Photosynthetic Thermotolerance Research Articles

Variation in thermotolerance of photosystem II energy trapping, intersystem electron transport, and photosystem I electron acceptor reduction for diverse cotton genotypes

Cotton breeding programs have focused on agronomically-desirable traits. Without targeted selection for tolerance to high temperature extremes, cotton will likely be more vulnerable to environment-induced yield loss. Recently-developed methods that couple chlorophyll fluorescence induction measurements with temperature response experiments could be used to identify genotypic variation in photosynthetic thermotolerance of specific photosynthetic processes for field-grown plants. It was hypothesized that diverse cotton genotypes would differ significantly in photosynthetic thermotolerance, specific thylakoid processes would exhibit differential sensitivities to high temperature, and that the most heat tolerant process would exhibit substantial genotypic variation in thermotolerance plasticity. A two-year field experiment was conducted at Tifton and Athens, Georgia, USA. Experiments included 10 genotypes in 2020 and 11 in 2021. Photosynthetic thermotolerance for field-collected leaf samples was assessed by determining the high temperature threshold resulting in a 15% decline in photosynthetic efficiency (T15) for energy trapping by photosystem II (ΦPo), intersystem electron transport (ΦEo), and photosystem I end electron acceptor reduction (ΦRo). Significant genotypic variation in photosynthetic thermotolerance was observed, but the response was dependent on location and photosynthetic parameter assessed. ΦEo was substantially more heat sensitive than ΦPo or ΦRo. Significant genotypic variation in thermotolerance plasticity of ΦEo was also observed. Identifying the weakest link in photosynthetic tolerance to high temperature will facilitate future selection efforts by focusing on the most heat-susceptible processes. Given the genotypic differences in environmental plasticity observed here, future research should evaluate genotypic variation in acclimation potential in controlled environments.

Chloroplast Inner Envelope Protein FtsH11 is Involved in the Adjustment of Assembly of Chloroplast ATP Synthase under Heat Stress.

The maintenance of a proton gradient across the thylakoid membrane is an integral aspect of photosynthesis that is mainly established by the splitting of water molecules in photosystem II and plastoquinol oxidation at the cytochrome complex, and it is necessary for the generation of ATP in the last step of photophosphorylation. Although environmental stresses, such as high temperatures, are known to disrupt this fundamental process, only a few studies have explored the molecular mechanisms underlying proton gradient regulation during stress. The present study identified a heat-sensitive mutant that displays aberrant photosynthesis at high temperatures. This mutation was mapped to AtFtsH11, which encodes an ATP-dependent AAA-family metalloprotease. We showed that AtFtsH11 localizes to the chloroplast inner envelope membrane and is capable of degrading the ATP synthase assembly factor BFA3 under heat stress. We posit that this function limits the amount of ATP synthase integrated into the thylakoid membrane to regulate proton efflux from the lumen to the stroma. Our data also suggest that AtFtsH11 is critical in stabilizing photosystem II and cytochrome complexes at high temperatures, and additional studies can further elucidate the specific molecular functions of this critical regulator of photosynthetic thermotolerance. This article is protected by copyright. All rights reserved.

Short-term warming does not affect intrinsic thermotolerance but induces strong sustaining photoprotection in tropical evergreen citrus genotypes.

Consequences of warming and postwarming events on photosynthetic thermotolerance (PT ) and photoprotective responses in tropical evergreen species remain elusive. We chose Citrus to answer some of the emerging questions related to tropical evergreen species' PT behaviour including (i) how wide is the genotypic variation in PT ? (ii) how does PT respond to short-term warming and (iii) how do photosynthesis and photoprotective functions respond over short-term warming and postwarming events? A study on 21 genotypes revealed significant genotypic differences in PT , though these were not large. We selected five genotypes with divergent PT and simulated warming events: Tmax 26/20°C (day-time highest maximum/night-time lowest maximum) (Week 1) < Tmax 33/30°C (Week 2) < Tmax 36/32°C (Week 3) followed by Tmax 26/16°C (Week 4, recovery). The PT of all genotypes remained unaltered despite strong leaf megathermy (leaf temperature > air temperature) during warming events. Though moderate warming showed genotype-specific stimulation in photosynthesis, higher warming unequivocally led to severe loss in net photosynthesis and induced higher nonphotochemical quenching. Even after a week of postwarming, photoprotective mechanisms strongly persisted. Our study points towards a conservative PT in evergreen citrus genotypes and their need for sustaining higher photoprotection during warming as well as postwarming recovery conditions.

