Differential Responses of Stomata and Photosynthesis to Elevated Temperature in Two Co-occurring Subtropical Forest Tree Species

Impact of climate-driven changes in temperature on stomatal anatomy and physiology

Climate-driven changes in temperature are likely to have major implications for plant performance including impacts on stomatal conductance (gs), gaseous exchange, photosynthesis, leaf temperature and plant water use. Stomatal conductance is not only vital for carbon assimilation but also plays a key role in maintaining optimum leaf temperatures for cellular processes. Higher gs facilitates both CO2 uptake and enhanced evaporative leaf cooling, however, most likely at the cost of greater water loss and lower water-use efficiency. Lower gs helps to maintain overall plant water status but at the expense of C uptake with reduced evaporative cooling which, at elevated temperatures, could be lethal. It is therefore important for gs to balance these competing demands; however, with rapid changes in temperature due to climate change, stomatal engineering may be required to ensure that this balance is achieved and different strategies for different crops in different environments may be needed. Here, we review current knowledge of stomatal anatomical and behavioural responses to temperature-driven changes, focusing on both rising temperatures and extreme heat events and potential genetic targets for manipulation of relevant stomatal characteristics.This article is part of the theme issue 'Crops under stress: can we mitigate the impacts of climate change on agriculture and launch the 'Resilience Revolution'?'.

Some Like It Hot: Differential Photosynthetic Performance and Recovery of English Walnut Accessions Under Emerging California Heat Waves

Heat waves (HWs) pose a significant threat to California agriculture, with potential adverse effects on crop photosynthetic capacity, quality and yield, all of which contribute to significant economic loss. Lack of heat-resilient cultivars puts perennial crop production under severe threat due to increasing HW frequency, duration and intensity. Currently, available walnut cultivars are highly sensitive to abiotic stress, and germplasm collections provide potential solutions via genotypes native to varied climates. We screened nine English walnut accessions (Juglans regia) for physiological heat stress resilience and recovery in the USDA-ARS National Clonal Germplasm over 2-years, and identified accessions with superior resilience to heat stress. Heat stress impacted photosynthetic capacity in most accessions, as evidenced by reductions in net (An) and maximum (Amax) assimilation rates, quantum efficiency of PSII, and changes in stomatal conductance (gs). However, two accessions exhibited either higher or complete recovery post-irrigation. This aligns with the established practice of using irrigation to mitigate heat stress, as it improved recovery for several accessions, with A3 and A5 demonstrating the most resilience. One of these two superior accessions is native to one of the hottest and driest habitats of all studied accessions. These same accessions exhibited the highest An under non-stressed conditions and at higher temperatures of 35° to 45°C. Higher performance for A3 and A5 under HWs was associated with greater carboxylation rates, electron transport rates, and Amax. All accessions suffered significant declines in photosynthetic performance at 45°C, which were the ambient leaf temperatures approached during record-setting heat waves in California during September 2022.

Hydraulic and structural traits of trees across light gradients in the Amazon secondary forest

Amazonian species are generally unable to adapt to long drought periods, indicating a low capacity to adjust their hydraulic traits. Secondary forests account for 20% of forest cover in the Amazon, making natural regeneration species crucial under climate change scenarios. In this study, we compared the hydraulic traits of five species, including non-pioneers (Bertholletia excelsa Bonpl., Carapa guianensis Aubl., Hymenaea courbaril L.) and pioneers [Cedrela fissilis Vell., Tabebuia rosea (Bertol.) Bertero ex A.DC.], across light conditions (understory, intermediate, gap) in a 22-year-old secondary forest in Central Amazon, Brazil. Twenty-five saplings were planted and monitored in 3 plots × 5 blocks. Five years after the plantation, we assessed growth, wood density, leaf water potential at predawn and midday, xylem embolism resistance (P50), and hydraulic safety margins (HSM). The leaf water potential ranged from -2.9 to 0 MPa. The non-pioneer species C. guianensis and H. courbaril exhibited the lowest P50 (-4.06 MPa), indicating higher embolism resistance, whereas the pioneer T. rosea had the highest P50 (-1.25 MPa), indicating lower resistance. The HSM varied from -1.60 to 3.26 MPa, with lower values in gap conditions during the dry period (-1.60 MPa), especially affecting pioneer species. Wood density was influenced by both light and species type, with non-pioneers showing a generally higher density, with H. courbaril reaching 0.75 g cm-3 in the understory while the pioneer T. rosea showed the lowest density (0.27 g cm-3). These results highlight that light conditions affect hydraulic traits differently across species strategies, especially during early growth. Non-pioneer, slow-growing native species appear more resilient to light variation, making them suitable for future plantations aimed at climate adaptation in secondary forests.

Cooling benefits from urban planting depend on local background canopy cover: A continental cross-city comparison

Numerous studies have shown that the cooling efficiency (CE) of urban trees varies by cities with different climate backgrounds, and recent research further indicated that there may be large within-city variations in CE. However, how such within-city variations differ across cities, and their relations to the local percent of urban tree canopy (Ptree) remain poorly understood. This study aims to fill this gap based on a comparison study across 118 cities with different biomes and climates in the continental USA. We used the tree canopy layer of the National Land Cover Dataset (NLCD) 2011 to measure urban tree canopy (UTC), and calculated land surface temperature (LST) based on Landsat thermal bands. We found: 1) CE had larger within-city and cross-city spatial variations in cities located in arid and semi-arid biomes. 2) CE was related to Ptree in nonlinear ways in >90 % of the study cities. In most cities (approximately 70 %), CE had an L-shaped relationship with Ptree, showing that CE first declined quickly with the increase of Ptree, but then gradually dropped in a slower way or became relatively stable after Ptree reached a certain threshold. 3) While there was no significant difference in the types of CE-Ptree relationship among biomes and climates, the threshold of Ptree in CE-Ptree nonlinear ways was smaller in arid cities. The results of this threshold linking cooling benefits and current UTC can serve as a useful tool to prioritize locations for urban planting to maximize cooling benefits.

