Temperature acclimation of net photosynthesis and its underlying component processes in four tropical tree species

Acclimation to Warming Shapes Gas Exchange and Metabolic Responses to Heat Shock in Pinus massoniana Seedlings

The sensitivity of physiological and metabolic processes in subtropical trees to temperature remains uncertain, limiting our ability to predict how subtropical forests will acclimate to future climates. In particular, our understanding of gas exchange and metabolic activity responses to warming and heat shocks is quite limited. Here, we exposed Pinus massoniana seedlings to three daytime growth temperatures (25°C, 3°C, and 35°C) for 65 days, followed by a heat shock up to 40°C, then immediately reduced to 25°C, to investigate physiological and metabolic responses. The optimal temperature of photosynthesis (ToptA) did not exhibit a significant shift with warming. Metabolism acclimated to rising growth temperature, resulting in enriched levels of key metabolites (tryptophan, indole, indoleacetate, and o-Phospho-L-serine) and key pathways (tryptophan metabolism). At 25°C, leaf dark respiration (Rd) decreased in warm-grown seedlings. At 40°C (heat shock period), warming reduced Rd, accumulated flavonoid metabolites, and upregulated tryptophan metabolism. After recovery to 25°C, higher growth temperatures decreased the net photosynthetic rate (Asat), accumulated prenol lipid metabolites, and led to enrichment in tryptophan metabolism, flavone, and flavonol biosynthesis pathways. Our findings suggest that photosynthesis in P. massoniana seedlings exhibits limited thermal acclimation, while respiration and metabolism can acclimate under short-term warming. However, acclimation to warming altered both physiological and metabolic responses to heat shock and during the subsequent recovery phase in seedlings.

Effect of daily thinning on biweekly increment of growth and yield of maize (Zea mays L.) in mountainous agroecosystem

Maize (Zea mays L.) growth and yield are severely affected due to intraspecific competition and agroclimatic conditions when cultivated with high plant densities. Field trials comprising four daily thinning patterns (0, 0.5, 1.0, &1.5% till silking) carried out in three consecutive years (2019-2021) using RCBD experimental design consisting of three replicates. Growth variables, dry matter allocation and growth rates in maize were examined during five biweekly periods starting from the emergence (15, 30, 45, 60 and 75 days after emergence, DAE) till silking whereas yield parameters i.e., biological yield, economic yield, and sustainability yield index (SYI) were recorded. Biweekly increase in growth variables, dry matter partitioning and growth rates of maize differed significantly due to the influence of daily thinning computed for the periods from 31 to 45, 46 to 60 and 61 to 75 DAE but the same parameters didn't differ significantly during the first two biweekly periods (1-15 & 16-30 DAE). Increase in growth variables, dry matter distribution, absolute growth rate (AGR), yield and SYI was the greatest where maize was established with 1% daily thinning. This increase in growth and dry matter partition observed highly associated to economic yield and biological yield. Current research highlighted that 3, 4th, and 5th biweekly periods are the most critical stages and daily thinning of 1% is suggested for higher and sustainable economic and biological returns from high density broadcasted maize.

Nitrogen Supply Mitigates Temperature Stress Effects on Rice Photosynthetic Nitrogen Use Efficiency and Water Relations

Climate-change-induced temperature fluctuations pose significant threats to global rice production, particularly through their impact on photosynthetic efficiency. The differential mechanisms by which low and high temperatures affect leaf photosynthetic processes in rice remain poorly understood. Here, we investigate the effects of temperature stress (15 °C, 30 °C, 45 °C) on rice photosynthetic performance across a gradient of nitrogen supply levels: low nitrogen (LN), medium nitrogen (MN), and high nitrogen (HN). The low temperature exhibited stronger negative impacts on photosynthesis than the high temperature, primarily through increased mesophyll limitation and disrupted cellular CO2 diffusion, while the high temperature showed less pronounced effects, particularly under HN and MN conditions. While photosynthetic nitrogen use efficiency (PNUE) decreased with increasing nitrogen under the optimal temperature, moderate nitrogen supply maintained optimal PNUE under temperature stress, suggesting that a balanced nitrogen level is crucial for maximizing both photosynthetic capacity and nitrogen use efficiency. Plants with adequate nitrogen maintained higher intrinsic water use efficiency (iWUE) under both temperature extremes through improved coordination between CO2 uptake and water loss. Our findings reveal distinct mechanisms underlying low- and high-temperature stress effects on photosynthesis and highlight the importance of optimizing nitrogen management for enhancing crop resilience to temperature extremes under climate change.