Differential Leaf Age-Dependent Thermal Plasticity in the Keystone Seagrass Posidonia oceanica.

Introduction: Gene-expression patterns and their upstream regulatory mechanisms (e.g. epigenetic) are known to modulate plant acclimatability and thus tolerance to heat stress. Within species, thermal plasticity (i.e. temperature-sensitive phenotypic plasticity) and differential thermo-tolerance are recognized among different genotypes, development stages, organs or tissues. Leaf age and lifespan have been demonstrated to strongly affect photosynthetic thermo-tolerance in terrestrial species, whereas there is no information available for marine plants. Materials and Methods: Here, we investigated how an intense warming event affects molecular and photo-physiological functions in the large-sized seagrass Posidonia oceanica, at fine spatial resolution. Plants were exposed for one week at 34°C in a controlled-mesocosm system. Subsequent variations in the expression of 12 target genes and global DNA methylation level were evaluated in three leaf-age sections (i.e. basal, medium and high) established along the longitudinal axis of youngest, young and fully mature leaves of the shoot. Targeted genes were involved in photosynthesis, chlorophyll biosynthesis, energy dissipation mechanisms, stress response and programmed cell death. Molecular analyses paralleled the assessment of pigment content and photosynthetic performance of the same leaf segments, as well as of plant growth inhibition under acute warming. Results: Our data revealed, for the first time, the presence of variable leaf age-dependent stress-induced epigenetic and gene-expression changes in seagrasses, underlying photo-physiological and growth responses to heat stress. An investment in protective responses and growth arrest was observed in immature tissues; while mature leaf sections displayed a higher ability to offset gene down-regulation, possibly through the involvement of DNA methylation changes, although heat-induced damages were visible at photo-physiological level. Discussion: Overall, mature and young leaf tissues exhibited different strategies to withstand heat stress and thus a variable thermal plasticity. This should be taken in consideration when addressing seagrass response to warming and other stressors, especially in large-sized species, where sharp age differences are present within and among leaves, and other gradients of environmental factors (e.g. light) could be at play. Molecular and physiological evaluations conducted only on adult leaf tissues, as common practice in seagrass research, could give inadequate estimates of the overall plant state, and should not be considered as a proxy for the whole shoot.

Effects of high temperature and high light on photosystem II activity in leaves of two Bauhinia species

It is, so far, unclear whether lianas and trees differ in photosynthetic thermotolerance and response to high temperature and high light. Here, photosynthetic thermotolerance was compared in the leaves of Bauhinia brachycarpa (tree) and Bauhinia comosa (liana). Moreover, the combined treatments of four temperatures (25, 35, 40, and 45°C) and two light intensities [500 and 2,000 μmol(photon) m-2 s-1] were used to investigate the effects of high temperature and high light on PSII activity. The results showed that B. brachycarpa leaves exhibited higher photosynthetic thermotolerance, leaf mass per area, and Ca2+ concentration than those of B. comosa. As the temperature and light intensity increased, maximum quantum yield of PSII decreased significantly, particularly with greater declines in B. comosa than that in B. brachycarpa. Our results suggested that PSII was more sensitive to high temperature and high light in liana species B. comosa than in the congener tree species B. brachycarpa.

