Photosynthesis in cold acclimated leaves of plants with various degrees of freezing tolerance

The objective of this study was to compare the photosynthetic changes during cold acclimation in various plant types able to acquire different degrees of freezing tolerance. Four herbaceous and six woody plants were hardened under natural or artificial conditions and – after determination of their frost resistance (LT₅₀) – the net photosynthetic rate at an ambient CO₂ of 33 Pa (P_n33), the dependencies of P_n to light and to CO₂ and the room temperature chlorophyll a fluorescence were recorded under optimal conditions. Herbaceous plants acquired freezing tolerances to temperatures between ?10 and ?15°C when hardened at temperatures around 0°C. Most leaves fully developed prior to frost hardening exhibited typical symptoms of senescence after frost hardening. In non-senescing leaves P_n33 was reduced by 15 to 50% mainly due to a reduced stomatal conductance. After hardening at temperatures around ?10°C Brassica survived down to ?24°C, but P_n33 was almost abolished as a result of disturbances in the chloroplasts. After transferring the plants to 20/15°C P_n33 recovered completely within a few days. Woody plants hardened at temperatures around 0°C tolerated – 15 to ?36°C: P_n33 was reduced by 25 to 60% and hardly recovered at 20/15°C. Hardening at ?10°C induced a tolerance of ?32 to n33 was almost totally blocked, but at 20/15°C it returned to the values of the plants hardened at 0°C within a few days. In woody plants disturbances were invariably localized in the chloroplasts. Thus, conifers, and especially Pinus cembra, can survive much lower temperatures than herbaceous plants and, at the same level of freezing tolerance, exhibit appreciably less restriction in relative P_n33.     

The tolerance of black currant flowers to induced frosts

Flower buds on potted plants of 17 varieties of black currant were frosted to -3.3, -4.5 and -5.2 °C between the grape stage and full flower in 1979 and 1980. In all varieties more flower buds died after the -5.2 °C frosts and at full flower, and less after the -3.3 °C frosts and at the grape stage. Varieties related to Ben More and Ojebyn tolerated the -4.5 °C frosts until after first flower while Baldwin and Magnus became susceptible at the grape stage. Seabrooks Black, Greens Black and Ben Lomond and its relatives were intermediate. In both years flower buds tolerated frosts to similar growth stages but in 1980 the varieties flowered about 2 wk earlier than in 1979 and suffered more frost damage at full flower. The frosted plants had slightly larger fruits than the unfrosted ones in 1979. The immature fruit drop was similar in frosted and unfrosted plants in both years except when it was increased after -5.2 °C in 1980. It is pointed out that for reliable cropping, varieties should flower late as well as tolerate spring frosts and that tests of frost tolerance should be done for at least three growth stages.     

Frost in a future climate: modelling interactive effects of warmer temperatures and rising atmospheric [CO2] on the incidence and severity of frost damage in a temperate evergreen (Eucalyptus pauciflora)

Although plants are more susceptible to frost damage under elevated atmospheric [CO₂], the importance of frost damage under future, warmer climate scenarios is unknown. Accordingly, we used a model to examine the incidence and severity of frost damage to snow gum (Eucalyptus pauciflora) in a sub‐alpine region of Australia for current and future conditions using the A2 IPCC elevated CO₂ and climate change scenario. An existing model for predicting frost effects on E. pauciflora seedlings was adapted to include effects of elevated [CO₂] on acclimation to freezing temperatures, calibrated with field data, and applied to a study region in Victoria using climate scenario data from CSIRO's Global Climate Model C‐CAM for current (1975–2004) and future (2035–2064) 30 years climate sequences. Temperatures below 0 °C were predicted to occur less frequently while the coldest temperatures (i.e. those below ?8 °C) were almost as common in the future as in the current climate. Both elevated [CO₂] and climate warming affected the timing and rates of acclimation and de‐acclimation of snow gum to freezing temperatures, potentially reducing the length of time that plants are fully frost tolerant and increasing the length of the growing season. Despite fewer days when temperatures fall below 0 °C in the future, with consequently fewer damaging frosts with lower average levels of impact, individual weather sequences resulting in widespread plant mortality may still occur. Furthermore, delayed acclimation due to either warming or rising [CO₂] combined with an early severe frost could lead to more frost damage and higher mortality than would occur in current conditions. Effects of elevated [CO₂] on frost damage were greater in autumn, while warming had more effect in spring. Thus, frost damage will continue to be a management issue for plantation and forest management in regions where frosts persist.     