Reduced soil moisture drives leaf anatomical shifts more than chronically elevated temperatures in European temperate trees

Chronic reductions in soil moisture combined with high air temperatures can modify tree carbon and water relations. However, little is known about how trees acclimate their foliar structure to the individual and combined effects of these two climate drivers. We used open-top chambers to determine the multi-year effects of chronic air warming (+5 °C) and soil moisture reduction (-50%) alone and in combination on the foliar anatomy of two European tree species. We further investigated how these climate drivers affected the relationship between foliar anatomy and physiology/chemistry in young downy oak and European beech trees. After 4 years, reduced soil moisture led to development of thinner leaves with a narrower epidermis and lower gas exchange for oak and beech, but to a lesser extent in the latter. In contrast, prolonged warming did not affect the anatomical and physiological/chemical traits in either species. Warming also did not exacerbate the impacts of dry soils, highlighting soil moisture as the key driver in leaf anatomical shifts. While soil moisture altered oak foliar anatomy, and the physiology and chemistry of both species, our work revealed a limited acclimation potential towards more drought- and heat-tolerant leaves as conditions become drier and warmer, suggesting potentially high vulnerability of both species to future climate predictions.

Stomatal development in the changing climate

Stomata, microscopic pores flanked by symmetrical guard cells, are vital regulators of gas exchange that link plant processes with environmental dynamics. The formation of stomata involves the multi-step progression of a specialized cell lineage. Remarkably, this process is heavily influenced by environmental factors, allowing plants to adjust stomatal production to local conditions. With global warming set to alter our climate at an unprecedented pace, understanding how environmental factors impact stomatal development and plant fitness is becoming increasingly important. In this Review, we focus on the effects of carbon dioxide, high temperature and drought - three environmental factors tightly linked to global warming - on stomatal development. We summarize the stomatal response of a variety of plant species and highlight the existence of species-specific adaptations. Using the model plant Arabidopsis, we also provide an update on the molecular mechanisms involved in mediating the plasticity of stomatal development. Finally, we explore how knowledge on stomatal development is being applied to generate crop varieties with optimized stomatal traits that enhance their resilience against climate change and maintain agricultural productivity.

Soil environmental anomalies dominate the responses of net ecosystem productivity to heatwaves in three Mongolian grasslands

Climate change is causing more frequent and intense heatwaves. Therefore, it is important to understand how heatwaves affect the terrestrial carbon cycle, especially in grasslands, which are especially susceptible to climate extremes. This study assessed the impact of naturally occurring, simultaneous short-term heatwaves on CO2 fluxes in three ecosystems on the Mongolia Plateau: meadow steppe (MDW), typical steppe (TPL), and shrub-grassland (SHB). During three heatwaves, net ecosystem productivity (NEP) was reduced by 86 %, 178 %, and 172 % at MDW, TPL, and SHB, respectively. The changes in ecosystem respiration, gross primary production, evapotranspiration, and water use efficiency were divergent, indicating the mechanisms underlying the observed NEP decreases among the sites. The impact of the heatwave in MDW was mitigated by the high soil water content, which enhanced evapotranspiration and subsequent cooling effects. However, at TPL, insufficient soil water led to combined thermal and drought stress and low resilience. At SHB, the ecosystem's low tolerance to an August heatwave was heavily influenced by species phenology, as it coincided with the key phenological growing phase of plants. The potential key mechanism of divergent NEP response to heatwaves lies in the divergent stability and varying importance of environmental factors, combined with the specific sensitivity of NEP to each factor in ecosystems. Furthermore, our findings suggest that anomalies in soil environment, rather than atmospheric anomalies, are the primary determinants of NEP anomalies during heatwaves. This challenges the conventional understanding of heatwaves as a discrete and ephemeral periods of high air temperatures. Instead, heatwaves should be viewed as chronologically variable, compound, and time-sensitive environmental stressors. The ultimate impact of heatwaves on ecosystems is co-determined by a complex interplay of environmental, biological, and heatwave features.

Response of stomatal density and size in Betula ermanii to contrasting climate conditions: The contributions of genetic and environmental factors

As plant distribution and performance are determined by both environmental and genetic factors, clarifying the contribution of these two factors is a key for understanding plant adaptation and predicting their distribution under ongoing global warming. Betula ermanii is an ideal species for such research because of its wide distribution across diverse environments. Stomatal density and size are crucial traits that plants undergo changes in to adapt to different environments as these traits directly influence plant photosynthesis and transpiration. In this study, we conducted a multi-location common garden experiment using B. ermanii to (1) clarify the contribution of both environmental and genetic factors to the variation in stomatal density and size of B. ermanii, (2) demonstrate the differences in the plasticity of stomatal density and size among B. ermanii populations, and (3) understand how stomatal density and size of B. ermanii would respond to increased temperature and changing precipitation patterns. Genetic factors played a more significant role in stomatal size than environmental factors, suggesting that B. ermanii struggles to adjust its stomatal size in response to a changing environment. Our results also revealed a positive correlation between stomatal size plasticity and original habitat suitability, indicating that in B. ermanii populations in harsh environments exhibit lower adaptability to environmental shifts. Although stomatal density and size of B. ermanii showed the significant responses to increased temperature and shifting precipitation patterns, the response ranges of stomatal density and size to the environmental factors varied among populations. Our findings highlighted the interplay between genetic and environmental factors in determining the intraspecific variation in stomatal density and size in B. ermanii. This indicated that certain populations of B. ermanii exhibit limited stomatal plasticity and adaptability, which could directly affect photosynthesis and transpiration, suggesting potential population-specific fitness implications for B. ermanii under future climate change.

