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.     

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.     

The temperature acclimation of photosynthetic responses to CO2 in Zea mays and its relationship to the activities of photosynthetic enzymes and the CO2-concentrating mechanism of C4 photosynthesis

Abstract Associations between photosynthetic responses to CO₂ at rate-saturating light and photosynthetic enzyme activities were compared for leaves of maize grown under constant air temperatures of 19, 25 and 31°C. Key photosynthetic enzymes analysed were ribulose bisphosphatc (RuBP) carboxylase, phosphoenolpyruvate (PEP) carboxylase, NADP-malic enzyme and pyruvate, P_i dikinasc. Rates of CO₂-saturated photosynthesis were similar in leaves developed at 19°C and 25°C but were decreased significantly by growth at 31°C. In contrast, carboxylation efficiency differed significantly between all three temperature regimes. Carboxylation efficiency was greatest in leaves developed at 19°C and decreased with increasing temperature during growth. The changes of carboxylation efficiency were highly correlated with changes in the activity of pyruvate, P_i dikinase (r= 0.95), but not with other photosynthetic enzyme activities. The activities of these latter enzymes, including that of RuBP carboxylase, were relatively insensitive to temperature during growth. The sensitivity of quantum yield to O₂ concentration was lower in leaves grown at 19°C than in leaves grown at 31°C. These observations support the novel hypothesis that variation in the capacity for CO₂ delivery to the bundle sheath by the C₄ cycle, relative to the capacity for net assimilation by the C₂ cycle, can be a principal determinant of C₄ photosynthetic responses to CO₂.     

Photosynthetic performance at low temperature of Bouteloua gracilis Lag., a high-altitude C4 grass from the Rocky Mountains, USA

The mechanisms controlling the photosynthetic performance of C₄ plants at low temperature were investigated using ecotypes of Bouteloua gracilis Lag. from high (3000 m) and low (1500 m) elevation sites in the Rocky Mountains of Colorado. Plants were grown in controlled‐environment cabinets at a photon flux density of 700 μ mol m^?2 s^?1 and day/night temperatures of 26/16 °C or 14/7 °C. The thermal response of the net CO₂ assimilation rate (A) was evaluated using leaf gas‐exchange analysis and activity assays of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco), phosphoenolpyruvate carboxylase (PEPCase) and pyruvate,orthophosphate dikinase (PPDK). In both ecotypes, a reduction in measurement temperature caused the CO₂‐saturated rate of photosynthesis to decline to a greater degree than the initial slope of A versus the intercellular CO₂ response, thereby reducing the photosynthetic CO₂ saturation point. As a consequence, A in normal air was CO₂‐saturated at sub‐optimal temperatures. Ecotypic variation was low when grown at 26/16 °C, with the major difference between the ecotypes being that the low‐elevation plants had higher A; however, the ecotypes responded differently when grown at cool temperature. At temperatures below the thermal optimum, A in high‐elevation plants grown at 14/7 °C was enhanced relative to plants grown at 26/16 °C, while A in low‐elevation plants grown at 14/7 °C was reduced compared to 26/16 °C‐grown plants. Photoinhibition at low growth temperature was minor in both ecotypes as indicated by small reductions in dark‐adapted F_v/F_m. In both ecotypes, the activity of Rubisco was equivalent to A below 17 °C but well in excess of A above 25 °C. Activities of PEPCase and PPDK responded to temperature in a similar proportion relative to Rubisco, and showed no evidence for dissociation that would cause them to become principal limitations at low temperature. Because of the similar temperature response of Rubisco and A, we propose that Rubisco is a major limitation on C₄ photosynthesis in B. gracilis below 17 °C. Based on these results and for theoretical reasons associated with how C₄ plants use Rubisco, we further suggest that Rubisco capacity may be a widespread limitation upon C₄ photosynthesis at low temperature.     