How endangered is sexual reproduction of high-mountain plants by summer frosts? Frost resistance, frequency of frost events and risk assessment

In temperate-zone mountains, summer frosts usually occur during unpredictable cold spells with snow-falls. Earlier studies have shown that vegetative aboveground organs of most high-mountain plants tolerate extracellular ice in the active state. However, little is known about the impact of frost on reproductive development and reproductive success. In common plant species from the European Alps (Cerastium uniflorum, Loiseleuria procumbens, Ranunculus glacialis, Rhododendron ferrugineum, Saxifraga bryoides, S. moschata, S. caesia), differing in growth form, altitudinal distribution and phenology, frost resistance of reproductive and vegetative shoots was assessed in different reproductive stages. Intact plants were exposed to simulated night frosts between ?2 and ?14 °C in temperature-controlled freezers. Nucleation temperatures, freezing damage and subsequent reproductive success (fruit and seed set, seed germination) were determined. During all reproductive stages, reproductive shoots were significantly less frost resistant than vegetative shoots (mean difference for LT₅₀ ?4.2 ± 2.7 K). In most species, reproductive shoots were ice tolerant before bolting and during fruiting (mean LT₅₀ ?7 and ?5.7 °C), but were ice sensitive during bolting and anthesis (mean LT₅₀ around ?4 °C). Only R. glacialis remained ice tolerant during all reproductive stages. Frost injury in reproductive shoots usually led to full fruit loss. Reproductive success of frost-treated but undamaged shoots did not differ significantly from control values. Assessing the frost damage risk on the basis of summer frost frequency and frost resistance shows that, in the alpine zone, low-statured species are rarely endangered as long as they are protected by snow. The situation is different in the subnival and nival zone, where frost-sensitive reproductive shoots may become frost damaged even when covered by snow. Unprotected individuals are at high risk of suffering from frost damage, particularly at higher elevations. It appears that ice tolerance in reproductive structures is an advantage but not an absolute precondition for colonizing high altitudes with frequent frost events.     

The after-effects of temperature and irradiance during early growth of winter oilseed rape (Brassica napus L. var. oleifera, cv. Górczański) seedlings on the progress of their cold acclimation

Experiments performed under controlled conditions showed that level of PPFD (photosynthetic photon flux density) during early seedlings growth (preceding cold acclimation at +2 °C) was not the key factor for the development of frost resistance. It did not modify the beneficial effects of prehardening (Rapacz 1997, in this issue) at moderately low (+12 °C) day temperature. Now I have shown that the increase of PPFD may replace to some extent prehardening in the development of frost resistance. It was particularly seen in non-prehardened plants, which had been grown under warm-day (+20 °C) conditions. Prehardening performed under controlled conditions, as well as seedlings growth under natural autumn conditions in the field, allowed to maintain a high net-photosynthesis rate at chilling temperatures. A net-photosynthesis rate during cold acclimation at +2 °C corresponded well with higher frost resistance. As a result, seedlings non subjected to prehardening and grown before cold acclimation under low PPFD acclimated better, if the cold treatment was applied only at nights (+20/2 °C day/night). Only under such conditions the photosynthetic rate was sufficiently high to allow plants to reach a higher level of frost resistance. All other plants acclimated better when they were exposed to the hardening temperature continuously during days and nights (+2/2 °C day/night).     

Freezing tolerance of winter wheat plants depends on adaptation of photosynthesis and respiration in different time intervals

This study is devoted to CO₂ gas exchange (true photosynthesis at light saturation (P), dark respiration (R), and P/R ratio) in vegetating and cold-hardened winter wheat (Triticum aestivum L.) plants (cultivar Mironovskaya 808) in relation to their freezing tolerance. Under natural cultivation conditions, freezing tolerance of plants depended on adaptive changes in the shape of P and R curves in the temperature range from 20 to ?2°C. These changes, induced by cold hardening and treatment of plants with the photosynthesis inhibitor diuron, were observed within month and week ranges. Under laboratory conditions, the P/R ratio in vegetating plants increased three times within an hour range as the temperature decreased from 22 to 0°C. The P/R ratio also decreased within a minute range as a result of partial inhibition of photosynthesis with diuron and immediately decreased when CO₂ concentration in the air was reduced from 419 to 0 μl/l. The P/R ratio decreased primarily at the expense of a decrease in P. The decrease in P/R was more pronounced at low temperatures, indicating variability of low-temperature tolerance of photosynthesis within a minute range. The possibility of plant adaptation to nonsimultaneous temperature changes under natural conditions via adaptive changes in temperature tolerance of the photosynthetic apparatus is discussed.     