Soil warming affects sap flow and stomatal gas exchange through altering functional traits in a subtropical forest

Climate warming influences the structure and function of ecosystems. However, the mechanisms of plant water use and gas exchange responses to climate warming have been less studied, especially from the perspective of different functional traits. We conducted a field experiment to investigate how soil warming (+2 °C) affects sap flow and stomatal gas exchange through plant functional traits and nutrient characteristics in a subtropical forest. We measured stomatal gas exchange of trees (Acacia auriculiformis and Schima superba) and shrubs (Castanea henryi and Psychotria asiatica), and monitored long-term sap flow of both tree species. Besides, plant leaf nutrient contents, functional traits, and soil nutrients were also studied. It is demonstrated that soil warming significantly increased maximum sap flow density (Js_max, 35.1 %) and whole-tree transpiration (EL, 46.0 %) of A. auriculiformis, but decreased those of S. superba (15.6 % and 14.9 %, respectively). Warming increased the photosynthetic rate of P. asiatica (18.0 %) and water use efficiency of S. superba (47.2 %). Leaf nutrients and stomatal anatomical characteristics of shrubs were less affected by soil warming. Soil warming increased (+42.7 %) leaf K content of A. auriculiformis in dry season. Decomposition of soil total carbon, total nitrogen, and available nitrogen was accelerated under soil warming, and soil exchangeable Ca2+ and Mg2+ were decreased. Trees changed stomatal and anatomic traits to adapt to soil warming, while shrubs altered leaf water content and specific leaf area under soil warming. Warming had a greater effect on sap flow of trees, as well as on their leaf gas exchange (total effect: -0.27) than on that of shrubs (total effect: 0.06). In summary, our results suggest that the combination of functional and nutrient traits can help to better understand plant water use and gas exchange responses under climate warming.

Leaf trait responses to global change factors in terrestrial ecosystems

Global change influences plant growth by affecting plant morphology and physiology. However, the effects of global change factors vary based on the climate gradient. Here, we established a global database of leaf traits from 192 experiments on elevated CO2 concentrations (eCO2), drought, N deposition, and warming. The results showed that the leaf mass per area (LMA) significantly increased under eCO2 and drought conditions but decreased with N deposition, whereas eCO2 levels and drought conditions reduced stomatal conductance and increased and decreased photosynthetic rates, respectively. Leaf dark respiration (Rd) increased in response to global change, excluding N deposition. Leaf N concentrations declined with eCO2 but increased with N deposition. Leaf area increased with eCO2, N deposition, and warming but decreased with drought. Leaf thickness increased with eCO2 but decreased with warming. eCO2 and N deposition enhanced plant water-use efficiency (WUE), eCO2 and warming increased photosynthetic N-use efficiency (PNUE), while N fertilization reduced PNUE significantly. eCO2 produced a positive relationship between WUE and PNUE, which were limited under drought but increased in areas with high humidity and high temperature. Trade-offs were observed between WUE and PNUE under drought, N deposition, and warming. These findings suggest that the effects of global change factors on plants can be altered by complex environmental changes; moreover, diverse plant water and nutrient strategy responses can be interpreted against the background of their functional traits.

Trade-off of leaf-scale resource-use efficiencies along the vertical canopy of the subtropical forest

Leaf resource-use efficiencies are key indicators of plant adaptability to climate change, as they depend on both photosynthetic carbon assimilation and available resources. However, accurately quantifying the response of the coupled carbon and water cycles is challenging due to the canopy vertical variability in resource-use efficiencies, which introduces greater uncertainty into the calculations. Here we experimented to ascertain the vertical variations of leaf resource-use efficiencies along three canopy gradients of coniferous (Pinus elliottii Engelmann.) and broad-leaved (Schima Superba Gardn & Champ.) forests over one year in the subtropical region of China. The efficiency of water (WUE), and nitrogen (NUE) showed higher values in the top canopy level for the two species. The maximum efficiency of light (LUE) occurred in the bottom canopy level for both species. The impact of photosynthetic photon flux density (PPFD), leaf temperature (T_leaf), and vapor pressure deficit (VPD) on leaf resource-use efficiencies varied with canopy gradients in slash pine and schima superba. We also observed a trade-off between NUE and LUE for slash pine and between NUE and WUE for schima superba. Moreover, the variation in the correlation between LUE and WUE indicated a change in resource-use strategies for slash pine. These results emphasize the significance of vertical variations in resource-use efficiencies to enhance the prediction of future carbon-water dynamics in the subtropical forest.

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.

Photosynthetic Plasticity and Stomata Adjustment in Chromosome Segment Substitution Lines of Rice Cultivar KDML105 under Drought Stress

The impact of increasing drought periods on crop yields as a result of global climate change is a major concern in modern agriculture. Thus, a greater understanding of crop physiological responses under drought stress can guide breeders to develop new cultivars with enhanced drought tolerance. In this study, selected chromosome segment substitution lines of KDML105 (KDML105-CSSL) were grown in the Plant Phenomics Center of Kasetsart University in Thailand under well-watered and drought-stressed conditions. Physiological traits were measured by observing gas exchange dynamics and using a high-throughput phenotyping platform. Furthermore, because of its impact on plant internal gas and water regulation, stomatal morphological trait variation was recorded. The results show that KDML105-CSS lines exhibited plasticity responses to enhance water-use efficiency which increased by 3.62%. Moreover, photosynthesis, stomatal conductance and transpiration decreased by approximately 40% and plant height was reduced by 17.69%. Stomatal density tended to decrease and was negatively correlated with stomatal size, and stomata on different sides of the leaves responded differently under drought stress. Under drought stress, top-performing KDML105-CSS lines with high net photosynthesis had shorter plant height and improved IWUE, as influenced by an increase in stomatal density on the upper leaf side and a decrease on the lower leaf side.

Effect of Heat Stress on Some Physiological and Anatomical Characteristics of Rice (Oryza sativa L.) cv. KDML105 Callus and Seedling

Simple Summary

Climate change is currently threatening agriculture all around the world, resulting in a lack of water and restricting the growth of plants, especially rice. Rice production decreases with the increase in temperature. An improvement in fundamental knowledge is necessary to comprehend plant adaptation mechanisms as responses to heat stress. Physiological and anatomical responses of Khao Dawk Mali 105 (KDML105) rice to artificial heat stress were studied. Our findings offer useful data for projects aimed at improving heat stress tolerance in rice to enhance long-term global food security.