Photosynthetic performance and resistance to photoinhibition of Zea mays L. leaves grown at sub-optimal temperature

The performance of the photosynthetic apparatus was examined in the third leaves of Zea mays L. seedlings grown at near-optimal (25 °C) or at suboptimal (15 °C) temperature by measuring chlorophyll (ChI) a fluorescence parameters and oxygen evolution in different temperature and light conditions. In leaf tissue grown at 25 and 15 °C, the quantum yield of PSII electron transport (ψ_PSII) and the rate of O₂ evolution decreased with decreasing temperature (from 25 to 4 °C) at a photon flux density of 125 μmol m^?2 s^?1. In leaves grown at 25 °C, the decrease of ψ_PSII correlated with a decrease of photochemical ChI fluorescence quenching (q_p), whereas in leaves crown at 15 °C q_p was largely insensitive to the temperature decrease. Compared with leaves grown at 25 °C, leaves grown at 15 °C were also able to maintain a higher fraction of oxidized to reduced Q_A (greater q_p) at high photon flux densities (up to 2000 μmol m^?2 s^?1), particularly when the measurements were performed at high temperature (25 °C). With decreasing temperature and/or increasing light intensity, leaves grown at 15 °C exhibited a substantial quenching of the dark level of fluorescence F₀ (q₀) whereas this type of quenching was virtually absent in leaves grown at 25 °C. Furthermore, leaves grown at 15 °C were able to recover faster from photo inhibition of photosynthesis after a photoinhibitory treatment (1200 μmol m^?2 s^?1 at 25, 15 or 6 °C for 8 h) than leaves grown at 25 °C. The results suggest that, in spite of having a low photosynthetic capacity, Z. mays leaves grown at sub optimal temperature possess efficient mechanisms of energy dissipation which enable them to cope better with photoinhibition than leaves grown at near-optimal temperature. It is suggested that the resistance of Z. mays leaves grown at 15 °C to photoinhibition is related to the higher content of carotenoids of the xanthophyll cycle (violaxanthin + antheraxanthin + zeaxanthin) measured in these leaves than in leaves grown at 25 °C.     

Photosynthetic downregulation in leaves of the Japanese white birch grown under elevated CO2 concentration does not change their temperature‐dependent susceptibility to photoinhibition

To determine the effects of elevated CO₂ concentration ([CO₂]) on the temperature‐dependent photosynthetic properties, we measured gas exchange and chlorophyll fluorescence at various leaf temperatures (15, 20, 25, 30, 35 and 40°C) in 1‐year‐old seedlings of the Japanese white birch (Betula platyphylla var. japonica), grown in a phytotron under natural daylight at two [CO₂] levels (ambient: 400 µmol mol^?1 and elevated: 800 µmol mol^?1) and limited N availability (90 mg N plant^?1). Plants grown under elevated [CO₂] exhibited photosynthetic downregulation, indicated by a decrease in the carboxylation capacity of Rubisco. At temperatures above 30°C, the net photosynthetic rates of elevated‐CO₂‐grown plants exceeded those grown under ambient [CO₂] when compared at their growth [CO₂]. Electron transport rates were significantly lower in elevated‐CO₂‐grown plants than ambient‐CO₂‐grown ones at temperatures below 25°C. However, no significant difference was observed in the fraction of excess light energy [(1 ? q_P)× F_v′/F_m′] between CO₂ treatments across the temperature range. The quantum yield of regulated non‐photochemical energy loss was significantly higher in elevated‐CO₂‐grown plants than ambient, when compared at their respective growth [CO₂] below 25°C. These results suggest that elevated‐CO₂‐induced downregulation might not exacerbate the temperature‐dependent susceptibility to photoinhibition, because reduced energy consumption by electron transport was compensated for by increased thermal energy dissipation at low temperatures.     

Electron transport is the functional limitation of photosynthesis in field-grown Pima cotton plants at high temperature