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.     

No increase in alpine snowbed productivity in response to experimental lengthening of the growing season

Climate change effects on snow cover and thermic regime in alpine tundra might lead to a longer growing season, but could also increase risks to plants from spring frost events. Alpine snowbeds, i.e. alpine tundra from late snowmelt sites, might be particularly susceptible to such climatic changes. Snowbed communities were grown in large monoliths for two consecutive years, under different manipulated snow cover treatments, to test for effects of early (E) and late (L) snowmelt on dominant species growth, plant functional traits, leaf area index (LAI) and aboveground productivity. Spring snow cover was reduced to assess the sensitivity of snowbed alpine species to severe early frost events, and dominant species freezing temperatures were measured. Aboveground biomass, productivity, LAI and dominant species growth did not increase significantly in E compared to L treatments, indicating inability to respond to an extended growing season. Edapho‐climatic conditions could not account for these results, suggesting that developmental constraints are important in controlling snowbed plant growth. Impaired productivity was only detected when harsher and more frequent frost events were experimentally induced by early snowmelt. These conditions exposed plants to spring frosts, reaching temperatures consistent with the estimated freezing points of the dominant species (～?10 °C). We conclude that weak plasticity in phenological response and potential detrimental effects of early frosts explain why alpine tundra from snowbeds is not expected to benefit from increased growing season length.     

The role of the photosynthetic apparatus in cold acclimation of <Emphasis Type="Italic">Lolium multiflorum</Emphasis>. Characteristics of novel genotypes low-sensitive to PSII over-reduction

During cold acclimation by higher plants, temperature perception via changes in redox state of Photosystem II (PSII) and subsequent acclimation of the photosynthetic apparatus to cold is very important for achieving freezing tolerance. These properties were studied in two groups (A and B) of the same backcross 3 (BC₃) progeny derived from a triploid hybrid of Festuca pratensis (2×) × Lolium multiflorum (4×) backcrossed three times onto diploid L. multiflorum cultivars. Leaves of Group A plants formed at 20°C at medium-low light were unable to acclimate their photosynthetic apparatus to cold. Compared to Group B, the Group A plants were also more frost sensitive. This acclimation ability correlated with the freezing tolerance of the plants. However, leaves of the same Group A plants developed at 20°C, but under higher-light conditions had increased ability to acclimate their photosynthetic apparatus to cold. It was concluded that Group A plants may have impaired PSII temperature perception, and this then resulted in their poor capability to cold acclimate.     

Photosynthetic temperature response of the Antarctic vascular plants Colobanthus quitensis and Deschampsia antarctica