Abstract

Global warming is a serious problem, with significant negative impacts on agricultural productivity. To better understand plant anatomical adaptation mechanisms as responses to heat stress, improved basic knowledge is required. This research studied the physiological and anatomical responses of Khao Dawk Mali 105 (KDML105) to artificial heat stress. Dehusked seeds were sterilized and cultured on Murashige and Skoog (MS) medium, supplemented with 3 mg/L 2,4-Dichlorophenoxyacetic acid (2,4-D) for callus induction. The cultures were maintained at 25 °C and 35 °C for 4 weeks, while the other culture was treated with heat shock at 42 °C for 1 week before further incubation at 25 °C for 3 weeks. Results revealed that elevated temperatures (35 °C and 42 °C) adversely impacted seedling growth. Plant height, root length, leaf number per plant, fresh and dry weight, chlorophyll a, chlorophyll b and total chlorophyll content decreased after heat stress treatment, while malondialdehyde (MDA) and electrolyte leakage percentage significantly increased, compared to the control. Heat stress induced ROS accumulation, leading to lipid peroxidation and membrane instability. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) results also confirmed negative correlations between MDA, electrolyte leakage and other parameters. MDA content and electrolyte leakage are effective indicators of heat stress in rice. Surface anatomical responses of rice seedlings to heat stress were studied but significant alterations were not observed, and heat stress had no significant negative effects on KDML105 calli. Size and mass of calli increased because heat stress stimulated gene expression that induced thermotolerance. Our results provide useful information for rice breeding and heat stress tolerance programs to benefit long-term global food security.

Responses of Microstructure, Ultrastructure and Antioxidant Enzyme Activity to PEG-Induced Drought Stress in Cyclocarya paliurus Seedlings

Drought is one of the most important abiotic constraints on agricultural productivity, while global warming leads to the occurrence of more frequent drought events. Cyclocarya paliurus is a multiple-function tree species with medicinal value and timber production, but no information is available on its drought tolerance. In this hydroponic experiment, variations in leaf anatomical morphology, chloroplast ultrastructure, stomatal characteristics, and antioxidant enzyme activities were investigated under six levels of polyethylene glycol 6000 (PEG)-induced drought treatments to assess the drought adaption and physiological response of C. paliurus seedlings. The results showed that PEG-induced drought treatments reduced leaf epidermis, spongy tissue, leaf vein diameter, and spongy ratio, whereas the ratio of palisade tissue to spongy tissue, cell tense ratio, and vein protuberant degree all increased with enhancing the PEG6000 concentrations. Significant differences in stomatal width, stomatal aperture, and stomatal density existed among the treatments (p < 0.01). The stomatal aperture decreased significantly with the increase in PEG6000 concentrations, whereas the greatest stomatal density was observed in the 15% PEG6000 treatment. Compared with the control, higher drought stresses (20% and 25% PEG concentrations) caused damage at the cellular level and chloroplast lysis occurred. PEG6000 treatments also promoted the activities of SOD, POD, and CAT in C. paliurus seedlings, but this increase was insufficient to deal with the membrane lipid peroxidative damage under the high PEG concentrations. Correlation analysis indicated that in most cases there were significant relationships between leaf anatomical characteristics and antioxidant enzyme activities. Our results suggested that C. paliurus seedlings would not survive well when the PEG6000 concentration was over 15% (equal to soil water potential of −0.30 MPa).

Plant growth, physiological variation and homological relationship of Cyclocarya species in ex situ conservation

Natural forests of Cyclocarya paliurus have been seriously damaged because of the extreme demand for leaf medicinal uses, making conservation of this valuable, medicinal woody species necessary. Because of geographical differentiation and diverse adaptability, in this study we analysed the variations in plant growth and physiological response to environmental factors at a resource plantation of ex situ conservation and determined the homological relationships between local provenance (from Fujian Province, FJ) and introduced provenances showing high-survival rate and better growth (from Zhejiang, Hubei, Guizhou and Jiangxi Province). Our results suggested the following: (i) Plant growth: FJ had the highest plant height but not the largest basal diameter in comparison to that of other provenances. (ii) Physiological responses during the growth periods: water content in leaf of FJ had similar change with that of other provenances, except for the provenance from Guizhou Province; total soluble sugar content in leaf of FJ was more than that of other provenances; calcium content in leaf of all provenances was higher as compared to K, Mg and Na; the highest activity among four kinds of antioxidant enzymes in all provenances was superoxide dismutase, then was polyphenol oxidase and peroxidase, finally was catalase; and total flavonoid among three kinds of secondary metabolites in all provenances showed the greatest content, followed by polysaccharides and total triterpenoid. (iii) Relation analysis: plant growth and physiological responses related with environmental factors, especially temperature and precipitation. (iv) Homological relationships: leaf characteristics among six provenances varied in colour, area and common petiole length, but not the shape of leaf base or apex. Cyclocarya paliurus distributed in Fujian Province showed a very close homological relationship with that distributed in Zhejiang Province by simple sequence repeat. These findings will provide knowledge on physiological response to environmental factors and aid to select suitable provenances for Cyclocarya cultivation.

Leaf physiological and anatomical responses of two sympatric Paphiopedilum species to temperature

Paphiopedilum dianthum and P. micranthum are two endangered orchid species, with high ornamental and conservation values. They are sympatric species, but their leaf anatomical traits and flowering period have significant differences. However, it is unclear whether the differences in leaf structure of the two species will affect their adaptabilities to temperature. Here, we investigated the leaf photosynthetic, anatomical, and flowering traits of these two species at three sites with different temperatures (Kunming, 16.7 ± 0.2 °C; Puer, 17.7 ± 0.2 °C; Menglun, 23.3 ± 0.2 °C) in southwest China. Compared with those at Puer and Kunming, the values of light-saturated photosynthetic rate (Pmax), stomatal conductance (gs), leaf thickness (LT), and stomatal density (SD) in both species were lower at Menglun. The values of Pmax, gs, LT, adaxial cuticle thickness (CTad) and SD in P. dianthum were higher than those of P. micranthum at the three sites. Compared with P. dianthum, there were no flowering plants of P. micranthum at Menglun. These results indicated that both species were less resistance to high temperature, and P. dianthum had a stronger adaptability to high-temperature than P. micranthum. Our findings can provide valuable information for the conservation and cultivation of Paphiopedilum species.

