The effect of heat on photosynthesis,dark respiration and cellular ultrastructure of the arctic-alpine psychrophyte Ranunculus glacialis

Effects of high temperatures on the leaves of Ranunculus glacialis were studied in plants taken from sites located between 2400-2550 m in the Central Alps. Changes in CO2 exchange rates, in vivo chlorophyll fluorescence, and cellular ultrastructure were investigated during and after an experimental heat exposure. The earliest heat stress effect was inactivation of the net photosynthetic rate at 38-39 °C. Between 40-42 °C, disorders appeared in the photosynthetic apparatus and in the tonoplast. Heat shock granules were observed at 42 °C in chloroplasts, and at 44 °C also in mitochondria. In this temperature range, the dark respiration rate was reversibly enhanced, and an increased number of polyribosomes indicated repair after the primary injury. Above 44 °C, the degradation progress entered the phase of chronic impairment leading to irreversible damage at 45-46 °C. An unusually wide temperature range from the start of reversible photosynthetic inhibition to incipient necrosis indicated a pronounced heat sensitivity, particularly in cellular functions, of this arctic-alpine species.     

Effect of heat stress on development in vitro and in vivo and on synthesis of heat shock proteins in porcine embryos

The present study was conducted (1) to examine the effect of an acute increase in ambient temperature on the development of porcine day 6 embryos in culture and after transfer to recipient gilts, and (2) to analyze intracellular production of heat shock proteins (hsps). The viability of porcine day 6 embryos following a temporary acute elevation in ambient temperature (at 42°–45.5°C and for 10–180 min) was examined. Synthesis of 70 kDa hsp (hsp 70) and 90 kDa hsp (hsp90) was determined by SDS-PAGE and Western blot analysis in porcine day 6 embryos subjected to heat stresses. Nonheat-stressed embryos were considered as control. Significantly higher numbers of viable nuclei were observed in treatment groups of 42°C-10 min (236.6 ± 71.4; P < 0.05) and 43°C-30 min (276.8 ± 89.4; P < 0.005) compared to control (173.9 ± 53.9). The 42°C-180 min group (158.0 ± 27.1 μm) had a greater increase in diameter after 24 hr in culture following heat stress compared to control (82.5 ± 47.3 μm), while heat stress with 43°C for ≧60 min, 44°–44.5°C for ≧30 min, or 45°-45.5°C for ≧10 min impaired their survival, as assessed by differences in number of viable nuclei. The embryos subjected to heat stresses under the conditions of 42°C-180 min, 43°C-10 min, 43°C-30 min, 44°C-10 min, or 45°C-10 min developed to normal piglets after transfer to recipient gilts. Overall pregnancy rate was 75% (6/8), and farrowing rate 62.5% (5/8). Of heat-stressed embryos transferred, 59% (36/61) developed to normal piglets. Heat-stress conditions of 42°C for 180 min, 43°C for 30 min, 44°C for 10 min, and 45°C for 10 min were determined as critical with respect to the in vitro and in vivo survival of porcine embryos. Porcine day 6 embryos constitutively synthesized hsp70 even without heat stress, while hsp90 was detected only at trace level. Neither hsp70 nor hsp90 levels increased in the embryos subjected to heat stresses. In conclusion, porcine day 6 embryos could continue to develop in vivo or during in vitro culture after exposure to acute and temporary rise in temperature. However, no increase of hsp70 and hsp90 was observed in the heat-stressed porcine embryos, while hsp70 was detected in the nonheat-stressed porcine embryos. The precise mechanism of the thermotolerance was unclear. © 1996 Wiley-Liss, Inc.     