Changes in the Responses of Leaf Gas Exchange to Temperature and Photosynthesis Model Parameters in Four C3 Species in the Field

Substantial variation in the temperature dependence of parameters of the Farquhar, von Caemmerer, and Berry C3 photosynthesis model, as well as those of in vitro Rubisco kinetic characteristics, have been observed in controlled conditions but have seldom been systematically examined in the field. In this work, A vs. Ci curves were measured over a 15 or 20 °C range of temperature in four C3 species growing outdoors on two occasions about three weeks apart early in the growing season and also once near mid-season when air temperatures were more stable. The two early season occasions were chosen for having contrasting temperatures for 3 to 4 days preceding the measurements. Low temperatures (mean maximum/minimum temperatures of 19/11 °C) resulted in higher values of the VCmax of Rubisco and Jmax at a given measurement temperature in most species compared with higher temperatures (max/min 31/25 °C). The apparent activation energy of VCmax of Rubisco ranged from 56 to 82 kJ mol-1, and that of electron transport (Jmax) ranged from 28 to 56 kJ mol-1 across species and temperatures. In three of the four species, the activation energy of VCmax decreased and that of Jmax increased after the cooler temperatures. Stomatal conductance measured at 20 and 25 °C increased strongly with the prior warm temperatures in all species. Measurements made near mid-season, after a period of relatively stable temperatures (mean maximum/minimum temperatures of 27/18 °C), also indicated a wide range of values of the activation energies of VCmax and Jmax among these species.

The thermal optimum of photosynthetic parameters is regulated by leaf nutrients in neotropical savannas

Global warming significantly threatens species in the Cerrado, the world's largest savannah. Therefore, understanding how plants respond to temperature change, particularly in relation to leaf-level photosynthetic capacity, is crucial to understanding the future of Cerrado vegetation. Here, we determined the optimum temperature of the maximum rate of RuBP-carboxylation and maximum electron transport rate (TOptV and TOptJ, respectively) of 12 tree species in two opposite borders (northeastern and southeastern) of the Cerrado with distinct temperature regimes. We focused on four widespread species found in both sites, four restricted to the northeast, and four to the southeast. We compared TOptV and TOptJ between regions and between widespread species (co-occurring in both sites) and species restricted to each ecoregion. Additionally, we also explored the relationship between TOptV and TOptJ with leaf nitrogen (N), phosphorus (P) and potassium (K). As a result, we found that TOptV and TOptJ values were similar across species, regardless of the study region or species distribution range. The similarity of TOpt values among species suggests that photosynthetic performance is optimized to current temperatures. Additionally, we also observed that the TOptV and TOptJ were similar to the local maximum ambient temperatures. Therefore, if these species do not have enough plasticity, the increasing temperature predicted for this region may reduce their photosynthetic performance. Finally, the studied species exhibited general relationships between the TOptV and TOptJ and foliar key nutrients, particularly with P, suggesting the nutrient availability has an important role in the thermal acclimation of leaves. These findings offer valuable insights into physiological and ecological mechanisms in photosynthesis performance present in the Cerrado species.

On the importance of vapor pressure deficit for the determination of the photosynthetic temperature optimum in tropical trees

This article is a Commentary on Slot et al. (2024), 244: 1238–1249.