Restrictions to photosynthesis can limit plant growth at high temperature in a variety of ways. In addition to increasing photorespiration, moderately high temperatures (35–42 °C) can cause direct injury to the photosynthetic apparatus. Both carbon metabolism and thylakoid reactions have been suggested as the primary site of injury at these temperatures. In the present study this issue was addressed by first characterizing leaf temperature dynamics in Pima cotton (Gossypium barbadense) grown under irrigation in the US desert south‐west. It was found that cotton leaves repeatedly reached temperatures above 40 °C and could fluctuate as much as 8 or 10 °C in a matter of seconds. Laboratory studies revealed a maximum photosynthetic rate at 30–33 °C that declined by 22% at 45 °C. The majority of the inhibition persisted upon return to 30 °C. The mechanism of this limitation was assessed by measuring the response of photosynthesis to CO₂ in the laboratory. The first time a cotton leaf (grown at 30 °C) was exposed to 45 °C, photosynthetic electron transport was stimulated (at high CO₂) because of an increased flux through the photorespiratory pathway. However, upon cooling back to 30 °C, photosynthetic electron transport was inhibited and fell substantially below the level measured before the heat treatment. In the field, the response of assimilation (A) to various internal levels of CO₂ (C_i) revealed that photosynthesis was limited by ribulose‐1,5‐bisphosphate (RuBP) regeneration at normal levels of CO₂ (presumably because of limitations in thylakoid reactions needed to support RuBP regeneration). There was no evidence of a ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) limitation at air levels of CO₂ and at no point on any of 30 A–C_i curves measured on leaves at temperatures from 28 to 39 °C was RuBP regeneration capacity measured to be in substantial excess of the capacity of Rubisco to use RuBP. It is therefore concluded that photosynthesis in field‐grown Pima cotton leaves is functionally limited by photosynthetic electron transport and RuBP regeneration capacity, not Rubisco activity.     

Temperature acclimation of photosynthesis in spinach leaves: analyses of photosynthetic components and temperature dependencies of photosynthetic partial reactions

Spinach (Spinacia oleracea) plants were grown under the day/night temperature regime of 15/10 °C (LT) or 30/25 °C (HT). The plants were also transferred from HT to LT when the sample leaves were at particular developmental stages (HL-transfer). With fully mature leaves, the light-saturated photosynthetic rate (A) at the ambient CO₂ concentration (C_a) of 1500 µL L⁻¹ (A₁₅₀₀) and the initial slope of A versus intercellular CO₂ concentration (C_i) at low C_i region (IS) were obtained to assess capacities of RuBP regeneration and carboxylation. Photosynthetic components including Rubisco and cytochrome f (Cyt f) were also determined. The optimum temperatures for A at C_a of 360 µL L⁻¹ (A₃₆₀), A₁₅₀₀ and IS in HT leaves were 27, 36 and 24 °C, whereas those in LT leaves were 18, 30 and 18 °C. The optimum temperatures in HL-transfer leaves approached those of LT leaves with the increase in the duration at LT. The shift in the optimum temperature was greater and quicker for IS than A₁₅₀₀. By the HL-transfer, the maximum values of A₁₅₀₀ and IS also increased. The maximum A₁₅₀₀ and Cyt f content increased more promptly than IS and Rubisco content. Changes in the Cyt f/Rubisco ratio were reflected to those in the A₁₅₀₀/IS ratio. Taken together, photosynthetic acclimation to low temperature in spinach leaves was due not only to the change in the balance of the absolute rates of RuBP regeneration and carboxylation but also to the large change in the optimum temperature of RuBP carboxylation.     

Response of the photosynthetic apparatus in maize leaves grown at low temperature on transfer to normal growth temperature

Low temperatures are known to restrict chloroplast development and prevent the attainment of photosynthetic competence in maize leaves. The responses of the photosynthetic apparatus of mature maize leaves grown at 14°C on transfer of the plants to 25°C are examined. The synthesis of thylakoid proteins increased immediately on transfer of leaves from 14 to 25°C, with a dramatic accumulation of thylakoid proteins and chlorophylls occurring after 3 d at 25°C. Thylakoid structure and organization also became similar to those observed in leaves grown at 25°C over this period. However, no comparable development of photosynthetic competence in photosystems I and II or in the rate of CO₂ assimilation was observed on transfer of leaves from 14 to 25°C. Immunocytological analyses demonstrated heterogeneity in the distribution of a range of thylakoid proteins (cy tochrome f, the α and β subunits of the coupling factor, Dl of the photosytem II reaction centre, the 33kDa protein of the extrinsic oxygen-evolving complex of photosystem II, and subunit II of photosystem I between mesophyll cells in leaves grown at 14°C, and in the responses of individual proteins to transfer of the leaves to 25°C. Such heterogeneity between mcsophyll cells would account for the inability of the leaves to develop the expected degree of photosynthetic competence on transfer to 25°C. The effects of low growth temperatures on chloroplast biogenesis are complex, as are the changes induced by the transfer ofleaves grown at low temperatures to optimal growth temperature, and both these factors may limit the canopy development and photosynthetic productivity of crops in temperate regions.     