Spatial variations in stomatal traits and their coordination with leaf traits in Quercus variabilis across Eastern Asia

The stomatal traits influence ecosystem carbon-water fluxes and play essential roles that enable plants to adapt to changing environmental conditions. However, how stomatal traits vary along a large climate gradient and whether stomatal traits coordinated with other leaf functional traits in response to environmental changes remain unclear. We investigated the stomatal density (SD), stomatal size (SS), and leaf traits (leaf area (LA), leaf mass per area (LMA), and vein density (VD)) of 44 in situ Quercus variabilis populations across Eastern Asia (24 to 51.8°N, 99 to 137°E) and 15 populations grown in a common garden, and evaluated their relationships with environmental factors. Stepwise multiple regression showed that the SD was significantly associated with mean annual precipitation (MAP), LMA, and VD, and the SS with latitude, mean annual temperature (MAT), mean monthly solar radiation (MMSR), and VD. The SD was positively correlated with the LMA, while the SS was negatively correlated with the VD. The SD and LMA increased with decreasing precipitation, which indicated that they may coordinate to commonly enhance plant resistance against drought. The SS decreased; however, the VD increased with temperature. This implied that plants might further reduce their SS by increasing VD limitations under global warming. In the common garden, plants exhibited a higher SD and VD and lower SS and LA compared to those in the field; however, no relation between the stomatal and leaf traits was observed. Our results suggested that stomatal traits have high environmental plasticity and are highly coordinated with other leaf functional traits in response to environmental changes. Nevertheless, this coordination may have been formed through long-term adaptations, rather than over short time spans.

Variations in Rainfall Affect the Responses of Foliar Chemical Properties of Cunninghamia lanceolata Seedlings to Soil Warming

Climate warming is becoming an increasingly serious threat. Understanding plant stoichiometry changes under climate warming is crucial for predicting the effects of future warming on terrestrial ecosystem productivity. Nevertheless, how plant stoichiometry responds to warming when interannual rainfall variation is considered, remains poorly understood. We performed a field soil warming experiment (+5°C) using buried heating cables in subtropical areas of China from 2015 to 2018. Stoichiometric patterns of foliar C:N:P:K:Ca:Mg, non-structural carbohydrate, and stable isotope of Cunninghamia lanceolata seedlings were studied. Our results showed that soil warming decreased foliar P and K concentrations, C:Ca, P:Ca, and P:Mg ratios. However, soil warming increased foliar Ca concentration, δ15N value, C:P and N:P ratios. The response ratios of foliar N, C:N, and δ15N to soil warming were correlated with rainfall. Our findings indicate that there was non-homeostasis of N and C:N under warming conditions. Three possible reasons for this result are considered and include interannual variations in rainfall, increased loss of N, and N limitation in leaves. Piecewise structural equation models showed that stoichiometric non-homeostasis indirectly affected the growth of C. lanceolata seedlings in response to soil warming. Consequently, the growth of C. lanceolata seedlings remained unchanged under the warming treatment. Taken together, our results advance the understanding of how altered foliar stoichiometry relates to changes in plant growth in response to climate warming. Our results emphasize the importance of rainfall variations for modulating the responses of plant chemical properties to warming. This study provides a useful method for predicting the effects of climate warming on economically important timber species.

Photosynthetic plasticity of a tropical tree species, Tabebuia rosea, in response to elevated temperature and [CO2 ]

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) CO₂ . 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.

Leaf hydraulic acclimation to nitrogen addition of two dominant tree species in a subtropical forest

Plant hydraulic traits have been shown to be sensitive to changes in nitrogen (N) availability in short-term studies largely using seedlings or saplings. The extent and the magnitude of N-sensitivity of the field grown mature trees in long-term experiments, however, are relatively unknown. Here, we investigated responses of leaf water relations and morphological and anatomical traits of two dominant tree species (Castanopsis chinensis and Schima superba) to a six-year canopy N addition in a subtropical forest. We found that N addition increased leaf hydraulic conductivity in both species along with higher transpiration rate and less negative water potential at 50% loss of leaf hydraulic conductivity and at leaf turgor loss point. Examination of leaf morphological and anatomical traits revealed that increased leaf hydraulic efficiency was at least in part due to increased vessel diameter which also compromised the hydraulic safety under increased water stress. Moreover, reduced vessel reinforcement and increased thickness shrinkage index further interpreted the increases in leaf hydraulic vulnerability under N addition. Our results demonstrated that N deposition may lead to increases of plant water loss to the atmosphere as well as tree vulnerability to drought.

Ozone Response of Leaf Physiological and Stomatal Characteristics in Brassica juncea L. at Supraoptimal Temperatures

Plants are affected by the features of their surrounding environment, such as climate change and air pollution caused by anthropogenic activities. In particular, agricultural production is highly sensitive to environmental characteristics. Since no environmental factor is independent, the interactive effects of these factors on plants are essential for agricultural production. In this context, the interactive effects of ozone (O3) and supraoptimal temperatures remain unclear. Here, we investigated the physiological and stomatal characteristics of leaf mustard (Brassica juncea L.) in the presence of charcoal-filtered (target concentration, 10 ppb) and elevated (target concentration, 120 ppb) O3 concentrations and/or optimal (22/20 °C day/night) and supraoptimal temperatures (27/25 °C). Regarding physiological characteristics, the maximum rate of electron transport and triose phosphate use significantly decreased in the presence of elevated O3 at a supraoptimal temperature (OT conditions) compared with those in the presence of elevated O3 at an optimal temperature (O conditions). Total chlorophyll content was also significantly affected by supraoptimal temperature and elevated O3. The chlorophyll a/b ratio significantly reduced under OT conditions compared to C condition at 7 days after the beginning of exposure (DAE). Regarding stomatal characteristics, there was no significant difference in stomatal pore area between O and OT conditions, but stomatal density under OT conditions was significantly increased compared with that under O conditions. At 14 DAE, the levels of superoxide (O2-), which is a reactive oxygen species, were significantly increased under OT conditions compared with those under O conditions. Furthermore, leaf weight was significantly reduced under OT conditions compared with that under O conditions. Collectively, these results indicate that temperature is a key driver of the O3 response of B. juncea via changes in leaf physiological and stomatal characteristics.

