosystem II light saturation

Estimates of thylakoid electron transport rates (J_e) from chlorophyll fluorometry are often used in combination with leaf gas exchange measurements to provide detailed information about photosynthetic activity of leaves in situ. Estimating J_e requires accurate determination of the quantum efficiency of Photosystem II (Φ_P), which in turn requires momentary light saturation of the Photosystem II light harvesting complex to induce the maximum fluorescence signal (F_M′). In practice, full saturation is often difficult to achieve, especially when incident photosynthetic photon flux density (Q) is high and energy is effectively dissipated by non-photochemical quenching. In the present work, a method for estimating the true F_M′ under high Q was developed, using multiple light pulses of varying intensity (Q′). The form of the expected relationship between the apparent F_M′ and Q′ was derived from theoretical considerations. This allowed the true F_M′ at infinite Q′ to be estimated from linear regression. Using a commercially available leaf gas exchange/ chlorophyll fluorescence measurement system, J_e was compared to gross photosynthetic CO₂ assimilation (A_G) under conditions where the relationship between J_e and A_G was expected to be linear. Both in C₄ leaves (Zea mays) in ambient air and also in C₃ leaves (Gossypium hirsutum) under non-photorespiratory conditions the apparent ratio between J_e and A_G declined at high Q when Φ_P was calculated from F_M′ measured simply using the highest available saturating pulse intensity. When F_M′ was determined using the multiple pulse / linear regression technique, the expected relationship between J_e and A_G at high Q was restored, indicating that the Φ_P estimate was improved. This method of determining F_M′ should prove useful for verifying when saturating pulse intensities are sufficient, and for accurately determining Φ_P when they are not. This revised version was published online in June 2006 with corrections to the Cover Date.     

Photoinhibition of Photosynthesis and its Recovery in Red Algae

In different marine red algae (Chondrus crispus, Delesseria sanguinea, Membranoptera alata, Phycodrys rubens, Phyllophora truncata, Polyneura hilliae) photoinhibition of photosynthesis has been investigated by means of both fluorescence and oxygen measurements. Measurements of absolute oxygen production show that photoinhibition causes a decline in the initial slope and in the rate of bending of the fluence rate-response curve (i.e. the photosynthetic efficiency at non-saturating fluence rates), as well as a decline in the photosynthetic capacity (Pm) at saturating fluence rates. Fluorescence data (Fv/Fm) were consistent with the results of oxygen measurements. Under excessive light photoinhibition protects photosynthesis against photo-damage in red algae. However, an increase in the initial fluorescence (Fo) after photoinhibitory treatment indicates that it could not prevent photodamage entirely. Action spectra of photoinhibition demonstrate that the main photoinhibition site in Polyneura hiliae is PS II, because far red light absorbed by PS I was ineffective. The strong increase of Fo in the blue wavelength range and the slight and partial recovery in weak blue light indicate that blue light especially causes photodamage. Recovery of photosynthesis requires dim white light conditions. Experiments with monochromatic light also show a wavelength dependence of recovery. Moreover, the recovery of photosynthesis after a photoinhibitory treatment is strongly temperature dependent, indicating participation of enzymatic processes. The comparison of fluorescence and oxygen measurement of the recovery shows different results in some species. The rate of oxygen production in red control light increased immediately after photoinhibited algae were exposed to weak light conditions. Surprisingly, the ratio of variable to maximum fluorescence (Fv/Fm) of Phyllophora truncata and the maximum fluorescence (Fm) of Polyneura hilliae show first a delay of the recovery under weak light conditions. Thus, in recovery experiments fluorescence and oxygen data are not quite consistent.     