Photosynthetic capacity and temperature responses of photosynthesis of rubber trees (Hevea brasiliensis Müll. Arg.) acclimate to changes in ambient temperatures

The aim of this study was to assess the temperature response of photosynthesis in rubber trees (Hevea brasiliensis Müll. Arg.) to provide data for process-based growth modeling, and to test whether photosynthetic capacity and temperature response of photosynthesis acclimates to changes in ambient temperature. Net CO₂ assimilation rate (A) was measured in rubber saplings grown in a nursery or in growth chambers at 18 and 28°C. The temperature response of A was measured from 9 to 45°C and the data were fitted to an empirical model. Photosynthetic capacity (maximal carboxylation rate, V _cmax, and maximal light driven electron flux, J _max) of plants acclimated to 18 and 28°C were estimated by fitting a biochemical photosynthesis model to the CO₂ response curves (A–C _i curves) at six temperatures: 15, 22, 28, 32, 36 and 40°C. The optimal temperature for A (T _opt) was much lower in plants grown at 18°C compared to 28°C and nursery. Net CO₂ assimilation rate at optimal temperature (A _opt), V _cmax and J _max at a reference temperature of 25°C (V _cmax25 and J _max25) as well as activation energy of V _cmax and J _max (E _aV and E _aJ) decreased in individuals acclimated to 18°C. The optimal temperature for V _cmax and J _max could not be clearly defined from our response curves, as they always were above 36°C and not far from 40°C. The ratio J _max25/V _cmax25 was larger in plants acclimated to 18°C. Less nitrogen was present and photosynthetic nitrogen use efficiency (V _cmax25/N _a) was smaller in leaves acclimated to 18°C. These results indicate that rubber saplings acclimated their photosynthetic characteristics in response to growth temperature, and that higher temperatures resulted in an enhanced photosynthetic capacity in the leaves, as well as larger activation energy for photosynthesis.     

Effects of chilling and high temperatures on photosynthesis and chlorophyll fluorescence in leaves of watermelon seedlings

The effects of chilling (CT, day/night temperatures of 12/10 °C, an irradiance of 250 μmol m^?2 s^?1), chilling combined with a low irradiance (CL, 12/10 °C, 80 μmol m^?2 s^?1), and a high temperature (HT, 42/40 °C, 250 μmol m^?2 s^?1) on chlorophyll content, chlorophyll fluorescence, and gas exchange were studied in two watermelon cultivars, ZJ8424 and YS01, differing in their resistance. The chlorophyll content, net photosynthetic rate (P_N), stomatal conductance (g_s), and transpiration rate (E) decreased substantially, whereas the intercellular CO₂ concentration (c_i) increased when the two watermelon cultivars were grown under these stresses. The photosynthetic parameters showed greater changes at chilling than at the high temperature, and the CL caused a more pronounced inhibition in PN compared with the CT. After 2 d exposure to the CT, YS01 had higher P_N, g_s, and E, but a lower ci compared with ZJ8424. The maximum efficiency of photosystem (PS) II photochemistry (F_v/F_m), effective quantum yield of PS II photochemistry (Φ_PSII), photochemical quenching (qP), and electron transport rate (ETR) decreased under the CT and CL but showed only a slight drop under the HT. All these stresses significantly increased non-photochemical quenching (NPQ). The CT brought more damage to the photosynthetic apparatus of leaves compared with the CL. In addition, after returning to normal conditions (25/15 °C, 250 μmol m^?2 s^?1) for 3 d, the photosynthetic parameters recovered to pre-stress levels in HT treated seedlings but not in CT treated seedlings. In conclusion, the low irradiance could help to alleviate the extent of photoinhibition of PS II photochemistry caused by chilling and cv. ZJ8424 was more sensitive to the extreme temperatures than cv. YS01.     

