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Modelling the response of plants to climate change is limited by our incomplete understanding of the component processes of photosynthesis and their temperature responses within and among species. For ≥20 individuals, each of six common subtropical tree species occurring across steep urban thermal gradients in Miami, Florida, USA, we determined rates of net photosynthesis (A net ), maximum RuBP carboxylation, maximum RuBP regeneration and stomatal conductance, and modelled the optimum temperature (T opt ) and process rate of each parameter to address two questions: (1) Do the T opt of A net (T optA ) and the maximum A net (A opt ) of subtropical trees reflect acclimation to elevated growth temperatures? And (2) What limits A net in subtropical trees? Against expectations, we did not find significant acclimation of T optA , A opt  or the T opt of any of the underlying photosynthetic parameters to growth temperature in any of the focal species. Model selection for the single best predictor of A net both across leaf temperatures and at T optA revealed that the A net of most trees was best predicted by stomatal conductance. Our findings are in accord with those of previous studies, especially in the tropics, that have identified stomatal conductance to be the most important factor limiting A net , rather than biochemical thermal responses., (© 2023 John Wiley & Sons Ltd.)

Tropical forests are experiencing unprecedented high-temperature conditions due to climate change that could limit their photosynthetic functions. We studied the high-temperature sensitivity of photosynthesis in a rainforest site in southern Amazonia, where some of the highest temperatures and most rapid warming in the Tropics have been recorded. The quantum yield (F v /F m ) of photosystem II was measured in seven dominant tree species using leaf discs exposed to varying levels of heat stress. T 50 was calculated as the temperature at which F v /F m was half the maximum value. T 5 is defined as the breakpoint temperature, at which F v /F m decline was initiated. Leaf thermotolerance in the rapidly warming southern Amazonia was the highest recorded for forest tree species globally. T 50 and T 5 varied between species, with one mid-storey species, Amaioua guianensis, exhibiting particularly high T 50 and T 5 values. While the T 50 values of the species sampled were several degrees above the maximum air temperatures experienced in southern Amazonia, the T 5 values of several species are now exceeded under present-day maximum air temperatures., (© 2020 The Authors. Plant, Cell & Environment published by John Wiley & Sons Ltd.)

Atmospheric and climate change will expose tropical forests to conditions they have not experienced in millions of years. To better understand the consequences of this change, we studied photosynthetic acclimation of the neotropical tree species Tabebuia rosea to combined 4°C warming and twice-ambient (800 ppm) CO 2 . We measured temperature responses of the maximum rates of ribulose 1,5-bisphosphate carboxylation (V CMax ), photosynthetic electron transport (J Max ), net photosynthesis (P Net ), and stomatal conductance (g s ), and fitted the data using a probabilistic Bayesian approach. To evaluate short-term acclimation plants were then switched between treatment and control conditions and re-measured after 1-2 weeks. Consistent with acclimation, the optimum temperatures (T Opt ) for V CMax , J Max and P Net were 1-5°C higher in treatment than in control plants, while photosynthetic capacity (V CMax , J Max , and P Net at T Opt ) was 8-25% lower. Likewise, moving control plants to treatment conditions moderately increased temperature optima and decreased photosynthetic capacity. Stomatal density and sensitivity to leaf-to-air vapour pressure deficit were not affected by growth conditions, and treatment plants did not exhibit stronger stomatal limitations. Collectively, these results illustrate the strong photosynthetic plasticity of this tropical tree species as even fully developed leaves of saplings transferred to extreme conditions partially acclimated., (© 2021 John Wiley & Sons Ltd.)

Exceeding thermal thresholds causes irreversible damage and ultimately loss of leaves. The lowland tropics are among the warmest forested biomes, but little is known about heat tolerance of tropical forest plants. We surveyed leaf heat tolerance of sun-exposed leaves from 147 tropical lowland and pre-montane forest species by determining the temperatures at which potential photosystem II efficiency based on chlorophyll a fluorescence started to decrease (T Crit ) and had decreased by 50% (T 50 ). T Crit averaged 46.7°C (5th-95th percentile: 43.5°C-49.7°C) and T 50 averaged 49.9°C (47.8°C-52.5°C). Heat tolerance partially adjusted to site temperature; T Crit and T 50 decreased with elevation by 0.40°C and 0.26°C per 100 m, respectively, while mean annual temperature decreased by 0.63°C per 100 m. The phylogenetic signal in heat tolerance was weak, suggesting that heat tolerance is more strongly controlled by environment than by evolutionary legacies. T Crit increased with the estimated thermal time constant of the leaves, indicating that species with thermally buffered leaves maintain higher heat tolerance. Among lowland species, T 50 increased with leaf mass per area, suggesting that in species with structurally more costly leaves the risk of leaf loss during hot spells is reduced. These results provide insight in variation in heat tolerance at local and regional scales., (© 2021 John Wiley & Sons Ltd.)

Experimental research shows that isoprene emission by plants can improve photosynthetic performance at high temperatures. But whether species that emit isoprene have higher thermal limits than non-emitting species remains largely untested. Tropical plants are adapted to narrow temperature ranges and global warming could result in significant ecosystem restructuring due to small variations in species' thermal tolerances. We compared photosynthetic temperature responses of 26 co-occurring tropical tree and liana species to test whether isoprene-emitting species are more tolerant to high temperatures. We classified species as isoprene emitters versus non-emitters based on published datasets. Maximum temperatures for net photosynthesis were ~1.8°C higher for isoprene-emitting species than for non-emitters, and thermal response curves were 24% wider; differences in optimum temperatures (T opt ) or photosynthetic rates at T opt were not significant. Modelling the carbon cost of isoprene emission, we show that even strong emission rates cause little reduction in the net carbon assimilation advantage over non-emitters at supraoptimal temperatures. Isoprene emissions may alleviate biochemical limitations, which together with stomatal conductance, co-limit photosynthesis above T opt . Our findings provide evidence that isoprene emission may be an adaptation to warmer thermal niches, and that emitting species may fare better under global warming than co-occurring non-emitting species., (© 2019 John Wiley & Sons Ltd.)

Net photosynthetic carbon uptake of Panamanian lowland tropical forest species is typically optimal at 30-32 °C. The processes responsible for the decrease in photosynthesis at higher temperatures are not fully understood for tropical trees. We determined temperature responses of maximum rates of RuBP-carboxylation (V CMax ) and RuBP-regeneration (J Max ), stomatal conductance (G s ), and respiration in the light (R Light ) in situ for 4 lowland tropical tree species in Panama. G s had the lowest temperature optimum (T Opt ), similar to that of net photosynthesis, and photosynthesis became increasingly limited by stomatal conductance as temperature increased. J Max peaked at 34-37 °C and V CMax ~2 °C above that, except in the late-successional species Calophyllum longifolium, in which both peaked at ~33 °C. R Light significantly increased with increasing temperature, but simulations with a photosynthesis model indicated that this had only a small effect on net photosynthesis. We found no evidence for Rubisco-activase limitation of photosynthesis. T Opt of V CMax and J Max fell within the observed in situ leaf temperature range, but our study nonetheless suggests that net photosynthesis of tropical trees is more strongly influenced by the indirect effects of high temperature-for example, through elevated vapour pressure deficit and resulting decreases in stomatal conductance-than by direct temperature effects on photosynthetic biochemistry and respiration., (© 2017 John Wiley & Sons Ltd.)