Sub-optimal emergence temperature alters thermotolerance of thylakoid component processes in cotton seedlings

Cotton (Gossypium hirsutum) is often planted under sub-optimal early season temperatures, yet it is unknown whether this increases susceptibility to acute high temperature exposure. To address this, cotton seedlings were grown under optimal (30/20 °C) and sub-optimal (20/15 °C) growth temperature regimes, and comprehensive fluorescence analysis was conducted for cotyledons from both temperature regimes in response to incubation temperatures ranging from 30 to 50 °C. Results indicated that low growth temperature did not alter reaction center density (RC/CS0), but increased antenna size (ratio of antenna chlorophyll to reaction center chlorophyll; ABS/RC) and decreased the amplitude of the I to P phase of the fluorescence transient [ΔVIP; associated with the photosystem (PS) I content]. Photosynthetic performance indices (PItotal and PIABS), quantum yield of energy trapping (φPo), the contribution of the thylakoid reactions to PIABS (Fv/F0), electron transport to intersystem electron acceptors (φE0) and reduction of PSI end electron acceptors (φRo) were all substantially lower in plants grown under sub-optimal temperature than in plants grown at optimal temperature. Furthermore, thermotolerance of all the aforementioned parameters was significantly reduced by low early season growth temperatures. Among different OJIP-derived parameters, PIABS and PItotal were more heat sensitive than other fluorescence-based parameters, and φPo and φRo were the most heat tolerant. In addition, thermotolerance of PItotal (overall photosynthetic thermotolerance) was significantly correlated with thermotolerance of all other OJIP-derived parameters. Regression analysis indicated that heat tolerance trends for φE0 were more consistent with the changes in thermotolerance of PItotal. We conclude that 1) low growth temperature in the seedling phase of cotton results in decreased photosynthetic thermotolerance and 2) overall photosynthetic thermotolerance is primarily constrained by the ability of intersystem electron transport to acclimate to growth environment.

An attempt to interpret a biochemical mechanism of C4 photosynthetic thermo-tolerance under sudden heat shock on detached leaf in elevated CO2 grown maize.

Detached leaves at top canopy structures always experience higher solar irradiance and leaf temperature under natural conditions. The ability of tolerance to high temperature represents thermotolerance potential of whole-plants, but was less of concern. In this study, we used a heat-tolerant (B76) and a heat-susceptible (B106) maize inbred line to assess the possible mitigation of sudden heat shock (SHS) effects on photosynthesis (PN) and C4 assimilation pathway by elevated [CO2]. Two maize lines were grown in field-based open top chambers (OTCs) at ambient and elevated (+180 ppm) [CO2]. Top-expanded leaves for 30 days after emergence were suddenly exposed to a 45°C SHS for 2 hours in midday during measurements. Analysis on thermostability of cellular membrane showed there was 20% greater electrolyte leakage in response to the SHS in B106 compared to B76, in agreement with prior studies. Elevated [CO2] protected PN from SHS in B76 but not B106. The responses of PN to SHS among the two lines and grown CO2 treatments were closely correlated with measured decreases of NADP-ME enzyme activity and also to its reduced transcript abundance. The SHS treatments induced starch depletion, the accumulation of hexoses and also disrupted the TCA cycle as well as the C4 assimilation pathway in the both lines. Elevated [CO2] reversed SHS effects on citrate and related TCA cycle metabolites in B106 but the effects of elevated [CO2] were small in B76. These findings suggested that heat stress tolerance is a complex trait, and it is difficult to identify biochemical, physiological or molecular markers that accurately and consistently predict heat stress tolerance.

Temperature modulation of thermal tolerance of a CAM-tank bromeliad and the relationship with acid accumulation in different leaf regions.

Physiological changes that increase plant performance during exposure to high temperatures may play an inverse role during exposure to low temperatures. The objective of this study was to test variations in photosystem II response to heat and cold stress in the leaves of a bromeliad with crassulacean acid metabolism submitted to high or low temperatures. Leaves were maintained under constant temperatures of 10 and 35°C and used to examine possible relationships among physiological responses to high and low temperatures and organic acid accumulation. We also tested if distinct parts of bromeliad leaves show differences in photosynthetic thermotolerance. The samples from leaves maintained at 35°C showed greater heat tolerance values, while those from leaves maintained at 10°C showed lower cold tolerance values. Our results identified a strong negative relationship between the organic acid accumulation and thermal tolerance of bromeliad leaves that largely explained the differences in thermal tolerance among groups. One of these differences occurred among regions of a single leaf, with the base showing critical heat values of up to 8°C higher than the top region, suggesting a possible partitioning of leaf response among its regions. Differences in thermal tolerance were also observed between sampling times, with higher values observed in the morning.