EVOLUTIONARY ADAPTATION TO TEMPERATURE II. THERMAL NICHES OF EXPERIMENTAL LINES OF ESCHERICHIA COLI

Groups of replicated lines of the bacterium Escherichia coli were propagated for 2,000 generations at constant 32, 37, or 42°C, or in an environment that alternated between 32 and 42°C. Here, we examine the performance of each group across a temperature range of 12-44°C measuring the temperatures over which each line can maintain itself in serial dilution culture (the thermal niche). Thermal niche was not affected by selection history: average lower and upper limits remained about 19 and 42°C for all groups. In addition, no significant differences among groups were observed in rate of extinction at more extreme temperatures. Within the thermal niche, we measured the mean fitness of the evolved groups relative to their common ancestor. Increases in mean fitness were temperature specific, with the largest increase for each group occurring near its selected temperature. Thus, the temperature at which mean fitness relative to the ancestor was greatest (the thermal optimum) diverged by about 10°C for the groups selected at constant 32°C versus constant 42°C. Tradeoffs in relative fitness (decrements relative to the ancestor elsewhere within the thermal niche) did not necessarily accompany fitness improvements, although tradeoffs were observed for a few of the lines. We conclude that adaptation in this system was quite temperature specific, but substantial divergence among groups in thermal optima had little effect on the limits of their thermal niches and did not necessarily involve tradeoffs in fitness at other temperatures.     

Influence of heat shock on chlorophyll fluorescence of white oak (<Emphasis Type="Italic">Quercus pubescens</Emphasis> Willd.) leaves

Chlorophyll fluorescence parameters of Quercus pubescens Willd. as response to heat shock (HS) by immersing leaves for 5 and 15 min in water of temperatures between 38 and 59 °C were examined. Fluorescence was measured after different periods of recovery (15, 30, 90, 210, and 1 440 min at 24/26 °C night/day temperature and 100 % humidity). The effective quantum yield of photosystem 2 (Y) in control and HS-treated leaves was always measured after previous 15 min irradiation. Under a 5 min HS, Y did not change after using temperatures below 44 °C, was rapidly restored after HS of moderate temperatures (44–48 °C), and progressively decreased and recovered eventually to the initial value after HS of high temperatures (48–52 °C). Y did not recover after HS with temperatures higher than 52 °C. Increase in the duration of HS from 5 to 15 min lead to change of the initial Y at each HS temperature, but the recovery processes were similar to those characteristic after 5 min incubation. The processes of recovery may depend mainly on the specificity of injuries caused by different heat shock temperatures. Thus Q. pubescens is able to preserve and recover the functional potential of its photosynthetic apparatus in response to HS up to 52 °C.     

Thermostability of photosynthesis in two new chlorophyllb-less rice mutants

Leaves of the two new chlorophyll b-less rice mutants VG28-1, VG30-5 and the wild type rice cv. Zhonghua 11 were subjected to temperatures 28, 36, 40, 44 and 48℃ in the dark for 30 min or gradually elevated temperature from 30℃ to 80℃ at 0.5℃/min. The thermostability of photosynthetic apparatus was estimated by the changes in chlorophyll fluorescence parameters, photosynthetic rate and pigment content, chloroplast ultrastructure and tissue location of H2O2 accumulation. There were different patterns of Fo-temperature curves between the Chl b-less mutants and the wild type plant, and the temperature of F_o rising threshold was shifted 3℃ lower in the Chl b-less mutants (48℃) than in the wild type (51℃). At temperature up to about 45℃, chloroplasts were swollen and thylakoid grana became misty accompanied with the complete loss of photosynthetic oxygen evolution in the two Chl b-less mutants, but chloroplast ultrastruc-ture in the wild type showed no obvious alteration. After 55℃ exposure, the disordered thylakoid and significant H₂O₂ accumulation in leaves were found in the two Chl _b-less mutants, whereas in the wild type plant, less H₂O₂ was accumulated and the swollen thylakoid still maintained a cer-tain extent of stacking. A large extent of the changes in qP, NPQ and F_v/F_m was consistent with the Pn decreasing rate in the Chl b-less mutants during high temperature treatment as compared with the wild type. The results indicated that the Chl b-less mutants showed a tendency for higher thermosensitivity, and loss of Chl b in LHC II could lead to less thermostability of PSII structure and function. Heat damage to photosynthetic apparatus might be partially attributed to the in-ternal oxidative stress produced at severely high temperature.     