Contributions of diffusional limitation, photoinhibition and photorespiration to midday depression of photosynthesis in Arisaema heterophyllum in natural high light

Diurnal changes in photosynthetic gas exchange and chlorophyll fluorescence were measured under full sunlight to reveal diffusional and non‐diffusional limitations to diurnal assimilation in leaves of Arisaema heterophyllum Blume plants grown either in a riparian forest understorey (shade leaves) or in an adjacent deforested open site (sun leaves). Midday depressions of assimilation rate (A) and leaf conductance of water vapour were remarkably deeper in shade leaves than in sun leaves. To evaluate the diffusional (i.e. stomatal and leaf internal) limitation to assimilation, we used an index [1–A/A₃₅₀], in which A₃₅₀ is A at a chloroplast CO₂ concentration of 350 μ mol mol ^{? 1}. A₃₅₀ was estimated from the electron transport rate (J_T), determined fluorometrically, and the specificity factor of Rubisco (S), determined by gas exchange techniques. In sun leaves under saturating light, the index obtained after the ‘peak’ of diurnal assimilation was 70% greater than that obtained before the ‘peak’, but in shade leaves, it was only 20% greater. The photochemical efficiency of photosystem II ( Δ F/F_m ′ ) and thus J_T was considerably lower in shade leaves than in sun leaves, especially after the ‘peak’. In shade leaves but not in sun leaves, A at a photosynthetically active photon flux density (PPFD) > 500 μ mol m ^{? 2} s ^{? 1} depended positively on J_T throughout the day. Electron flows used by the carboxylation and oxygenation (J_O) of RuBP were estimated from A and J_T. In sun leaves, the J_O/J_T ratio was significantly higher after the ‘peak’, but little difference was found in shade leaves. Photorespiratory CO₂ efflux in the absence of atmospheric CO₂ was about three times higher in sun leaves than in shade leaves. We attribute the midday depression of assimilation in sun leaves to the increased rate of photorespiration caused by stomatal closure, and that in shade leaves to severe photoinhibition. Thus, for sun leaves, increased capacities for photorespiration and non‐photochemical quenching are essential to avoid photoinhibitory damage and to tolerate high leaf temperatures and water stress under excess light. The increased Rubisco content in sun leaves, which has been recognized as raising photosynthetic assimilation capacity, also contributes to increase in the capacity for photorespiration.     

Canopy gradients in leaf intercellular CO2 mole fractions revisited: interactions between leaf irradiance and water stress need consideration

Intercellular CO₂ mole fractions (C_i) are lower in the upper canopy relative to the lower canopy leaves. This canopy gradient in C_i has been associated with enhanced rates of carbon assimilation at high light, and concomitant greater draw‐downs in C_i. However, increases in irradiance in the canopy are generally also associated with decreases in leaf water availability. Thus, stress effects on photosynthesis rates (A) and stomatal conductance (G), may provide a further explanation for the observed C_i gradients. To test the hypotheses of the sources of canopy variation in C_i, and quantitatively assess the influence of within‐canopy differences in stomatal regulation on A, the seasonal and diurnal variation in G was studied in relation to seasonal average daily integrated quantum flux density (Q_int) in tall shade‐intolerant Populus tremula L. trees. Daily time‐courses of A were simulated using the photosynthesis model of Farquhar et al. (Planta 149, 78–90, 1980). Stable carbon isotope composition of a leaf carbon fraction with rapid turnover rate was used to estimate canopy gradient in C_i during the simulations. Daily maximum G (G_max) consistently increased with increasing Q_int. However, canopy differences in G_max decreased as soil water availability became limiting during the season. In water‐stressed leaves, there were strong mid‐day decreases in G that were poorly associated with vapour pressure deficits between the leaf and atmosphere, and the magnitude of the mid‐day decreases in G occasionally interacted with long‐term leaf light environment. Simulations indicated that the percentage of carbon lost due to mid‐day stomatal closure was of the order of 5–10%, and seasonal water stress increased this percentage up to 20%. The percentage of carbon lost due to stomatal closure increased with increasing Q_int. Canopy differences in light environment resulted in a gradient of daily average C_i of approximately 20 µmol mol^?1. The canopy variation in seasonal and diurnal reductions in G led to a C_i gradient of approximately 100 µmol mol^?1, and the actual canopy C_i gradient was of the same magnitude according to leaf carbon isotope composition. This study demonstrates that stress effects influence C_i more strongly than within‐canopy light gradients, and also that leaves acclimated to different irradiance and water stress conditions may regulate water use largely independent of foliar photosynthetic potentials.     