Elevated [CO2] and growth temperature have a small positive effect on photosynthetic thermotolerance of Pinus taeda seedlings

Key message Growth temperature had little effect on the response of net photosynthesis to high temperatures (up to 47 °C). On the other hand, elevated [CO 2 ] increased net photosynthesis at high temperatures.

Acclimation of photosynthetic tolerance to acute heat stress at elevated CO2 and N

Determining interactive effects of pre-heat-stress, CO2 and N on photosynthetic thermotolerance is necessary for predicting plant responses to global change. We grew Hordeum vulgare (barley, C3) and Zea mays (corn, C4) at current or elevated CO2 (370 and 700ppm) and limiting or optimal soil N (0.5 and 7.5mM). We assessed basal and inducible thermotolerance of net photosynthesis (Pn), photosystem II efficiency (F′v/F′m), photochemical quenching (qp), carboxylation efficiency (CE), and rubisco activase content. Inducible thermotolerance was measured on plants which were pre-heat-stressed (PHS) for 4h before heat stress. We also assayed content of several major heat-shock proteins (HSPs), as HSPs are primary adaptations to heat stress and affected by N. Acclimation of photosynthetic thermotolerance was dependent on species, CO2 and N treatment and the component in the photosynthetic processes. PHS had a positive effect on the production of HSP60 and sHSP in low-N barley and corn. These results indicate that stimulatory effects of elevated CO2 at normal temperatures on photosynthesis may be partly changed by the different interactive effects of CO2, heat stress and N for species with different photosynthetic pathways. Thus, PHS, CO2 and N effects on photosynthetic thermotolerance may contribute to changes in plant productivity, distribution, and diversity.

Effects of CO2 on the tolerance of photosynthesis to heat stress can be affected by photosynthetic pathway and nitrogen

Determining effects of elevated CO2 and N on photosynthetic thermotolerance is critical for predicting plant responses to global warming. We grew Hordeum vulgare (barley, C3) and Zea mays (corn, C4) at current or elevated CO2 (370, 700 ppm) and limiting or optimal soil N (0.5, 7.5 mmol/L). We assessed thermotolerance of net photosynthesis (Pn), photosystem II efficiency in the light (Fv'/Fm'), photochemical quenching (qp), carboxylation efficiency (CE), and content of rubisco activase and major heat-shock proteins (HSPs). For barley, elevated CO2 had no effect on Pn, qp, and CE at both high and low N and only a positive effect on Fv'/Fm' at high N. However, for corn, Pn, Fv'/Fm', qp, and CE were decreased substantially by elevated CO2 under high and low N, with greater decreases at high N for all but qp. The negative effects of high CO2 during heat stress on photosynthesis were correlated with rubisco activase and HSPs content, which decreased with heat stress, especially for low-N corn. These results indicate that stimulatory effects of elevated CO2 at normal temperatures on photosynthesis and growth (only found for high-N barley) may be partly offset by neutral or negative effects during heat stress, especially for C4 species. Thus, CO2 and N effects on photosynthetic thermotolerance may contribute to changes in plant productivity, distribution, and diversity in future.

Photosynthetic thermotolerance of woody savanna species in China is correlated with leaf life span.

Photosynthetic thermotolerance (PT) is important for plant survival in tropical and sub-tropical savannas. However, little is known about thermotolerance of tropical and sub-tropical wild plants and its association with leaf phenology and persistence. Longer-lived leaves of savanna plants may experience a higher risk of heat stress. Foliar Ca is related to cell integrity of leaves under stresses. In this study it is hypothesized that (1) species with leaf flushing in the hot-dry season have greater PT than those with leaf flushing in the rainy season; and (2) PT correlates positively with leaf life span, leaf mass per unit area (LMA) and foliar Ca concentration ([Ca]) across woody savanna species. The temperature-dependent increase in minimum fluorescence was measured to assess PT, together with leaf dynamics, LMA and [Ca] for a total of 24 woody species differing in leaf flushing time in a valley-type savanna in south-west China. The PT of the woody savanna species with leaf flushing in the hot-dry season was greater than that of those with leaf flushing in the rainy season. Thermotolerance was positively associated with leaf life span and [Ca] for all species irrespective of the time of flushing. The associations of PT with leaf life span and [Ca] were evolutionarily correlated. Thermotolerance was, however, independent of LMA. Chinese savanna woody species are adapted to hot-dry habitats. However, the current maximum leaf temperature during extreme heat stress (44·3 °C) is close to the critical temperature of photosystem II (45·2 °C); future global warming may increase the risk of heat damage to the photosynthetic apparatus of Chinese savanna species.