The photosynthetic temperature response of the Antarctic vascular plants Colobanthus quitensis and Deschampsia antarctica was examined by measuring whole-canopy CO₂ gas exchange and chlorophyll (Chl) a fluorescence of plants growing near Palmer Station along the Antarctic Peninsula. Both species had negligible midday net photosynthetic rates (P_n) on warm, usually sunny, days (canopy air temperature [T_c]> 20°C), but had relatively high P_n on cool days (T_c<10°C). Laboratory measurements of light and temperature responses of P_n showed that high temperature, not visible irradiance, was responsible for depressions in P_n on warm sunny days. The optimal leaf temperatures (T_l) for P_n in C. quitensis and D. antarctica were 14 and 10°C, respectively. Both species had substantial positive P_n at 0°C T_l, which were 28 (C. quitensis) and 32% (D. antarctica) of their maximal P_n, and we estimate that their low-temperature compensation points occurred at ?2°C T_l (C. quitensis) and ?3°C (D. antarctica). Because of the strong warming trend along the peninsula over recent decades and predictions that this will continue, we were particularly interested in the mechanisms responsible for their negligible rates of P_n on warm days and their unusually low high-temperature compensation points (i.e., 26°C in C. quitensis and 22°C in D. antarctica). Low P_n at supraoptimal temperature (25°C) appeared to be largely due to high rates of temperature-enhanced respiration. However, there was also evidence for direct impairment of the photosynthetic apparatus at supraoptimal temperature, based on Chl fluorescence and P_n/intercellular CO₂ concentration (c_i) response curve analyses. The breakpoint or critical temperature (T_cr) of minimal fluorescence (F_o) was ≈42°C in both species, which was well above the temperatures where reductions in P_n were evident, indicating that thylakoid membranes were structurally intact at supraoptimal temperatures for P_n. The optimal T_l for photochemical quenching (q_p) and the quantum yield of photosystem II (PSII) electron transfer (φ_PSII) were 9 and 7°C in C. quitensis and D. antarctica, respectively. Supraoptimal temperatures resulted in lower q_p and greater non-photochemical quenching (q_NP), but had little effect on F_o, maximal fluorescence (F_m) or the ratio of variable to maximal fluorescence (F_v/F_m) in both species. In addition, carboxylation efficiencies or initial slopes of their P_n/c_i response were lower at supraoptimal temperatures, suggesting reduced activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Although continued warming along the peninsula will increase the frequency of supraoptimal temperatures, T_c at our field site averaged 4.3°C and was below the temperature optima for P_n in these species for the majority of diurnal periods (86%) during the growing season, suggesting that continued warming will usually improve their rates of P_n.     

The effects of long-term elevation of air temperature and CO on the frost hardiness of Scots pine

The frost hardiness of 20 to 25-year-old Scots pine (Pinus sylvestris L.) saplings was followed for 2 years in an experiment that attempted to simulate the predicted climatic conditions of the future, i.e. increased atmospheric CO₂ concentration and/or elevated air temperature. Frost hardiness was determined by an electrolyte leakage method and visual damage scoring on needles. Elevated temperatures caused needles to harden later and deharden earlier than the controls. In the first year, elevated CO₂ enhanced hardening at elevated temperatures, but this effect disappeared the next year. Dehardening was hastened by elevating CO₂ in both springs. The frost hardiness was high (相似文献   

An investigation of the frost hardiness of grapevine (Vitis vinifera) during bud break

The aim of this investigation was to assess ice nucleation and frost resistance of two varieties of grapevine (Siegrebbe and Madeleine Angevine) during bud burst under radiative freezing conditions analogous to those during Spring in the UK. During bud burst, grapevines were almost entirely resistant to freezing during frosts of less than -3°C by virtue of their ability to supercool. The risk of frost damage increased significantly as bud development progressed, and once buds had passed growth stage DS3 they became more sensitive to freezing and freezing damage was more extensive. The two varieties did not differ in frost resistance but, because of its earlier developing habit, variety Siegrebbe could be more prone to frost damage in the field. Buds were more prone to damage after freezing once bud burst had commenced and the damage could not be reversed by acclimating plants for periods of 7 to 21 days at 4°C in an 8 h photoperiod. Such acclimation appeared to predispose frozen buds to more extensive damage.     

Effects of low nocturnal temperature on photosynthetic characteristics and chloroplast ultrastructure of winter rapeseed

We investigated the effects of low nocturnal temperature on photosynthetic apparatus of winter rapeseed (Brassica campestris L.). An artificial climate chamber was used to simulate the effects of low nocturnal temperature on seedling and stomatal morphology, chloroplast ultrastructure, photosynthetic parameters, and dry matter distribution and accumulation in two winter rapeseed cultivars, Longyou-7 (ultra coldresistant) and Tianyou-2 (weak cold resistance). Compared with those at diurnal/nocturnal temperatures of 20°/10°C (control), rapeseed seedlings at 20°/5°C had increased leaf chlorophyll content, deepened green leaf color, decreased stomatal conductance (G_s), intercellular CO₂ concentration (C_i), and photosynthetic rate (P_n), and improved root/shoot ratio; the majority of stomata remained open in Longyou-7 while those in Tianyou-2 were mostly closed or semi-closed. At diurnal/nocturnal temperatures of 20°/–5°C, rapeseed seedlings had decreased leaf chlorophyll content with increased C_i but decreased G_s and P_n; Tianyou-2 exhibited ruptured chloroplast membrane, dissolved grana, broken stroma lamella, and decreased root/shoot ratio, whereas Longyou-7 had chloroplasts retaining partial structure of grana with a small amount of starch granules in guard cells. Low nocturnal temperature damaged the photosynthetic membrane of chloroplasts and reduced P_n in the leaves of winter rapeseed influencing photosynthetic processes in this crop. The reduction of P_n was mainly related to stomatal limitation at diurnal/nocturnal temperatures of 20°/5°C and non-stomatal limitation at diurnal/nocturnal temperatures of 20°/–5°C.     