Light-dependent damage to photosynthesis in olive leaves during chilling and high temperature stress

Abstract The leaves of olive are long lived and likely to experience both chilling and high temperature stress during their life. Changes in photosynthetic CO₂ assimilation resulting from chilling and high temperature stress, in both dim and high light, are investigated. The quantum yield (φ) of photosynthesis at limiting light levels was reduced following chilling (at 5°C for 12 h), in dim light by approximately 10%, and in high light by 75%; the difference being attributed to photoinhibition. Similar reductions were observed in the light-saturated rate of CO₂ uptake (A_max). Decrease in A_max correlated with a halving of the leaf internal CO₂ concentration (c_i), suggesting an increased limitation by stomata following photoinhibition. Leaves were apparently more susceptible to photoinhibitory damage if the whole plant, rather than the leaf alone, was chilled. On return to 26 °C, I he photosynthetic capacity recovered to pre-stress levels within a few hours if leaves had been chilled in high light for 8 h or less, but did not fully recover from longer periods of chilling when loss of chlorophyll occurred. Leaves which were recovering from chilling in high light showed far more damage on being chilled a second time in high light. Three hours in high light at 38 °C reduced φ by 80%, but φ recovered within 4h of return to 26 °C. Although leaves of Olive are apparently less susceptible to photoinhibitory damage during chilling stress than the short-lived leaves of chilling-sensitive annual? crops, the results nevertheless show that photoinhibition during temperature stress is potentially a major factor influencing the photosynthetic productivity of Olive in the field.     

Functional relationships between hydraulic traits and the timing of diurnal depression of photosynthesis

The hydraulic coordination along the water transport pathway helps trees provide adequate water supply to the canopy, ensuring that water deficits are minimized and that stomata remain open for CO₂ uptake. We evaluated the stem and leaf hydraulic coordination and the linkages between hydraulic traits and the timing of diurnal depression of photosynthesis across seven evergreen tree species in the southern Andes. There was a positive correlation between stem hydraulic conductivity (k_s) and leaf hydraulic conductance (K_Leaf) across species. All species had similar maximum photosynthetic rates (A_max). The species with higher k_s and K_Leaf attained A_max in the morning, whereas the species with lower k_s and K_Leaf exhibited their A_max in the early afternoon concurrently with turgor loss. These latter species had very negative leaf water potentials, but far from the pressure at which the 88% of leaf hydraulic conductance is lost. Our results suggest that diurnal gas exchange dynamics may be determined by leaf hydraulic vulnerability such that a species more vulnerable to drought restrict water loss and carbon assimilation earlier than species less vulnerable. However, under stronger drought, species with earlier CO₂ uptake depression may increase the risk of hydraulic failure, as their safety margins are relatively narrow.     

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.     