Rubisco activity is associated with photosynthetic thermotolerance in a wild rice (Oryza meridionalis)

Oryza meridionalis is a wild species of rice, endemic to tropical Australia. It shares a significant genome homology with the common domesticated rice Oryza sativa. Exploiting the fact that the two species are highly related but O. meridionalis has superior heat tolerance, experiments were undertaken to identify the impact of temperature on key events in photosynthesis. At an ambient CO(2) partial pressure of 38 Pa and irradiance of 1500 µmol quanta m(-2) s(-1), the temperature optimum of photosynthesis was 33.7 ± 0.8°C for O. meridionalis, significantly higher than the 30.6 ± 0.7°C temperature optimum of O. sativa. To understand the basis for this difference, we measured gas exchange and rubisco activation state between 20 and 42°C and modeled the response to determine the rate-limiting steps of photosynthesis. The temperature response of light respiration (R(light)) and the CO(2) compensation point in the absence of respiration (Γ(*)) were determined and found to be similar for the two species. C3 photosynthesis modeling showed that despite the difference in susceptibility to high temperature, both species had a similar temperature-dependent limitation to photosynthesis. Both rice species were limited by ribulose-1,5-bisphosphate (RuBP) regeneration at temperatures of 25 and 30°C but became RuBP carboxylation limited at 35 and 40°C. The activation state of rubisco in O. meridionalis was more stable at higher temperatures, explaining its greater heat tolerance compared with O. sativa.

Physiological ecology of desert biocrust moss following 10 years exposure to elevated CO2: evidence for enhanced photosynthetic thermotolerance

In arid regions, biomes particularly responsive to climate change, mosses play an important biogeochemical role as key components of biocrusts. Using the biocrust moss Syntrichia caninervis collected from the Nevada Desert Free Air CO₂ Enrichment Facility, we examined the physiological effects of 10 years of exposure to elevated CO₂, and the effect of high temperature events on the photosynthetic performance of moss grown in CO₂-enriched air. Moss exposed to elevated CO₂ exhibited a 46% decrease in chlorophyll, a 20% increase in carbon and no difference in either nitrogen content or photosynthetic performance. However, when subjected to high temperatures (35-40°C), mosses from the elevated CO₂ environment showed higher photosynthetic performance and photosystem II (PSII) efficiency compared to those grown in ambient conditions, potentially reflective of a shift in nitrogen allocation to components that offer a higher resistance of PSII to heat stress. This result suggests that mosses may respond to climate change in markedly different ways than vascular plants, and observed CO₂-induced photosynthetic thermotolerance in S. caninervis will likely have consequences for future desert biogeochemistry.

Mechanisms of Reproductive Thermotolerance in Gossypium hirsutum: The Effect of Genotype and Exogenous Calcium Application

Although photosynthetic thermotolerance has been investigated extensively in cotton leaves, reports on the biochemical influence of the pistil in promoting fertilization thermostability are limited. To evaluate the effect of temperature, genotype, and exogenous calcium application on fertilization and pistil biochemistry in cotton, thermosensitive (cv. ST4554 B2RF) and thermotolerant (cv. VH260) plants were grown under control (30/20 °C) or high-temperature (38/20 °C) conditions during flowering, and exogenous CaCl 2 was applied to flowers 1 day prior to anthesis. Measured pistil parameters included fertilization efficiency; protein concentration; glutathione reductase, superoxide dismutase (SOD) and NADPH oxidase activities; and ATP and calcium levels. Exogenous calcium had no effect on reproductive thermostability due to poor calcium uptake under high temperature. High temperature resulted in a 19.2 % decline in fertilization efficiency for ST4554 but no change in fertilization efficiency for VH260 relative to optimal temperature conditions. Pre-stress glutathione reductase and SOD activities were higher in VH260 than ST4554 pistils, and calcium and ATP content were higher in VH260 than ST4554 pistils. It was concluded that pre-stress antioxidant enzyme activity, ATP and calcium content of the pistil may be associated with reproductive thermotolerance in cotton. These findings should be confirmed in future experiments with a larger number of cultivars.