Strong Environmental Filtering Based on Hydraulic Traits Occurring in the Lower Water Availability of Temperate Forest Communities

The trait-based approaches have made progress in understanding the community assembly process. Here, we explore the key traits that may shape community assembly patterns of the same community type but within different water availabilities. Natural Quercus wutaishanica forests were chosen as a suitable study system to test the difference between economic and hydraulic traits across water availability on the Loess Plateau (LP, drought region) and Qinling Mountains (QL, humid region) of China. A total of 75 plots were established separately in two sites, and 12 functional traits (seven hydraulic traits and five economic traits) of 167 species were studied. Community-weighted mean trait values and functional diversity indices were compared between the two sites. Canonical component analysis was performed to infer whether the changes of community traits and their relationships are driven by intraspecific variation or species turnover. Evidence for likely community assembly processes was tested using the null model to determine whether functional structure among seven hydraulic traits and five economic traits was dominated by different ecological processes between two sites. We found that forests in the Loess Plateau and Qinling Mountains showed different hydraulic and economic traits. Hydraulic and economic traits coupled at the community level were driven by species turnover. Hydraulic traits showed more significant convergent patterns on LP than that in QL. Our results suggest a strong environmental filtering process occurred in hydraulic-based community assembly in the temperate forest with low water availability. Reveal the relationship of hydraulic and economic traits at the community level. Emphasize the critical role of multi-dimensional traits selecting like hydraulic traits in community ecology.

A stomata classification and detection system in microscope images of maize cultivars

Plant stomata are essential structures (pores) that control the exchange of gases between plant leaves and the atmosphere, and also they influence plant adaptation to climate through photosynthesis and transpiration stream. Many works in literature aim for a better understanding of these structures and their role in the evolution process and the behavior of plants. Although stomata studies in dicots species have advanced considerably in the past years, even there is not much knowledge about the stomata of cereal grasses. Due to the high morphological variation of stomata traits intra- and inter-species, detecting and classifying stomata automatically becomes challenging. For this reason, in this work, we propose a new system for automatic stomata classification and detection in microscope images for maize cultivars based on transfer learning strategy of different deep convolution neural netwoks (DCNN). Our performed experiments show that our system achieves an approximated accuracy of 97.1% in identifying stomata regions using classifiers based on deep learning features, which figures out as a nearly perfect classification system. As the stomata are responsible for several plant functionalities, this work represents an important advance for maize research, providing an accurate system in replacing the current manual task of categorizing these pores on microscope images. Furthermore, this system can also be a reference for studies using images from different cereal grasses.

Long-term effects of 7-year warming experiment in the field on leaf hydraulic and economic traits of subtropical tree species

Rising temperature associated with climate change may have substantial impacts on forest tree functions. We conducted a 7-year warming experiment in subtropical China by translocating important native forest tree species (Machilas breviflora, Syzygium rehderianum, Schima superba and Itea chinensis) from cooler high-elevation sites (600 m) to 1-2°C warmer low-elevation sites (300 and 30 m) to investigate warming effects on leaf hydraulic and economic traits. Here, we report data from the last 3 years (Years 5-7) of the experiment. Warming increased leaf hydraulic conductance of S. superba to meet the higher evaporative demand. M. breviflora (300 m), S. rehderianum, S. superba and I. chinensis (300 and 30 m) exhibited higher area-based and mass-based maximum photosynthetic rates (A_a and A_m , respectively) related to increasing stomatal conductance (g_s ) and stomatal density in the wet season, which led to rapid growth; however, we observed decreased growth of M. breviflora at 30 m due to lower stomatal density and decreased A_a in the wet season. Warming increased photosynthetic nitrogen-use efficiency and photosynthetic phosphorus-use efficiency, but reduced leaf dry mass per unit area due to lower leaf thickness, suggesting that these tree species allocated more resources into upregulating photosynthesis rather than into structural investment. Our findings highlight that there was trait variation in the capacity of trees to acclimate to warmer temperatures such that I. chinensis may benefit from warming, but S. superba may be negatively influenced by warming in future climates.

Effect of summer warming on growth, photosynthesis and water status in female and male Populus cathayana: implications for sex-specific drought and heat tolerances

Effects of climate warming on tree growth and physiology may be driven by direct thermal effects and/or by changes in soil moisture. Dioecious tree species usually show sexual spatial segregation along abiotic gradients; however, few studies have assessed the sex-specific responses to warming in dioecious trees. We investigated the sex-specific responses in growth, photosynthesis, nonstructural carbohydrate (NSC), water-use efficiency and whole-plant hydraulic conductance (KP) of the dioecious tree species Populus cathayana Rehd. under +4 °C elevated temperature with and without supplemental water. For both sexes, high-temperature treatments significantly decreased growth (height and biomass), photosynthetic rate (A), the ratio of A to dark respiration rate, stomatal conductance (gs), transpiration rate, NSC, leaf water potential and KP, but increased water-use efficiency (estimated from carbon isotope composition). Under warming with supplemental water, most traits of females did not change relative to ambient conditions, but traits of males decreased, resulting in greater sexual differences. Females showed a lower KP, and their gs and A responded more steeply with water-related traits than males. These results show that the effect of summer warming on growth and photosynthesis was driven mainly by soil moisture in female P. cathayana, while male performance was mainly related to temperature. Females may experience less thermal stress than males due to flexible water balance strategy via stomata regulation and water use.