RELATIONS BETWEEN SUNFLECK SEQUENCES AND PHOTOINHIBITION OF PHOTOSYNTHESIS IN A TROPICAL RAIN FOREST UNDERSTORY HERB

We report the photosynthetic characteristics of a C₃ shade plant native to the tropical rain forest understory. It was shown that Elatostema repens Lour. (Hall) f. (Urticaceae) presents a large light adjustment capacity. The effects of several lightfleck sequences on photoinhibition of photosynthesis and carbon gain are analyzed. Photoinhibition is measured both as a decrease in leaf net CO₂ uptake in limiting light (shown to be linearly correlated to quantum yield of O₂ evolution measured at saturating CO₂) and as a decrease of the ratio of variable fluorescence (Fv) to maximum fluorescence (Fmax) measured in liquid nitrogen. It is shown that lightflecks (from 10 to 30 min in duration) of 700 μmol m^–2 s^–1 (high light) induce photoinhibition, and that the effects of those successive high light periods are additive; there is apparently no recovery from photoinhibition during the low light periods (from 10 to 45 min in duration). In contrast, the Fv/Fmax ratio, though decreasing similarly to quantum yield of net CO₂ uptake on leaves submitted to a continuous illumination of 700 μmol m^–2 s^–1, is only decreased a little on leaves submitted to lightfleck sequences of the same photon flux density. Lightflecks of 250 μmol m^–2 s^–1 are not photoinhibitory. Compared to the control maintained under light growth condition (40 μmol m^–2 s^–1) carbon gain is increased on leaves submitted to lightflecks; this gain remains high throughout the light cycles on leaves submitted to nonphotoinhibitory lightflecks and to the photoinhibitory lightflecks followed by the shortest low light period. In the other cases, carbon gain, higher than that of the control at the beginning of the treatments, decreases and becomes lower than the control carbon gain. Finally, the relevance of photoinhibition in the tropical rain forest understory environment is discussed.     

Increased photoinhibition in dehydrated leaves of hot pepper (Capsicum annuum L.) is not accompanied by an incremental loss of functional PSII

We examined the photosynthetic responses to photoinhibition in dehydrated leaves of hot pepper (Capsicum annuum L.). Stress was induced by immersing the roots of whole plants in Hoaglands solution containing polyethylene glycol (PEG) under high light (900 μmol photons m^-2 · s^-1). This PEG-treatment lowered the leaf water potential and the maximal rate of photosynthetic O₂ evolution (Pmax) linearly, in a time-dependent manner, to about 50% inhibition after 6 h. Pmax also decreased linearly as the period of high-light treatment lengthened. That inhibitory response was not as extreme, showing about 30% inhibition after 6 h. However, when the treatments of dehydration and high light were simultaneously administered, Pmax decreased more rapidly, in a synergistic fashion, showing about 90% inhibition within 2 h. Dehydration, in contrast to the light treatment, did not lower the maximal photochemical efficiency (Fv/Fm). Furthermore, this decline in Fv/Fm for light-treated, dehydrated leaves was almost identical to the response of photoinhibited leaves that were not dehydrated. Similar changes were observed in the number of functional PSII complexes. The decrease in Pmax and the amount of functional PSII was linearly correlated in photoinhibited leaves, but not in dehydrated leaves, regardless of light treatment. Therefore, we have demonstrated that exacerbated photoinhibition in dehydrated leaves occurs without an incremental loss of functional PSII.     

Photoinhibition and Photooxidation in Leaves of indica and  japonica Rice Under Different Temperatures and Light Intensities

Physiological indices related to the efficiency （ Fv/Fm ） of light energy conversion in PSⅡ and the peroxidation of membrane lipid were measured in leaves of Oryza sativa L. sp. indica rice cv. “Shanyou 63” and sp. japonica rice cv. “9516” under different temperatures and light intensities for 4 days. No changes in Fv/Fm and membrane lipid peroxidation product (MDA) were observed, so neither photoinhibition nor photooxidation happened in both rice cultivars under moderate temperature and medium light intensity. However, Fv/Fm dropped obviously with no change in MDA contents, and photoinhibition appeared in indica rice cv. “Shanyou 63” under medium temperature and strong light intensity. Furthermore, both photoinhibition and photooxidation were observed in two rice cultivars under chilling temperature and strong light intensity. Experiments with inhibitors under chilling temperature and strong light intensity showed that indica rice had a decrease in D1 protein content and SOD activity, and the extent of inhibition of xanthophyll cycle and nonphotochemical quenching ( qN ) was larger, and a higher level of MDA was observed. The photoinhibition and photooxidation in indica rice were more distinct as compared with japonica rice. The authors suggested that PSⅡ light energy conversion efficiency ( Fv/Fm ) and membrane lipid peroxidation were the key indices for the detection of photooxidation.     