Immature leaves of Weinmannia racemosa are more heat tolerant than mature leaves based on differences in chlorophyll a fluorescence and solute leakage

Numerous studies have examined how leaf temperature affects photosynthesis, but few have investigated photosynthetic temperature sensitivity during leaf expansion. Here, we show that immature leaves (~18% of final leaf area) of the common New Zealand tree, Weinmannia racemosa L. f. (Cunoniaceae), are less sensitive than mature leaves to rapid increases in leaf temperature (from 25 to 35 °C) as measured by chlorophyll a fluorescence. Relative differences in the tolerance of photosystem II to heat were supported by solute leakage experiments, in which disks from immature leaves incubated at 47 °C lost significantly fewer solutes than did disks from mature leaves. In addition, levels of trienoic (18:3) fatty acids were far lower in immature than in mature leaves (low levels are known to be associated with photosynthetic thermotolerance). The developmental changes in photosynthetic tolerance to heat stress likely correspond to compositional differences of the thylakoid membranes during leaf expansion. This result may be useful in understanding photosynthetic responses to temperature in young leaves which lack functioning stomates, or under extreme conditions when leaf temperatures are high or fluctuate rapidly.

COMPARATIVE STUDY ON THE PHOTOSYNTHETIC PROPERTIES OF PRASIOLA (CHLOROPHYCEAE) AND NOSTOC (CYANOPHYCEAE) FROM ANTARCTIC AND NON‐ANTARCTIC SITES1

The terrestrial cyanobacterium Nostoc commune Vaucher ex Bornet et Flahault occurs worldwide, including in Japan and on the Antarctic continent. The terrestrial green alga Prasiola crispa (Lightf.) Kütz. is also distributed in Antarctica. These two species need to acclimate to the severe Antarctic climate including low ambient temperature and desiccation under strong light conditions. To clarify this acclimation process, the physiological characteristics of the photosynthetic systems of these two Antarctic terrestrial organisms were assessed. The relative rate of photosynthetic electron flow in N. commune collected in Japan and in Antarctica reached maxima at 900 and 1,100 μmol photons · m−2 · s−1, respectively. The difference seemed to reflect the presence of high amounts of UV‐absorbing substances within the Antarctic cyanobacterium. On the other hand, the optimal temperatures for photosynthesis at the two locations were 30°C–35°C and 20°C–25°C, respectively. This finding suggested a decreased photosynthetic thermotolerance in the Antarctic strain. P. crispa exhibited desiccation tolerance and dehydration‐induced quenching of PSII fluorescence. Re‐reduction of the photooxidized PSI reaction center, P700, was also inhibited at fully dry states. Photosynthetic electron flow in P. crispa reached a maximum at 20°C–25°C and at a light intensity of 700 μmol photons ḃ m−2 ḃ s−1. Interestingly, the osmolarity of P. crispa cells suggested that photosynthesis is performed using water absorbed in a liquid form rather than water absorbed from the air. Overall, these data suggest that these two species have acclimated to optimally photosynthesize under conditions of the highest light intensity and the highest temperature for their habitat in Antarctica.

Genotypic differences in thermotolerance are dependent upon prestress capacity for antioxidant protection of the photosynthetic apparatus inGossypium hirsutum