Growth and photosynthesis in seedlings of Hizikia fusiformis (Harvey) Okamura (Sargassaceae, Phaeophyta) cultured at two different temperatures

This study examined temperature acclimation, growth, and photosynthetic characteristics of the zygote-derived seedlings of Hizikia fusiformis (Harvey) Okamura (Sargassaceae). The seedlings were cultured at 15°C or 25°C for 4 weeks. The average relative growth rate was significantly higher in seedlings acclimated at 25°C. The photosynthetic rate measured at 15°C was much higher in seedlings grown at 15°C than those grown at 25°C, indicating photosynthetic acclimation to a lower temperature. At 35°C, the photosynthetic rate of 15°C-grown seedlings was drastically decreased, whereas that of 25°C-grown seedlings was significantly increased. The maximum relative electron transport rate (rETR_max) measured at the respective growth temperature was significantly higher in seedlings grown at 25°C than at 15°C. At a measuring temperature of 35°C, the rETR_max in both 15°C- and 25°C-grown seedlings were considerably reduced with regard to those measured at 15°C or 25°C. Our results suggested that, compared with the seedlings grown at 25°C, those acclimated at a lower temperature could be disadvantaged under adverse conditions such as increased temperatures.     

The effect of high temperatures on leaf cells of Valerianella: relative heat stability of the tonoplast membrane of mesophyll vacuoles

The effect of short-term heat stress on the tonoplast membrane of lamb's lettuce (Valerianella locusta (L.) Betcke) mesophyll vacuoles has been investigated. The maintainance of a proton concentration difference (δpH) across the tonoplast membrane served as a criterion for the integrity of the vacuoles. After heat treatment, δpH was measured at room temperature using the fluorescent amine, 9-aminoacridine. It was found with this method that thermal damage to isolated vacuoles mainly occurred in the temperature range above 50°C. Compared with this results, the photosynthetic functions of isolated lettuce protoplasts proved to be markedly more thermolabile, e.g. photosynthetic CO₂ fixation and light-induced chlorophyll fluorescence were drastically reduced at temperatures between 40° and 50°C. Heating of whole leaves and protoplasts and subsequent isolation of vacuoles showed that tonoplast-membrane integrity is not affected by heat stress in situ up to 45°C. Measurement of 9-aminoacridine fluorescence in protoplasts, which allowed conclusions to be drawn regarding the integrity of the tonoplast membrane in its natural cytoplasmic environment, revealed that heat treatment up to 55°C did not significantly affect vacuolar compartmentation. The data provide evidence that the tonoplast membrane is relatively heat stable compared with photosynthetic membranes.     

Effects of high temperature on photosynthesis, chlorophyll fluorescence, chloroplast ultrastructure, and antioxidant activities in fingered citron

Effects of heat stress on the photosynthesis system and antioxidant activities in Fingered citron (Citrus medica var. sarcodactylis Swingle) were investigated. Two-year-old Fingered citron plants were exposed to different temperature (28, 35, 40, and 45°C) for 6 h; then the photosynthetic capacity, chlorophyll fluorescence, chloroplast ultrastructure, and antioxidant activities in the leaves were evaluated. Exposure to 40 and 45°C for 6 h resulted in a significant decrease in the photosynthetic rate (P _n), carboxylation efficiency (CE), the maximal photochemical efficiency of photosystem II, and the light-saturated photosynthetic rate, which were related to the reduction of CO₂ assimilation, inactivation of photosystem II and photosynthetic electron transport. Moreover, transmission electron microscopy showed chloroplast ultrastructural alterations, including their swelling, matrix zone expanding, and lamella structure loosening. Furthermore, heat stress, especially at 45°C, caused oxidative damage resulted from ROS accumulation in Fingered citron leaves accompanied by increases in activities of superoxide dismutase, peroxidase, and catalase. However, exposure to 35°C for 6 h or 40°C for 4 h had no significant influence on the photosynthetic capacity at all. The results suggest that Fingered citron plants show no heat injury when temperature is below 40°C.     