Photosynthetic limitation of several representative subalpine species in the Catalan Pyrenees in summer

Information on the photosynthetic process and its limitations is essential in order to predict both the capacity of species to adapt to conditions associated with climate change and the likely changes in plant communities. Considering that high‐mountain species are especially sensitive, three species representative of subalpine forests of the Central Catalan Pyrenees: mountain pine (Pinus uncinata Mill.), birch (Betula pendula Roth) and rhododendron (Rhododendron ferrugineum L.) were studied under conditions associated with climate change, such as low precipitation, elevated atmospheric [CO₂] and high solar irradiation incident at Earth's surface, in order to detect any photosynthetic limitations. Short‐term high [CO₂] increased photosynthesis rates (A) and water use efficiency (WUE), especially in birch and mountain pine, whereas stomatal conductance (g_s) was not altered in either species. Birch showed photosynthesis limitation through stomatal closure related to low rainfall, which induced photoinhibition and early foliar senescence. Rhododendron was especially affected by high irradiance, showing early photosynthetic saturation in low light, highest chlorophyll content, lowest gas exchange rates and least photoprotection. Mountain pine had the highest A, photosynthetic capacity (A_max) and light‐saturated rates of net CO₂ assimilation (A_sat), which were maintained under reduced precipitation. Furthermore, maximum quantum yield (F_v/F_m), thermal energy dissipation, PRI and SIPI radiometric index, and ascorbate content indicated improved photoprotection with respect to the other two species. However, maximum velocity of carboxylation of RuBisco (V_cmax) indicated that N availability would be the main photosynthetic limitation in this species.     

热带雨林冠层树种绒毛番龙眼两种发育阶段叶片的光抑制

 通过测定西双版纳热带雨林冠层树种绒毛番龙眼(Pometia tomentosa)完全伸展嫩叶和成熟叶的叶片解剖、生理特征和雨季晴天自然条件下叶绿素a荧光以及午间强光对部分保护酶活性和膜脂过氧化作用的影响,探讨了两种不同发育阶段叶片光合作用的光抑制与强光和温度的关系。结果表明：绒毛番龙眼全展嫩叶和成熟叶表现出明显的解剖和生理特征差异。与全展嫩叶相比,成熟叶的叶片较厚、叶绿素含量高、气孔导度大、羧化效率高、最大净光合速率和光饱和点高,而气孔密度和保卫细胞长度没有显著差别。在雨季晴天自然条件下,午间最高光强可达2 200 μmol·m-2·s-1以上,最高叶温比气温高7～8 ℃,而成熟叶片的最高温度比全展嫩叶高1.5～2 ℃。上午随光强的增大,两种叶片的非光化学猝灭系数（NPQ）增大,PSⅡ原初光化学效率(Fv/Fm)、实际光化学效率［(Fm′_Fs)/Fm′］逐渐减小,在15∶30左右达最小。下午随着光强的减弱,Fv/Fm逐渐恢复,在傍晚基本恢复到清晨值。初始荧光(F0)在一天中变化很小。这表明绒毛番龙眼叶片光抑制是非辐射能量耗散增加引起的保护光合机构免受光破坏的保护性反应,而非光破坏。全展嫩叶比成熟叶有较低的光化学效率和非辐射耗散能力,对强光和高温处理的敏感性也较强,但在自然条件下一天中的光抑制程度与成熟叶没有显著差别。田间午间强光导致两种叶片的保护酶活性(超氧化物歧化酶,SOD;抗坏血酸过氧化物酶,APX)升高,而H2O2含量变化较小。其中,全展嫩叶的保护酶活性高,丙二醛(MDA)含量低。这表明自然条件下,与成熟叶相比,绒毛番龙眼全展嫩叶通过较低的光能利用效率、较低的叶温和高的保护酶活性减轻了强光高温的光抑制程度。