Deep supercooling enabled by surface impregnation with lipophilic substances explains the survival of overwintering buds at extreme freezing

The frost survival mechanism of vegetative buds of angiosperms was suggested to be extracellular freezing causing dehydration, elevated osmotic potential to prevent freezing. However, extreme dehydration would be needed to avoid freezing at the temperatures down to ?45°C encountered by many trees. Buds of Alnus alnobetula, in common with other frost hardy angiosperms, excrete a lipophilic substance, whose functional role remains unclear. Freezing of buds was studied by infrared thermography, psychrometry, and cryomicroscopy. Buds of A. alnobetula did not survive by extracellular ice tolerance but by deep supercooling, down to ?45°C. An internal ice barrier prevented ice penetration from the frozen stem into the bud. Cryomicroscopy revealed a new freezing mechanism. Until now, supercooled buds lost water towards ice masses that form in the subtending stem and/or bud scales. In A. alnobetula, ice forms harmlessly inside the bud between the supercooled leaves. This would immediately trigger intracellular freezing and kill the supercooled bud in other species. In A. alnobetula, lipophilic substances (triterpenoids and flavonoid aglycones) impregnate the surface of bud leaves. These prevent extrinsic ice nucleation so allowing supercooling. This suggests a means to protect forestry and agricultural crops from extrinsic ice nucleation allowing transient supercooling during night frosts.     

A chamber for the simulation of radiation freezing of plants

Frost injury to plants can occur following episodic radiation frosts. In the UK this is particularly important to spring sown crops such as potatoes. Most laboratory based frost studies simulate freezing using either conductive or convective freezing chambers. Such frost tests do not simulate overnight freezing events adequately. A freezing chamber based on radiative cooling is described which mimics overnight radiative freezing. The chamber is rectangular in design (1 m × lm × 2 m high) with a radiative cooling plate at the top of the chamber cooled to -40°C to -45°C using HFC coolants, which acts as a cold black body. The sides of the chamber are also cooled to variable temperatures down to -5°C in order to prevent the chamber walls radiating to the plant material during testing. Using thermocouples to measure air temperature and plant temperature the chamber has been characterised to simulate the radiative cooling conditions found in the UK during autumn and spring. Exotherm detection upon plant freezing is simplified by virtue of the reduction in temperature fluctuation normally experienced at the plant surface during natural freezing. Radiation frosts and subsequent frost damage to potatoes have been recorded in the temperature range -4°C to –5°C. The equipment is recommended for studies of frost damage to plants normally caused by episodic radiation frost events.     

Enhancement of frost tolerance in olive shoots in vitro by cold acclimation and sucrose increase in the culture medium

The evaluation of frost tolerance in olive shoots in vitro has been successfully accomplished. The behavior of in vitro shoots at freezing temperatures was comparable to that of intact plants. Cold acclimation was found to increase frost tolerance in cv. Moraiolo and the LT₅₀ was about 4 °C lower compared to nonacclimated shoots. Damage in acclimated shoots occurred at –15 °C, whereas control shoots were damaged at –10 °C. Olive shoots were unable to withstand freezing temperatures of –20 °C, even when acclimated. The effects of sucrose were also determined. 6% (w/v) sucrose in the medium conferred the highest frost tolerance in both acclimated and nonacclimated plants.     