Thermostability of photosynthesis in two new chlorophyllb-less rice mutants

Leaves of the two new chlorophyllb-less rice mutants VG28-1, VG30-5 and the wild type rice cv. Zhonghua 11 were subjected to temperatures 28, 36, 40, 44 and 48°C in the dark for 30 min or gradually elevated temperature from 30°C to 80°C at 0.5°C/min. The thermostability of photosynthetic apparatus was estimated by the changes in chlorophyll fluorescence parameters, photosynthetic rate and pigment content, chloroplast ultrastructure and tissue location of H₂O₂ accumulation. There were different patterns of F_o-temperature curves between the Chlb-less mutants and the wild type plant, and the temperature of F_o rising threshold was shifted 3°C lower in the Chlb-less mutants (48°C) than in the wild type (51°C). At temperature up to about 45°C, chloroplasts were swollen and thylakoid grana became misty accompanied with the complete loss of photosynthetic oxygen evolution in the two Chlb-less mutants, but chloroplast ultrastructure in the wild type showed no obvious alteration. After 55°C exposure, the disordered thylakoid and significant H₂O₂ accumulation in leaves were found in the two Chlb-less mutants, whereas in the wild type plant, less H₂O₂ was accumulated and the swollen thylakoid still maintained a certain extent of stacking. A large extent of the changes in qP, NPQ and F_v/F_m was consistent with the Pn decreasing rate in the Chlb-less mutants during high temperature treatment as compared with the wild type. The results indicated that the Chlb-less mutants showed a tendency for higher thermosensitivity, and loss of Chlb in LHC II could lead to less thermostability of PSII structure and function. Heat damage to photosynthetic apparatus might be partially attributed to the internal oxidative stress produced at severely high temperature.     

Heat stress responses and population genetics of the kelp Laminaria digitata (Phaeophyceae) across latitudes reveal differentiation among North Atlantic populations

To understand the thermal plasticity of a coastal foundation species across its latitudinal distribution, we assess physiological responses to high temperature stress in the kelp Laminaria digitata in combination with population genetic characteristics and relate heat resilience to genetic features and phylogeography. We hypothesize that populations from Arctic and cold‐temperate locations are less heat resilient than populations from warm distributional edges. Using meristems of natural L. digitata populations from six locations ranging between Kongsfjorden, Spitsbergen (79°N), and Quiberon, France (47°N), we performed a common‐garden heat stress experiment applying 15°C to 23°C over eight days. We assessed growth, photosynthetic quantum yield, carbon and nitrogen storage, and xanthophyll pigment contents as response traits. Population connectivity and genetic diversity were analyzed with microsatellite markers. Results from the heat stress experiment suggest that the upper temperature limit of L. digitata is nearly identical across its distribution range, but subtle differences in growth and stress responses were revealed for three populations from the species’ ecological range margins. Two populations at the species’ warm distribution limit showed higher temperature tolerance compared to other populations in growth at 19°C and recovery from 21°C (Quiberon, France), and photosynthetic quantum yield and xanthophyll pigment responses at 23°C (Helgoland, Germany). In L. digitata from the northernmost population (Spitsbergen, Norway), quantum yield indicated the highest heat sensitivity. Microsatellite genotyping revealed all sampled populations to be genetically distinct, with a strong hierarchical structure between southern and northern clades. Genetic diversity was lowest in the isolated population of the North Sea island of Helgoland and highest in Roscoff in the English Channel. All together, these results support the hypothesis of moderate local differentiation across L. digitata's European distribution, whereas effects are likely too weak to ameliorate the species’ capacity to withstand ocean warming and marine heatwaves at the southern range edge.     