The responses of photosynthetic gas exchange and chlorophyll a fluorescence to changes of irradiance and temperature in two species of Miscanthus

Photosynthetic CO₂ uptake and chlorophyll (Chl) a fluorescence of C₄ perennial grasses, Miscanthus floridulus (Labill) Warb and M. transmorrisonensis Hayata, from altitudes in central Taiwan of 390 and 2700 m, respectively, were studied at 10 and 25 °C to find if the species differ in their photosynthetic responses to a low temperature, and whether their photosystems 2 become more susceptible to the photoinhibition at low temperatures. For both species, the maximum photosynthetic rate (P_max) was reduced when the leaves were exposed to 10 °C. At irradiances higher than 400 µmol m^-2 s^-1, the values of F_v/F_m were reduced in both species at 10 °C but not at 25 °C, which indicated the photoinhibition at 10 °C. Reductions in P_max and the values of F_v/F_m at 10 °C were lesser in M. transmorrisonensis than in M. floridulus.     

The responses of photosynthetic gas exchange and chlorophyll a fluorescence to changes of irradiance and temperature in two species of Miscanthus

Photosynthetic CO₂ uptake and chlorophyll (Chl) a fluorescence of C₄ perennial grasses, Miscanthus floridulus (Labill) Warb and M. transmorrisonensis Hayata, from altitudes in central Taiwan of 390 and 2700 m, respectively, were studied at 10 and 25 °C to find if the species differ in their photosynthetic responses to a low temperature, and whether their photosystems 2 become more susceptible to the photoinhibition at low temperatures. For both species, the maximum photosynthetic rate (P_max) was reduced when the leaves were exposed to 10 °C. At irradiances higher than 400 μmol m^-2 s^-1, the values of F_v/F_m were reduced in both species at 10 °C but not at 25 °C, which indicated the photoinhibition at 10 °C. Reductions in P_max and the values of F_v/F_m at 10 °C were lesser in M. transmorrisonensis than in M. floridulus. This revised version was published online in June 2006 with corrections to the Cover Date.     

Consequences of growth at two carbon dioxide concentrations and two temperatures for leaf gas exchange in Pascopyrum smithii (C3) and Bouteloua gracilis (C4)*

Continually rising atmospheric CO₂ concentrations and possible climatic change may cause significant changes in plant communities. This study was undertaken to investigate gas exchange in two important grass species of the short-grass steppe, Pascopyrum smithii (western wheat-grass), C₃, and Bouteloua gracilis (blue grama), C4, grown at different CO₂ concentrations and temperatures. Intact soil cores containing each species were extracted from grasslands in north-eastern Colorado, USA, placed in growth chambers, and grown at combinations of two CO₂ concentrations (350 and 700 μmol mol⁻¹) and two temperature regimes (field average and elevated by 4°C). Leaf gas exchange was measured during the second, third and fourth growth seasons. All plants exhibited higher leaf CO₂ assimilation rates (A) with increasing measurement CO₂ concentration, with greater responses being observed in the cool-season C₃ species P. smithii. Changes in the shape of intercellular CO₂ response curves of A for both species indicated photosynthetic acclimation to the different growth environments. The photosynthetic capacity of P. smithii leaves tended to be reduced in plants grown at high CO₂ concentrations, although A for plants grown and measured at 700μmol mol⁻¹ CO₂ was 41% greater than that in plants grown and measured at 350 μmol mol⁻¹ CO₂. Low leaf N concentration may have contributed to photosynthetic acclimation to CO₂. A severe reduction in photosynthetic capacity was exhibited in P. smithii plants grown long-term at elevated temperatures. As a result, the potential response of photosynthesis to CO₂ enrichment was reduced in P. smithii plants grown long-term at the higher temperature.     

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.     

Genotypic variation within Zea mays for susceptibility to and rate of recovery from chill-induced photoinhibition of photosynthesis