Diverging Responses of Two Subtropical Tree Species (Schima superba and Cunninghamia lanceolata) to Heat Waves

The frequency and intensity of heat waves (HWs) has increased in subtropical regions in recent years. The mechanism underlying the HW response of subtropical trees remains unclear. In this study, we conducted an experiment with broad-leaved Schima superba (S. superba) and coniferous Cunninghamia lanceolata (C. lanceolata) seedlings to examine HW (5-day long) effects on stem water transport, leaf water use efficiency (WUE), morphology and growth, and to elucidate differences in the responses of both species. Our results indicated that HWs can significantly reduce hydraulic conductivity in both species. C. lanceolata experienced significant xylem embolism, with the percentage loss of conductivity (PLC) increasing by 40%, while S. superba showed a non-significant increase in PLC (+25%). Furthermore, HW also caused a reduction in photosynthesis rates (An), but transpiration rates (Tr) increased on the 5th day of the HW, together leading to a significant decrease in leaf WUE. From diurnal dynamics, we observed that the HW caused significant decrease of S. superba An only in the morning, but nearly the all day for C. lanceolata. During the morning, with a high vapor pressure deficit (VPD) environment, the HW increased Tr, which contributed a lot to latently cooling the foliage. In comparing the two tree species, we found that HW effects on S. superba were mostly short-term, with leaf senescence but limited or no xylem embolism. The surviving S. superba recovered rapidly, forming new branches and leaves, aided by their extensive root systems. For C. lanceolata, continued seedling growth initially but with subsequent xylem embolism and withering of shoots, led to stunted recovery and regrowth. In conclusion, apart from the direct thermal impacts caused by HW, drought stress was the main cause of significant negative effects on plant water transport and the photosynthetic system. Furthermore, S. superba and C. lanceolata showed clearly different responses to HW, which implies that the response mechanisms of broad-leaved and coniferous tree species to climate change can differ.

OCO-2 Solar-Induced Chlorophyll Fluorescence Variability across Ecoregions of the Amazon Basin and the Extreme Drought Effects of El Niño (2015–2016)

Amazonian ecosystems are major biodiversity hotspots and carbon sinks that may lose species to extinction and become carbon sources due to extreme dry or warm conditions. We investigated the seasonal patterns of high-resolution solar-induced chlorophyll fluorescence (SIF) measured by the satellite Orbiting Carbon Observatory-2 (OCO-2) across the Amazonian ecoregions to assess the area´s phenology and extreme drought vulnerability. SIF is an indicator of the photosynthetic activity of chlorophyll molecules and is assumed to be directly related to gross primary production (GPP). We analyzed SIF variability in the Amazon basin during the period between September 2014 and December 2018. In particular, we focused on the SIF drought response under the extreme drought period during the strong El Niño in 2015–2016, as well as the 6-month drought peak period. During the drought´s peak months, the SIF decreased and increased with different intensities across the ecoregions of the Amazonian moist broadleaf forest (MBF) biome. Under a high temperature, a high vapor pressure deficit, and extreme drought conditions, the SIF presented differences from −31.1% to +17.6%. Such chlorophyll activity variations have been observed in plant-level measurements of active fluorescence in plants undergoing physiological responses to water or heat stress. Thus, it is plausible that the SIF variations in the ecoregions’ ecosystems occurred as a result of water and heat stress, and arguably because of drought-driven vegetation mortality and collateral effects in their species composition and community structures. The SIF responses to drought at the ecoregional scale indicate that there are different levels of resilience to drought across MBF ecosystems that the currently used climate- and biome-region scales do not capture. Finally, we identified monthly SIF values of 32 ecoregions, including non-MBF biomes, which may give the first insights into the photosynthetic activity dynamics of Amazonian ecoregions.

Physiological, biochemical and gene-expressional responses to water deficit in apple subjected to partial root-zone drying (PRD)

Water scarcity is one of the major factors limiting apple production. Partial root-zone drying (PRD) is a water-saving irrigation technique necessary to improve the efficiency of irrigation techniques to optimize the amount of fruit produced with the volume of water used. The apple trees cv. Red Delicious were exposed to four treatments, including (1) control with 100% of the crop evapotranspiration (ETc) needs; (2) alternate partial root-zone drying with 75% of the ETc needs (APRD75); (3) fixed partial root-zone drying with 75% of the ETc needs (FPRD75); (4) fixed partial root-zone irrigation with 50% of the ETc needs (FPRD50) in a semiarid region of Iran. Results showed that leaf water potential (Ψ _leaf), and chlorophyll were significantly decreased in FPRD50 compared to control and other PRD treatments. APRD75 and FPRD75 treatments significantly enhanced (+) -catechin (+C), epicatechin (EC), chlorogenic acid (CGA), caffeic acid (CA) as well as increased water use efficiency (WUE) (by 30-40% compared to control) without significant reduction of yield. PRD reduced gibberellic acid (GA3) and kinetin, while, increased the abscisic acid (ABA) and salicylic acid (SA) levels. The abiotic stress-responsive transcription factors (TFs) MdoMYB121, MdoMYB155, MdbZIP2, and MdbZIP48 were highly expressed in all PRD treatments. Our results demonstrated that APRD75 and FPRD75 have the potential to stimulate antioxidant defense mechanisms, hormonal signaling pathways, and expression of drought-tolerance TFs to improve WUE while maintaining crop yield. Therefore, APRD75andFPRD75 with water savings as compared to full irrigation might be a suitable strategy for irrigation apple trees under water scarcity.