Arabidopsis: Sensitivity of growth to high temperature

Arabidopsis thaliana seedlings as measured by an electrolyte leakage assay, have been found to be extremely sensitive to high temperature stress as compared to a high temperature tolerant variety (Tracy) of soybean. Over 50% ion leakage occurred in Arabidopsis leaves during a 15-minute exposure to 50°C, indicating a heat killing time of less than 15 minutes. In contrast, the heat killing time for soybean at 50°C was over five times longer. When soybean or Arabidopsis seedlings in culture plates were exposed to 37°C for 2 hours and then returned to 23°C, they suffered no apparent short-term or long-term damage. Soybean seedlings given a 42°C, treatment for 2 hours also showed no damage. Arabidopsis seedlings after a 42°C treatment for 2 hours showed no apparent immediate damage, but 48 hours after return to 23°C severe damage symptoms were visible and after 96 hours all the seedlings were dead. Both soybean and Arabidopsis seedlings synthesize heat shock proteins (hsps) when exposed to 42°C for 2 hours. The hsps synthesized are of similar molecular weights, although the relative abundances of the different size classes are very different in the two plants. Even though hsps are produced in Arabidopsis seedlings after a 2 hour exposure to 42°C their presence is not sufficient for the seedlings to recover from the effects of rhe heat shock when returned to 23°C. Our results show that Arabidopsis has a heat sensitive genotype. This along with its other characteristics should make it a good model system in which to assay in transgenic plants, the functions of homologous and heterologous genes that might be candidates for determining heat tolerance in plants.     

Long‐term elevated air [CO2] strengthens photosynthetic functioning and mitigates the impact of supra‐optimal temperatures in tropical Coffea arabica and C. canephora species

The tropical coffee crop has been predicted to be threatened by future climate changes and global warming. However, the real biological effects of such changes remain unknown. Therefore, this work aims to link the physiological and biochemical responses of photosynthesis to elevated air [CO₂] and temperature in cultivated genotypes of Coffea arabica L. (cv. Icatu and IPR108) and Coffea canephora cv. Conilon CL153. Plants were grown for ca. 10 months at 25/20 °C (day/night) and 380 or 700 μl CO₂ l^?1 and then subjected to temperature increase (0.5 °C day^?1) to 42/34 °C. Leaf impacts related to stomatal traits, gas exchanges, C isotope composition, fluorescence parameters, thylakoid electron transport and enzyme activities were assessed at 25/20, 31/25, 37/30 and 42/34 °C. The results showed that (1) both species were remarkably heat tolerant up to 37/30 °C, but at 42/34 °C a threshold for irreversible nonstomatal deleterious effects was reached. Impairments were greater in C. arabica (especially in Icatu) and under normal [CO₂]. Photosystems and thylakoid electron transport were shown to be quite heat tolerant, contrasting to the enzymes related to energy metabolism, including RuBisCO, which were the most sensitive components. (2) Significant stomatal trait modifications were promoted almost exclusively by temperature and were species dependent. Elevated [CO₂], (3) strongly mitigated the impact of temperature on both species, particularly at 42/34 °C, modifying the response to supra‐optimal temperatures, (4) promoted higher water‐use efficiency under moderately higher temperature (31/25 °C) and (5) did not provoke photosynthetic downregulation. Instead, enhancements in [CO₂] strengthened photosynthetic photochemical efficiency, energy use and biochemical functioning at all temperatures. Our novel findings demonstrate a relevant heat resilience of coffee species and that elevated [CO₂] remarkably mitigated the impact of heat on coffee physiology, therefore playing a key role in this crop sustainability under future climate change scenarios.     

Heat shock increases levels of reactive oxygen species,autophagy and apoptosis

Hyperthermia is a promising anticancer treatment used in combination with radiotherapy and chemotherapy. Temperatures above 41.5 °C are cytotoxic and hyperthermia treatments can target a localized area of the body that has been invaded by a tumor. However, non-lethal temperatures (39–41 °C) can increase cellular defenses, such as heat shock proteins. This adaptive survival response, thermotolerance, can protect cells against subsequent cytotoxic stress such as anticancer treatments and heat shock (>41.5 °C). Autophagy is another survival process that is activated by stress. This study aims to determine whether autophagy can be activated by heat shock at 42 °C, and if this response is mediated by reactive oxygen species (ROS). Autophagy was increased during shorter heating times (<60 min) at 42 °C in cells. Levels of acidic vesicular organelles (AVO) and autophagy proteins Beclin-1, LC3-II/LC-3I, Atg7 and Atg12-Atg5 were increased. Heat shock at 42 °C increased levels of ROS. Increased levels of LC3 and AVOs at 42 °C were inhibited by antioxidants. Therefore, increased autophagy during heat shock at 42 °C (<60 min) was mediated by ROS. Conversely, heat shock at 42 °C for longer times (1?3 h) caused apoptosis and activation of caspases in the mitochondrial, death receptor and endoplasmic reticulum (ER) pathways. Thermotolerant cells, which were developed at 40 °C, were resistant to activation of apoptosis at 42 °C. Autophagy inhibitors 3-methyladenine and bafilomycin sensitized cells to activation of apoptosis by heat shock (42 °C). Improved understanding of autophagy in cellular responses to heat shock could be useful for optimizing the efficacy of hyperthermia in the clinic.     