Numerous studies have illustrated the need for antioxidant enzymes in acquired photosynthetic thermotolerance, but information on their possible role in promoting innate thermotolerance is lacking. We investigated the hypothesis that genotypic differences in source leaf photosynthetic thermostability would be dependent upon prestress capacity for antioxidant protection of the photosynthetic apparatus in Gossypium hirsutum. To test this hypothesis, thermosensitive (cv. ST4554) and reportedly thermotolerant (cv. VH260) G. hirsutum plants were exposed to control (30/20 degrees C) or high-day temperature (38/20 degrees C) conditions during flowering and source leaf gas exchange, chlorophyll content and maximum photochemical efficiency (F(v)/F(m)) were measured for each treatment. The relationship between source leaf thermostability and prestress antioxidant capacity was quantified by monitoring the actual quantum yield response of photosystem II (PSII) (Phi(PSII)) to a range of temperatures for both cultivars grown under the control temperature regime and measuring antioxidant enzyme activity for those same leaves. VH260 was more thermotolerant than ST4554 as evidenced by photosynthesis and F(v)/F(m) being significantly lower under high temperature for ST4554 but not VH260. Under identical growth conditions, VH260 had significantly higher optimal and threshold temperatures for Phi(PSII) and glutathione reductase (GR; EC 1.8.1.7) activity than ST4554, and innate threshold temperature was dependent upon endogenous GR and superoxide dismutase (SOD; EC 1.15.1.1) activity. We conclude that maintaining a sufficient antioxidant enzyme pool prior to heat stress is an innate mechanism for coping with rapid leaf temperature increases that commonly occur under field conditions.

Variation in Heat‐shock Proteins and Photosynthetic Thermotolerance among Natural Populations of Chenopodium album L. from Contrasting Thermal Environments: Implications for Plant Responses to Global Warming

Production of heat-shock proteins (Hsps) is a key adaptation to acute heat stress and will be important in determining plant responses to climate change. Further, intraspecifc variation in Hsps, which will influence species-level response to global warming, has rarely been examined in naturally occurring plants. To understand intraspecific variation in plant Hsps and its relevance to global warming, we examined Hsp content and thermotolerance in five naturally occurring populations of Chenopodium album L. from contrasting thermal environments grown at low and high temperatures. As expected, Hsp accumulation varied between populations, but this was related more to habitat variability than to mean temperature. Unexpectedly, Hsp accumulation decreased with increasing variability of habitat temperatures. Hsp accumulation also decreased with increased experimental growth temperatures. Physiological thermotolerance was partitioned into basal and induced components. As with Hsps, induced thermotolerance decreased with increasing temperature variability. Thus, populations native to the more stressful habitats, or grown at higher temperatures, had lower Hsp levels and induced thermotolerance, suggesting a greater reliance on basal mechanisms for thermotolerance. These results suggest that future global climate change will differentially impact ecotypes within species, possibly by selecting for increased basal versus inducible thermotolerance.

Interactive effects of elevated CO2 and growth temperature on the tolerance of photosynthesis to acute heat stress in C3 and C4 species.

Determining effects of elevated CO2 on the tolerance of photosynthesis to acute heat-stress (heat wave) is necessary for predicting plant responses to global warming, as photosynthesis is thermolabile and acute heat-stress and atmospheric CO2 will increase in the future. Few studies have examined this, and past results are variable, which may be due to methodological variation. To address this, we grew two C3 and two C4 species at current or elevated CO2 and three different growth temperatures (GT). We assessed photosynthetic thermotolerance in both unacclimated (basal tolerance) and pre-heat-stressed (preHS = acclimated) plants. In C3 species, basal thermotolerance of net photosynthesis (P(n)) was increased in high CO2, but in C4 species, P(n) thermotlerance was decreased by high CO2 (except Zea mays at low GT); CO2 effects in preHS plants were mostly small or absent, though high CO2 was detrimental in one C3 and one C4 species at warmer GT. Though high CO2 generally decreased stomatal conductance, decreases in P(n) during heat stress were mostly due to non-stomatal effects. Photosystem II (PSII) efficiency was often decreased by high CO2 during heat stress, especially at high GT; CO2 effects on post-PSII electron transport were variable. Thus, high CO2 often affected photosynthetic theromotolerance, and the effects varied with photosynthetic pathway, growth temperature, and acclimation state. Most importantly, in heat-stressed plants at normal or warmer growth temperatures, high CO2 may often decrease, or not benefit as expected, tolerance of photosynthesis to acute heat stress. Therefore, interactive effects of elevated CO2 and warmer growth temperatures on acute heat tolerance may contribute to future changes in plant productivity, distribution, and diversity.