Plant Cell Walls Tackling Climate Change: Insights into Plant Cell Wall Remodeling, Its Regulation, and Biotechnological Strategies to Improve Crop Adaptations and Photosynthesis in Response to Global Warming

Plant cell wall (CW) is a complex and intricate structure that performs several functions throughout the plant life cycle. The CW of plants is critical to the maintenance of cells' structural integrity by resisting internal hydrostatic pressures, providing flexibility to support cell division and expansion during tissue differentiation, and acting as an environmental barrier that protects the cells in response to abiotic stress. Plant CW, comprised primarily of polysaccharides, represents the largest sink for photosynthetically fixed carbon, both in plants and in the biosphere. The CW structure is highly varied, not only between plant species but also among different organs, tissues, and cell types in the same organism. During the developmental processes, the main CW components, i.e., cellulose, pectins, hemicelluloses, and different types of CW-glycoproteins, interact constantly with each other and with the environment to maintain cell homeostasis. Differentiation processes are altered by positional effect and are also tightly linked to environmental changes, affecting CW both at the molecular and biochemical levels. The negative effect of climate change on the environment is multifaceted, from high temperatures, altered concentrations of greenhouse gases such as increasing CO2 in the atmosphere, soil salinity, and drought, to increasing frequency of extreme weather events taking place concomitantly, therefore, climate change affects crop productivity in multiple ways. Rising CO2 concentration in the atmosphere is expected to increase photosynthetic rates, especially at high temperatures and under water-limited conditions. This review aims to synthesize current knowledge regarding the effects of climate change on CW biogenesis and modification. We discuss specific cases in crops of interest carrying cell wall modifications that enhance tolerance to climate change-related stresses; from cereals such as rice, wheat, barley, or maize to dicots of interest such as brassica oilseed, cotton, soybean, tomato, or potato. This information could be used for the rational design of genetic engineering traits that aim to increase the stress tolerance in key crops. Future growing conditions expose plants to variable and extreme climate change factors, which negatively impact global agriculture, and therefore further research in this area is critical.

The role of leaf water potential in the temperature response of mesophyll conductance

Variation in temperature (T) is usually accompanied by changes in leaf water potential (Ψ_leaf ), which may influence mesophyll conductance (g_m ). However, the effects of Ψ_leaf on g_m have not yet been considered in models of the g_m response to temperature. Temperature responses of g_m and Ψ_leaf and the response of g_m to Ψ_leaf were studied in rice (Oryza sativa) and wheat (Triticum aestivum), and then an empirical model of Ψ_leaf was incorporated into an existing g_m -T model. In wheat, Ψ_leaf was dramatically decreased with increasing T, whereas in rice Ψ_leaf was less sensitive or insensitive to T. Without taking Ψ_leaf into account, g_m for wheat showed no response to T. However, at a given Ψ_leaf , g_m was significantly higher at high temperature compared with low. After incorporating the function of Ψ_leaf into the g_m -T model, we suggest that the g_m -T relationship can be influenced by the activation and deactivation energy for membrane permeability, Ψ_leaf gradient between temperatures, and the sensitivity of g_m to Ψ_leaf , below a threshold (Ψ_leaf,0 ). The data presented here suggest that Ψ_leaf plays an important role in the g_m -T relationship and should be considered in future studies related to the temperature response of g_m and photosynthesis.

Accelerating Automated Stomata Analysis Through Simplified Sample Collection and Imaging Techniques

Digital image processing is commonly used in plant health and growth analysis, aiming to improve research efficiency and repeatability. One focus is analysing the morphology of stomata, with the aim to better understand the regulation of gas exchange, its link to photosynthesis and water use and how they are influenced by climatic conditions. Despite the key role played by these cells, their microscopic analysis is largely manual, requiring intricate sample collection, laborious microscope application and the manual operation of a graphical user interface to identify and measure stomata. This research proposes a simple, end-to-end solution which enables automatic analysis of stomata by introducing key changes to imaging techniques, stomata detection as well as stomatal pore area calculation. An optimal procedure was developed for sample collection and imaging by investigating the suitability of using an automatic microscope slide scanner to image nail polish imprints. The use of the slide scanner allows the rapid collection of high-quality images from entire samples with minimal manual effort. A convolutional neural network was used to automatically detect stomata in the input image, achieving average precision, recall and F-score values of 0.79, 0.85, and 0.82 across four plant species. A novel binary segmentation and stomatal cross section analysis method is developed to estimate the pore boundary and calculate the associated area. The pore estimation algorithm correctly identifies stomata pores 73.72% of the time. Ultimately, this research presents a fast and simplified method of stomatal assay generation requiring minimal human intervention, enhancing the speed of acquiring plant health information.

Effect of Cold Stress on Photosynthetic Traits, Carbohydrates, Morphology, and Anatomy in Nine Cultivars of Stevia rebaudiana

Stevia rebaudiana Bertoni is a sweet medicinal herb that is cultivated worldwide. This study aimed to identify the genotypic responses and function of nine cultivars of S. rebaudiana (accession numbers 1-9 from the EUSTAS Stevia Gene Bank) to low temperature. Plants were grown in vitro and incubated under controlled conditions at 5° or 25°C for 1 month. Cold stress significantly decreased the maximum quantum yield of photosystem II (Fv/Fm) in all cultivars, which was more pronounced in cultivars 5, 6, 8, and 9. The efficiency of photosystems I and II (PIABS) also declined in cold-stressed plants and was accompanied by reductions in net photosynthesis (PN), intercellular CO2 (Ci), water use efficiency (WUE), and chlorophyll a, chlorophyll b and carotenoid contents, more so in cultivars 5, 6, 8, and 9. Regardless of the downregulation of photosynthetic capacity, the cold stress increased water-soluble carbohydrates in all cultivars, which was accompanied by an increase in fresh leaf mass and area, more so in cultivars 5, 6, 8, and 9. Furthermore, cold stress increased the stomatal index and density, epidermal cell density, stem diameter, xylem vessel width, phloem tissue width, and number of sclerenchyma in all cultivars. Even though the nine cultivars of S. rebaudiana had lower PSII efficiencies at low temperatures, the increase in carbohydrates and leaf mass suggests that damage to PSII is not responsible for the reduction in its efficiency.