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The temperature sensitivity of physiological processes and growth of tropical trees remains a key uncertainty in predicting how tropical forests will adjust to future climates. In particular, our knowledge regarding warming responses of photosynthesis, and its underlying biochemical mechanisms, is very limited. We grew seedlings of two tropical montane rainforest tree species, the early‐successional species Harungana montana and the late‐successional species Syzygium guineense, at three different sites along an elevation gradient, differing by 6.8℃ in daytime ambient air temperature. Their physiological and growth performance was investigated at each site. The optimum temperature of net photosynthesis (ToptA) did not significantly increase in warm‐grown trees in either species. Similarly, the thermal optima (ToptV and ToptJ) and activation energies (EaV and EaJ) of maximum Rubisco carboxylation capacity (Vcmax) and maximum electron transport rate (Jmax) were largely unaffected by warming. However, Vcmax, Jmax and foliar dark respiration (Rd) at 25℃ were significantly reduced by warming in both species, and this decline was partly associated with concomitant reduction in total leaf nitrogen content. The ratio of Jmax/Vcmax decreased with increasing leaf temperature for both species, but the ratio at 25℃ was constant across sites. Furthermore, in H. montana, stomatal conductance at 25℃ remained constant across the different temperature treatments, while in S. guineense it increased with warming. Total dry biomass increased with warming in H. montana but remained constant in S. guineense. The biomass allocated to roots, stem and leaves was not affected by warming in H. montana, whereas the biomass allocated to roots significantly increased in S. guineense. Overall, our findings show that in these two tropical montane rainforest tree species, the capacity to acclimate the thermal optimum of photosynthesis is limited while warming‐induced reductions in respiration and photosynthetic capacity rates are tightly coupled and linked to responses of leaf nitrogen. [ABSTRACT FROM AUTHOR]

High night temperature (HNT) causes substantial yield loss in rice (Oryza sativa L.). In this study, the physiological processes related to flag leaf dark respiration (Rn) and grain filling under HNT were explored in a multi‐parent advanced generation intercross population developed for heat tolerance (MAGICheat) along with selected high temperature tolerant breeding lines developed with heat‐tolerant parents. Within a subset of lines, flag leaf Rn under HNT treatment was related to lower spikelet number per panicle and thus reduced yield. HNT enhanced the nighttime reduction of non‐structural carbohydrates (NSC) in stem tissue, but not in leaves, and stem nighttime NSC reduction was negatively correlated with yield. Between heading and harvest, the major difference in NSC concentration was found for starch, but not for soluble sugar. HNT weakened the relationship between NSC remobilization and harvest index at both the phenotypic and genetic level. By using genome‐wide association studies, an invertase inhibitor, MADS box transcription factors and a UDP‐glycosyltransferase that were identified as candidate genes orchestrating stem NSC remobilization in the control treatment were lost under HNT. With the identification of physiological and genetic components related to rice HNT response, this study offers promising prebreeding materials and trait targets to sustain yield stability under climate change. Physiological response to high night temperature (HNT) was explored in rice (Oryza sativa L.) heat prebreeding lines. In stems, nighttime reduction of non‐structural carbohydrates, as well as starch remobilization between heading and harvest, was affected by HNT and correlated with yield. [ABSTRACT FROM AUTHOR]

Summary: Leaf respiration in the dark (Rdark) is often measured at a single time during the day, with hot‐acclimation lowering Rdark at a common measuring temperature. However, it is unclear whether the diel cycle influences the extent of thermal acclimation of Rdark, or how temperature and time of day interact to influence respiratory metabolites.To examine these issues, we grew rice under 25°C : 20°C, 30°C : 25°C and 40°C : 35°C day : night cycles, measuring Rdark and changes in metabolites at five time points spanning a single 24‐h period.Rdark differed among the treatments and with time of day. However, there was no significant interaction between time and growth temperature, indicating that the diel cycle does not alter thermal acclimation of Rdark. Amino acids were highly responsive to the diel cycle and growth temperature, and many were negatively correlated with carbohydrates and with organic acids of the tricarboxylic acid (TCA) cycle. Organic TCA intermediates were significantly altered by the diel cycle irrespective of growth temperature, which we attributed to light‐dependent regulatory control of TCA enzyme activities.Collectively, our study shows that environmental disruption of the balance between respiratory substrate supply and demand is corrected for by shifts in TCA‐dependent metabolites. [ABSTRACT FROM AUTHOR]

The engineering process of C4 photosynthesis into C3 plants requires an increased activity of phosphoenolpyruvate carboxylase (PEPC) in the cytosol of leaf mesophyll cells. The literature varies on the physiological effect of transgenic maize (Zea mays) PEPC (ZmPEPC) leaf expression in Oryza sativa (rice). Therefore, to address this issue, leaf–atmosphere CO2 and 13CO2 exchanges were measured, both in the light (at atmospheric O2 partial pressure of 1.84 kPa and at different CO2 levels) and in the dark, in transgenic rice expressing ZmPEPC and wild-type (WT) plants. The in vitro PEPC activity was 25 times higher in the PEPC overexpressing (PEPC-OE) plants (~20% of maize) compared to the negligible activity in WT. In the PEPC-OE plants, the estimated fraction of carboxylation by PEPC (β) was ~6% and leaf net biochemical discrimination against 13CO2 Δ bio was ~ 2‰ lower than in WT. However, there were no differences in leaf net CO2 assimilation rates (A) between genotypes, while the leaf dark respiration rates (Rd) over three hours after light–dark transition were enhanced (~ 30%) and with a higher 13C composition δ 13 C Rd in the PEPC-OE plants compared to WT. These data indicate that ZmPEPC in the PEPC-OE rice plants contributes to leaf carbon metabolism in both the light and in the dark. However, there are some factors, potentially posttranslational regulation and PEP availability, which reduce ZmPEPC activity in vivo. [ABSTRACT FROM AUTHOR]