Canopy temperatures strongly overestimate leaf thermal safety margins of tropical trees

Current estimates of temperature effects on plants mostly rely on air temperature, although it can significantly deviate from leaf temperature (Tleaf). To address this, some studies have used canopy temperature (Tcan). However, Tcan fails to capture the fine-scale variation in Tleaf among leaves and species in diverse canopies. We used infrared radiometers to study Tleaf and Tcan and how they deviate from air temperature (ΔTleaf and ΔTcan) in multispecies tropical tree plantations at three sites along an elevation and temperature gradient in Rwanda. Our results showed high Tleaf (up to c. 50°C) and ΔTleaf (on average 8-10°C and up to c. 20°C) of sun-exposed leaves during 10:00 h-15:00 h, being close to or exceeding photosynthetic heat tolerance thresholds. These values greatly exceeded simultaneously measured values of Tcan and ΔTcan, respectively, leading to strongly overestimated leaf thermal safety margins if basing those on Tcan data. Stomatal conductance and leaf size affected Tleaf and Tcan in line with their expected influences on leaf energy balance. Our findings highlight the importance of leaf traits for leaf thermoregulation and show that monitoring Tcan is not enough to capture the peak temperatures and heat stress experienced by individual leaves of different species in tropical forest canopies.

Plasticity and implications of water-use traits in contrasting tropical tree species under climate change

Plants face a trade-off between hydraulic safety and growth, leading to a range of water-use strategies in different species. However, little is known about such strategies in tropical trees and whether different water-use traits can acclimate to warming. We studied five water-use traits in 20 tropical tree species grown at three different altitudes in Rwanda (RwandaTREE): stomatal conductance (gs), leaf minimum conductance (gmin), plant hydraulic conductance (Kplant), leaf osmotic potential (ψo) and net defoliation during drought. We also explored the links between these traits and growth and mortality data. Late successional (LS) species had low Kplant, gs and gmin and, thus, low water loss, while low ψo helped improve leaf water status during drought. Early successional (ES) species, on the contrary, used more water during both moist and dry conditions and exhibited pronounced drought defoliation. The ES strategy was associated with lower mortality and more pronounced growth enhancement at the warmer sites compared to LS species. While Kplant and gmin showed downward acclimation in warmer climates, ψo did not acclimate and gs measured at prevailing temperature did not change. Due to distinctly different water use strategies between successional groups, ES species may be better equipped for a warmer climate as long as defoliation can bridge drought periods.

Variable thermal plasticity of leaf functional traits in Andean tropical montane forests

Tropical montane forests (TMFs) are biodiversity hotspots and provide vital ecosystem services, but they are disproportionately vulnerable to climate warming. In the Andes, cold-affiliated species from high elevations are being displaced at the hot end of their thermal distributions by warm-affiliated species migrating upwards from lower elevations, leading to compositional shifts. Leaf functional traits are strong indicators of plant performance and at the community level have been shown to vary along elevation gradients, reflecting plant adaptations to different environmental niches. However, the plastic response of such traits to relatively rapid temperature change in Andean TMF species remains unknown. We used three common garden plantations within a thermosequence in the Colombian Andes to investigate the warming and cooling responses of key leaf functional traits in eight cold- and warm-affiliated species with variable thermal niches. Cold-affiliated species shifted their foliar nutrient concentrations when exposed to warming, while all other traits did not significantly change; contrastingly, warm-affiliated species were able to adjust structural, nutrient and water-use efficiency traits from acquisitive to conservative strategies in response to cooling. Our findings suggest that cold-affiliated species will struggle to acclimate functional traits to warming, conferring warm-affiliated species a competitive advantage under climate change.