Six genotypes of Zea mays L. were grown in pots inside a glasshouse at a mean temperature of 22±2°C and a minimum photosynthetic photon flux density (Q) during the daylight period of 400 μmol m^?2 s^?1. Chilling-dependent photoinhibition was induced by exposing plants to a temperature of 7°C and a Q of 1 000 μmol m^?2 s^?1 for 6 h. Recovery from photoinhibition was then followed at a temperature of 25°C and a Q of 200 μmol m^?2 s^?1. Leaf gas exchange and chlorophyll fluorescence were measured on attached leaves at room temperature prior to the photoinhibitory treatments and at 6 sampling intervals from 0 to 24 h during the recovery period. The relative water content (RWC) was also measured during the recovery period. The results showed a significant genotypic variation in the susceptibility to and rate of recovery from chilling-dependent photoinhibition of photosynthesis in Zea mays seedlings. The Highland Pool 1a from highland sites in Mexico was the least susceptible to chill-induced photoinhibition, but had the slowest rate of recovery. The hybrid variety LG11 showed the highest rate of recovery, whilst the inbred line ZPF307 was the most susceptible to chill-induced photoinhibition. Susceptibility to photoinhibition and subsequent recovery were at least partially independent, suggesting that selection for improved genotypes will require independent selection for both tolerance and capacity for recovery. Although chlorophyll fluorescence provided a more rapid method of assessing the occurrence of photoinhibition, it was not as effective as direct gas-exchange measurements of the maximum quantum yield of photosynthesis (φ) in separating genotypes with respect to their susceptibility to photoinhibition, especially in the most vulnerable genotypes such as ZPF307. Water stress induced by chilling and high Q treatments appeared to impair the recovery processes. Decreases in stomatal conductance (g_s) produce a significant decrease in intercellular CO₂ concentration (C_i), although this decrease was never so extreme that it limited photosynthetic rates at the light intensities used to determine φ. Nevertheless, closure of stomata in patches, producing local restriction of CO₂ supply, would explain the poor correlation between chlorophyll fluorescence and quantum yield measurements in some genotypes immediately after photoinhibitory treatments.     

Interaction between light and chilling temperature on the inhibition of photosynthesis in chilling-sensitive plants*

Abstract. Fully expanded leaves of 25°C grown Phaseolus vulgaris and six other species were exposed for 3 h to chilling temperatures at photon flux densities equivalent to full sunlight. In four of the species this treatment resulted in substantial inhibition of the subsequent quantum yield of CO₂ uptake, indicating reduction of the photochemical efficiency of photosynthesis. The extent of inhibition was dependent on the photon flux density during chilling and no inhibition occurred when chilling occurred at a low photon flux density. No inhibition occurred at temperatures above 11.5°C, even in the presence of the equivalent of full sunlight. This interaction between chilling and light to cause inhibition of photosynthesis was promoted by the presence of oxygen at normal air partial pressures and was unaffected by the CO₂ partial pressure present when chilling occurred in air. When chilling occurred at low O₂ partial pressures, CO₂ was effective in reducing the degree of inhibition. Apparently, when leaves of chilling-sensitive plants are exposed to chilling temperatures in air of normal composition then light is instrumental in inducing rapid damage to the photochemical efficiency of photosynthesis.     

Growth and photosynthesis of two eucalypt species during high temperature stress under ambient and elevated [CO2]

Two species of eucalypt (Eucalyptus macrorhyncha and E. rossii) were grown under conditions of high temperatures (45 °C, maximum) and high light (1500 μmol m^?2 s^?1, maximum) at either ambient (350 μL L^?1) or elevated (700 μL L^?1) CO₂ concentrations for 8 weeks. The growth enhancement, in terms of total dry weight, was 41% and 103% for E. macrorhyncha and E. rossii, respectively, when grown in elevated [CO₂]. A reduction in specific leaf area and increased concentrations of non-structural carbohydrates were observed for leaves grown in elevated [CO₂]. Plants grown in elevated [CO₂] had an overall increase in photosynthetic CO₂ assimilation rate of 27%; however, when measured at the same CO₂ concentration a down-regulation of photosynthesis was evident especially for E. macrorhyncha. During the midday period when temperatures and irradiances were maximal, photosynthetic efficiency as measured by chlorophyll fluorescence (F_v/F_m) was lower in E. macrorhyncha than in E. rossii. Furthermore, F_v/F_m was lower in leaves of E. macrorhyncha grown under elevated than under ambient [CO₂]. These reductions in F_v/F_m were accompanied by increases in both photochemical (qP) and nonphotochemical quenching (qN and NPQ), and by increases in the concentrations of xanthophyll cycle pigments with an increased proportion of the total xanthophyll cycle pool comprising of antheraxanthin and zeaxanthin. Thus, increased atmospheric [CO₂] may enhance photoinhibition when environmental stresses such as high temperatures limit the capacity of a plant to respond with growth to elevated [CO₂].     

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.