Critically reduced frost resistance of Picea abies during sprouting could be linked to cytological changes

Frost resistance of sprouting Picea abies shoots is insufficient for survival of naturally occurring late frosts. The cellular changes during sprouting appeared to be responsible for frost damage as frost events that damaged sprouting shoots did not damage older needles and stems. Whilst resting buds showed initial frost damage at ?15.0°C, 20 days later, current year’s growth was damaged at ?5.6°C. The decrease in frost resistance in sprouting shoots of P. abies was accompanied by a significant reduction of the cellular solute concentration, indicated by much less negative Ψ_oSAT values (increase from ?2.8 to ?1.2 MPa). ψ_oSAT decreased again after the final cell volume was reached and cell wall thickening began. After bud break, ice nucleation temperature increased from ?4.7°C to ?1.5°C. This increase was probably caused by the loss of bud scales, the onset of expansion growth of the central cylinder and the development of vascular tissue permitting the spread of ice from the stem into the growing needles. The onset of mesophyll cell wall thickening coincided with the lowest frost resistances. Cell wall thickening caused an increase in the modulus of elasticity, ε, indicating a decrease in tissue elasticity and after that frost resistance increased again. Metabolic and cytological changes that evidently leave little leeway for frost hardening are responsible for the low frost resistance in current year’s growth of P. abies. This low frost resistance will be significant in the future as the risk of frost damage due to earlier bud break is anticipated to even further increase.     

Overwintering temperatures affect freezing temperatures of turions of aquatic plants

The effect of different overwintering temperatures (2.5 ± 1 °C in a refrigerator or outdoor natural overwintering on wet topsoil with weak frosts) on the freezing temperature and survival rate of turions of 10 aquatic plant species with different ecological traits (free-floating habit or bottom rooting) was studied using mini thermocouples. Dormant, non-hardened turions of 9 species exhibited freezing within a narrow temperature range of ?7.0 to ?10.2 °C, while Hydrocharis morsus-ranae froze at ?3.6 °C. The survival rate of the turions after the measurements was, however, very low (0–38%). In several species, the freezing temperature of turions at the beginning of germination was not significantly different (at p < 0.05) from the dormant ones. The mean freezing temperature of outdoor hardened turions of 6 species was within a very narrow range of ?2.8 to ?3.3 °C and was thus significantly higher by 4–7 °C (p < 0.0002) than that for the non-hardened turions. It is assumed that the freezing temperatures indicate freezing of the extracellular water. The hardened turions of all 7 species were able to survive mild winter frosts under the topsoil conditions at a rate of 76–100%. These characteristics suggest that the turions of aquatic species can be hardened by weak frosts and that their frost hardiness is based on the shift from frost avoidance in non-hardened turions to frost tolerance.     

Growth at moderately elevated temperature alters the physiological response of the photosynthetic apparatus to heat stress in pea (Pisum sativum L.) leaves

The impact of heat stress on the functioning of the photosynthetic apparatus was examined in pea (Pisum sativum L.) plants grown at control (25 °C; 25 °C-plants) or moderately elevated temperature (35 °C; 35 °C-plants). In both types of plants net photosynthesis (P_n) decreased with increasing leaf temperature (LT) and was more than 80% reduced at 45 °C as compared to 25 °C. In the 25 °C-plants, LTs higher than 40 °C could result in a complete suppression of P_n. Short-term acclimation to heat stress did not alter the temperature response of P_n. Chlorophyll a fluorescence measurements revealed that photosynthetic electron transport (PET) started to decrease when LT increased above 35 °C and that growth at 35 °C improved the thermal stability of the thylakoid membranes. In the 25 °C-plants, but not in the 35 °C-plants, the maximum quantum yield of the photosystem II primary photochemistry, as judged by measuring the F_v/F_m ratio, decreased significantly at LTs higher than 38 °C. A post-illumination heat-induced reduction of the plastoquinone pool was observed in the 25 °C-plants, but not in the 35 °C-plants. Inhibition of P_n by heat stress correlated with a reduction of the activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). Western-blot analysis of Rubisco activase showed that heat stress resulted in a redistribution of activase polypeptides from the soluble to the insoluble fraction of extracts. Heat-dependent inhibition of P_n and PET could be reduced by increasing the intercellular CO₂ concentration, but much more effectively so in the 35 °C-plants than in the 25 °C-plants. The 35 °C-plants recovered more efficiently from heat-dependent inhibition of P_n than the 25 °C-plants. The results show that growth at moderately high temperature hardly diminished inhibition of P_n by heat stress that originated from a reversible heat-dependent reduction of the Rubisco activation state. However, by improving the thermal stability of the thylakoid membranes it allowed the photosynthetic apparatus to preserve its functional potential at high LTs, thus minimizing the after-effects of heat stress.