Absence of canopy temperature variation despite stomatal adjustment in Pinus sylvestris under multidecadal soil moisture manipulation

Global warming and droughts push forests closer to their thermal limits, altering tree carbon uptake and growth. To prevent critical overheating, trees can adjust their thermotolerance (Tcrit ), temperature and photosynthetic optima (Topt and Aopt ), and canopy temperature (Tcan ) to stay below damaging thresholds. However, we lack an understanding of how soil droughts affect photosynthetic thermal plasticity and Tcan regulation. In this study, we measured the effect of soil moisture on the seasonal and diurnal dynamics of net photosynthesis (A), stomatal conductance (gs ), and Tcan , as well as the thermal plasticity of photosynthesis (Tcrit , Topt , and Aopt ), over the course of 1 yr using a long-term irrigation experiment in a drought-prone Pinus sylvestris forest in Switzerland. Irrigation resulted in higher needle-level A, gs , Topt , and Aopt compared with naturally drought-exposed trees. No daily or seasonal differences in Tcan were observed between treatments. Trees operated below their thermal thresholds (Tcrit ), independently of soil moisture content. Despite strong Tcan and Tair coupling, we provide evidence that drought reduces trees' temperature optimum due to a substantial reduction of gs during warm and dry periods of the year. These findings provide important insights regarding the effects of soil drought on the thermal tolerance of P. sylvestris.

Tropical Trees Will Need to Acclimate to Rising Temperatures-But Can They?

For tropical forests to survive anthropogenic global warming, trees will need to avoid rising temperatures through range shifts and "species migrations" or tolerate the newly emerging conditions through adaptation and/or acclimation. In this literature review, we synthesize the available knowledge to show that although many tropical tree species are shifting their distributions to higher, cooler elevations, the rates of these migrations are too slow to offset ongoing changes in temperatures, especially in lowland tropical rainforests where thermal gradients are shallow or nonexistent. We also show that the rapidity and severity of global warming make it unlikely that tropical tree species can adapt (with some possible exceptions). We argue that the best hope for tropical tree species to avoid becoming "committed to extinction" is individual-level acclimation. Although several new methods are being used to test for acclimation, we unfortunately still do not know if tropical tree species can acclimate, how acclimation abilities vary between species, or what factors may prevent or facilitate acclimation. Until all of these questions are answered, our ability to predict the fate of tropical species and tropical forests-and the many services that they provide to humanity-remains critically impaired.

Boreal conifers maintain carbon uptake with warming despite failure to track optimal temperatures

Warming shifts the thermal optimum of net photosynthesis (ToptA) to higher temperatures. However, our knowledge of this shift is mainly derived from seedlings grown in greenhouses under ambient atmospheric carbon dioxide (CO2) conditions. It is unclear whether shifts in ToptA of field-grown trees will keep pace with the temperatures predicted for the 21st century under elevated atmospheric CO2 concentrations. Here, using a whole-ecosystem warming controlled experiment under either ambient or elevated CO2 levels, we show that ToptA of mature boreal conifers increased with warming. However, shifts in ToptA did not keep pace with warming as ToptA only increased by 0.26-0.35 °C per 1 °C of warming. Net photosynthetic rates estimated at the mean growth temperature increased with warming in elevated CO2 spruce, while remaining constant in ambient CO2 spruce and in both ambient CO2 and elevated CO2 tamarack with warming. Although shifts in ToptA of these two species are insufficient to keep pace with warming, these boreal conifers can thermally acclimate photosynthesis to maintain carbon uptake in future air temperatures.

Acclimation of photosynthetic capacity and foliar respiration in Andean tree species to temperature change

Climate warming is causing compositional changes in Andean tropical montane forests (TMFs). These shifts are hypothesised to result from differential responses to warming of cold- and warm-affiliated species, with the former experiencing mortality and the latter migrating upslope. The thermal acclimation potential of Andean TMFs remains unknown. Along a 2000 m Andean altitudinal gradient, we planted individuals of cold- and warm-affiliated species (under common soil and irrigation), exposing them to the hot and cold extremes of their thermal niches, respectively. We measured the response of net photosynthesis (A_net ), photosynthetic capacity and leaf dark respiration (R_dark ) to warming/cooling, 5 months after planting. In all species, A_net and photosynthetic capacity at 25°C were highest when growing at growth temperatures (T_g ) closest to their thermal means, declining with warming and cooling in cold-affiliated and warm-affiliated species, respectively. When expressed at T_g , photosynthetic capacity and R_dark remained unchanged in cold-affiliated species, but the latter decreased in warm-affiliated counterparts. R_dark at 25°C increased with temperature in all species, but remained unchanged when expressed at T_g . Both species groups acclimated to temperature, but only warm-affiliated species decreased R_dark to photosynthetic capacity ratio at T_g as temperature increased. This could confer them a competitive advantage under future warming.