INTERACTIVE EFFECTS OF IRRADIANCE AND TEMPERATURE ON THE PHOTOSYNTHETIC PHYSIOLOGY OF THE PENNATE DIATOM PSEUDO‐NITZSCHIA GRANII (BACILLARIOPHYCEAE) FROM THE NORTHEAST SUBARCTIC PACIFIC1

This study examined how light and temperature interact to influence growth rates, chl a, and photosynthetic efficiency of the oceanic pennate diatom Pseudo‐nitzschia granii Hasle, isolated from the northeast subarctic Pacific. Growth rates were modulated by both light and temperature, although for each irradiance tested, the growth rate was always the greatest at ～14°C. Chl a per cell was affected primarily by temperature, except at the maximum chl a per cell (at 10°C) where the effects of light were noticeable. At both ends of the temperature gradient, cells displayed evidence of chlorosis even at low light intensities. Chl fluorescence data suggested that cells at 8°C were significantly more efficient in their photosynthetic processes than cells at 20°C, despite having comparable concentrations of chl. Cells at low temperature showed photosynthetic characteristics similar to high‐irradiance‐adapted cells. The decline of growth rates beyond the optimum growth temperature coincided with the cell's inability to accumulate chl in response to increasing temperature. The decline in photosynthetic ability at 20°C was likely due to a combination of high‐temperature stress on cellular membranes and a decline in chl. Our results highlight the important interactions between light and temperature and the need to incorporate these interactions into the development of phytoplankton models for the subarctic Pacific.     

High temperature tolerance and thermal-adaptability plasticity of Asian corn borer (Ostrinia furnacalis Guenée) after a single extreme heat wave at the egg stage

Due to ongoing climate change, short-term extreme heat waves in the summer are expected to be more frequent. Insect eggs are sensitive to thermal stress. This raises the question of whether herbivore insects' thermal adaptability would be changed after a single extreme heat wave at the egg stage. In this study, we examined the developmental performance of Ostrinia furnacalis Guenée at 25?°C, 27?°C, 29?°C or 31?°C after a single extreme heat wave (42?°C) for 0?h (control), 1?h, 2?h, or 3?h at the egg stage. The results showed that O. furnacalis at the egg or larval stage was more sensitive to a single heat wave than it was at the pupae or adult stage. After a single heat wave, O. furnacalis showed a reduced egg-hatching rate or reduced larval survival rate, but the optimum temperature for egg hatching and larval survival was higher than that in the control. The upper temperature threshold and optimum temperature for larval development in the control were higher than that after a single extreme heat wave. Both male and female pupal weight decreased with increasing temperature, and pupal weight decreased faster in females than in males. The Cox proportional hazard model showed that when O. furnacalis developed at 25?°C, the instantaneous death risk of adults with a 3?h heat wave at the egg stage was higher than that of the control, but when O. furnacalis developed at 29?°C and 31?°C, the instantaneous death risk of adults after a heat wave was significantly lower than that of the control. Our study highlights the effect of a single heat wave on O. furnacalis eggs and the subsequent development of survival individuals.     