Determining the ecophysiological limits of a narrow niche tropical conifer tree (Podocarpus trinitensis)

Many tropical species live close to their thermal limits within a narrow niche. Here, we investigate the ecophysiological limits of the tropical tree Podocarpus trinitensis, which is endemic to Trinidad and Tobago where most populations exist as isolated stands on hilltops. Five wild stands from a range of elevations were compared in the field with measurements of leaf temperature, canopy cover, stomatal conductance (gs), chlorophyll content and several chlorophyll fluorescence parameters. A parallel greenhouse experiment was used to acclimate seedlings to 'CONTROL' and 'HEAT' treatments (with mid-day air temperatures of 34.5 and 37 °C respectively), after which the above parameters were measured along with photosynthetic light and temperature response curves, leaf morphology and in vitro Fv/Fm thermostability. There was a positive association between improved physiological performance and elevation. In the high elevation sites, leaf temperatures were significantly lower while most of the physiological parameters were higher (gs, chlorophyll content, ɸ PSII, ETRmax and Isat90). In the greenhouse, HEAT and CONTROL plants were similar for most parameters, except leaf temperature (which was coupled with air temperature) and leaf mass per unit area (which was higher in HEAT plants). Temperature response curves showed an optimum temperature for photosynthesis of 30 ± 0.5 °C (TOpt) and in vitro Fv/Fm indicated a critical temperature of 47.4 ± 0.38 °C for HEAT and 48.2 ± 0.24 °C for CONTROL (T50), with no indication of heat acclimation. Podocarpus trinitensis was found to be shade tolerant. In the field, seedlings established under a close canopy (>95% canopy cover) and had a low light saturation point (LCP). In the greenhouse, where more light was available, seedlings retained a low light compensation point, light saturation point (LSP) and maximum photosynthetic rate (Amax). The results suggest that P. trinitensis is moderately heat tolerant with the higher elevation sites being more habitable, but stands are also able to survive near sea level under a closed canopy. The narrow niche, along with the 30 ± 0.5 °C optimum temperature for photosynthesis and the lack of thermal plasticity in critical temperature, suggests that P. trinitensis has little room to acclimate to temperatures higher than those currently experienced.

Contrasting warming responses of photosynthesis in early- and late-successional tropical trees

The productivity and climate feedbacks of tropical forests depend on tree physiological responses to warmer and, over large areas, seasonally drier conditions. However, knowledge regarding such responses is limited due to data scarcity. We studied the impact of growth temperature on net photosynthesis (An), maximum rates of Rubisco carboxylation at 25°C (Vcmax25), stomatal conductance (gs) and the slope parameter of the stomatal conductance-photosynthesis model (g1), in ten early- (ES) and eight late-successional (LS) tropical tree species grown at three sites along an elevation gradient in Rwanda, differing by 6.8°C in daytime ambient air temperature. The effect of seasonal drought on An was also investigated. We found that warm climate decreased wet-season An in LS species, but not in ES species. Values of Vcmax25 were lower at the warmest site across both successional groups, and An and Vcmax25 were higher in ES compared to LS species. Stomatal conductance exhibited no significant site differences and g1 was similar across both sites and successional groups. Drought strongly reduced An at warmer sites but not at the coolest montane site and this response was similar in both ES and LS species. Our results suggest that warming has negative effects on leaf-level photosynthesis in LS species, while both LS and ES species suffer photosynthesis declines in a warmer climate with more pronounced droughts. The contrasting responses of An between successional groups may lead to shifts in species' competitive balance in a warmer world, to the disadvantage of LS trees.