How specialized volatiles respond to chronic and short‐term physiological and shock heat stress in Brassica nigra

Brassicales release volatile glucosinolate breakdown products upon tissue mechanical damage, but it is unclear how the release of glucosinolate volatiles responds to abiotic stresses such as heat stress. We used three different heat treatments, simulating different dynamic temperature conditions in the field to gain insight into stress‐dependent changes in volatile blends and photosynthetic characteristics in the annual herb Brassica nigra (L.) Koch. Heat stress was applied by either heating leaves through temperature response curve measurements from 20 to 40 °C (mild stress), exposing plants for 4 h to temperatures 25–44 °C (long‐term stress) or shock‐heating leaves to 45–50 °C. Photosynthetic reduction through temperature response curves was associated with decreased stomatal conductance, while the reduction due to long‐term stress and collapse of photosynthetic activity after heat shock stress were associated with non‐stomatal processes. Mild stress decreased constitutive monoterpene emissions, while long‐term stress and shock stress resulted in emissions of the lipoxygenase pathway and glucosinolate volatiles. Glucosinolate volatile release was more strongly elicited by long‐term stress and lipoxygenase product released by heat shock. These results demonstrate that glucosinolate volatiles constitute a major part of emission blend in heat‐stressed B. nigra plants, especially upon chronic stress that leads to induction responses.     

An analysis of photosynthetic response and adaptation to temperature in higher plants: temperature acclimation in the desert evergreen Nerium oleander L*

Abstract. Factors underlying the process of photosynthetic acclimation to temperature were investigated for the shrub Nerium oleander L. Ramets of a single clone were grown under day/night temperature regimes of 20°C/15°C or 45°C/32°C. Plants grown at the lower temperature regime possessed rates of photosynthesis twice that of the high-temperature grown plants when CO₂ fixation was measured at 20°C. In contrast, the plants grown at the high-temperature regime had twice the rate of CO₂ fixation of the 20°C/l 5°C-grown plants at a measurement temperature of 45° C. It was determined that the ability to acclimate to changes in temperature regime was present in fully mature leaves. A reciprocal transfer of plants between the two growth regimes resulted in the appearance of the CO₂ fixation characteristics appropriate to the new growth temperature after 12–14d. The response of CO₂ fixation to light, temperature, and CO₂ partial pressure and the temperature responses of soluble and membrane-bound photosynthetic enzyme systems were analysed to determine which components might be responsible for the superior photosynthetic performance of the 20°C/I5°C-grown plants at 20°C, and the enhanced high-temperature stability of the 45°C/32°C plants. The measured photosynthetic capacity of the 20°C/15°C plants could not be attributed to gross morphological, stomatal, or other physical changes, or to a general increase in the concentration of components of the photosynthetic process. Only a single enzyme, Fru-P₂ phosphatase, was affected to an extent similar to that of photosynthesis. The enhanced thermal stability of the 45°C/32°C plants may be attributed primarily to an enhanced stability of the chloroplast membrane-bound enzymatic activities and the stability of the photosynthetic carbon metabolism enzymes which require lighl for activation.     

When it is too hot for photosynthesis: heat-induced instability of photosynthesis in relation to respiratory burst, cell permeability changes and H₂O₂ formation

Photosynthesis rate (A_n) becomes unstable above a threshold temperature, and the recovery upon return to low temperature varies because of reasons not fully understood. We investigated responses of A_n, dark respiration and chlorophyll fluorescence to supraoptimal temperatures of varying duration and kinetics in Phaseolus vulgaris asking whether the instability of photosynthesis under severe heat stress is associated with cellular damage. Cellular damage was assessed by Evans blue penetration (enhanced membrane permeability) and by H₂O₂ generation [3,3′‐diaminobenzidine 4HCl (DAB)‐staining]. Critical temperature for dark fluorescence (F₀) rise (T_F) was at 46–48 °C, and a burst of respiration was observed near T_F. However, A_n was strongly inhibited already before T_F was reached. Membrane permeability increased with temperature according to a switch‐type response, with enhanced permeability observed above 48 °C. Experiments with varying heat pulse lengths and intensities underscored the threshold‐type loss of photosynthetic function, and indicated that the degree of photosynthetic deterioration and cellular damage depended on accumulated heat‐dose. Beyond the ‘point of no return’, propagation of cellular damage and reduction of photosynthesis continued upon transfer to lower temperatures and photosynthetic recovery was slow or absent. We conclude that instability of photosynthesis under severe heat stress is associated with time‐dependent propagation of cellular lesions.