The influence of reduced glycine decarboxylase complex (GDC) activity on leaf atmosphere CO2 and 13CO2 exchange was tested in transgenic Oryza sativa with the GDC H - subunit knocked down in leaf mesophyll cells. Leaf measurements on transgenic gdch knockdown and wild-type plants were carried out in the light under photorespiratory and low photorespiratory conditions (i.e. 18.4 kPa and 1.84 kPa atmospheric O2 partial pressure, respectively), and in the dark. Under approximately current ambient O2 partial pressure (18.4 kPa p O2), the gdch knockdown plants showed an expected photorespiratory-deficient phenotype, with lower leaf net CO2 assimilation rates (A) than the wild-type. Additionally, under these conditions, the gdch knockdown plants had greater leaf net discrimination against 13CO2 (Δo) than the wild-type. This difference in Δo was in part due to lower 13C photorespiratory fractionation (f) ascribed to alternative decarboxylation of photorespiratory intermediates. Furthermore, the leaf dark respiration rate (R d) was enhanced and the 13CO2 composition of respired CO2 (δ13CRd) showed a tendency to be more depleted in the gdch knockdown plants. These changes in R d and δ13CRd were due to the amount and carbon isotopic composition of substrates available for dark respiration. These results demonstrate that impairment of the photorespiratory pathway affects leaf 13CO2 exchange, particularly the 13C decarboxylation fractionation associated with photorespiration. [ABSTRACT FROM AUTHOR]

In the present study we investigated variations in leaf respiration in darkness (RD) and light (RL), and associated traits in response to season, and along a gradient of soil moisture, in Mediterranean woodland dominated by holm oak (Quercus ilex L.) in central and north-eastern Spain respectively. On seven occasions during the year in the central Spain site, and along the soil moisture gradient in north-eastern Spain, we measured rates of leaf RD, RL (using the Kok method), light-saturated photosynthesis (A) and related light response characteristics, leaf mass per unit area (MA) and leaf nitrogen (N) content. At the central Spain site, significant seasonal changes in soil water content and ambient temperature (T) were associated with changes in MA, foliar N, A and stomatal conductance. RD measured at the prevailing daily T and in instantaneous R-T responses, displayed signs of partial acclimation and was not significantly affected by time of year. RL was always less than, and strongly related to, RD, and RL/RD did not vary significantly or systematically with seasonal changes in T or soil water content. Averaged over the year, RL/RD was 0.66±0.05 s.e. (n = 14) at the central Spain site. At the north-eastern Spain site, the soil moisture gradient was characterised by increasing MA and RD, and reduced foliar N, A, and stomatal conductance as soil water availability decreased. Light inhibition of R occurred across all sites (mean RL/RD= 0.69±0.01 s.e. (n = 18)), resulting in ratios of RL/A being lower than for RD/A. Importantly, the degree of light inhibition was largely insensitive to changes in soil water content. Our findings provide evidence for a relatively constrained degree of light inhibition of R (RL/RD ~ 0.7, or inhibition of ~30%) across gradients of water availability, although the combined impacts of seasonal changes in both T and soil water content increase the range of values expressed. The findings thus have implications in terms of the assumptions made by predictive models that seek to account for light inhibition of R, and for our understanding of how environmental gradients impact on leaf trait relationships in Mediterranean plant communities. [ABSTRACT FROM AUTHOR]

The leaf respiration (RD) of grapevine (Vitis vinifera 'Chasselas') was measured under field conditions during the growing season in leaves of different physiological ages in relation to the temperature and plant water status. RD increased with the temperature and was particularly high in young growing leaves on primary and lateral shoots. The RD response to the temperature evolved over the season according to the type and age of the leaves and their phenology. Leaf aging (senescence) induced a decrease in RD at the end of the season. At constant temperatures (20 °C), the highest RD rates were measured during the rapid plant growth phase (the Q10 values were also the highest), and they progressively decreased to reach their lowest rates at the end of the growing season. The lowest RD values were measured on leaves that were inserted opposite the clusters of primary shoots at any period during the season. Water stress led to a reduction in RD, especially when the leaf temperature was above 20 °C. The nocturnal RD evolution showed that the RD rates were greatest at nightfall when the nocturnal temperatures were still high and leaf carbohydrate availability was at its highest; the rates gradually decreased to reach the lowest RD values just before dawn. [ABSTRACT FROM AUTHOR]

Quercus ilex L. seedlings were exposed in open-top chambers for one growing season to three levels of ozone (O3): charcoal filtered air, non-filtered air supplemented with +30% or +74% ambient air O3. Key functional parameters related to photosynthetic performance and stomatal density were measured to evaluate the response mechanisms of Q. ilex to chronic O3 exposure, clarifying how ecophysiological traits are modulated during the season in an ozone-enriched environment. Dark respiration showed an early response to O3 exposure, increasing approximately 45% relative to charcoal-filtered air in both O3 enriched treatments. However, at the end of the growing season, maximum rate of assimilation (Amax) and stomatal conductance (gs) showed a decline (−13% and −36%, for Amax and gs, respectively) only in plants under higher O3 levels. Photosystem I functionality supported the capacity of Q. ilex to cope with oxidative stress by adjusting the energy flow partitioning inside the photosystems. The response to O3 was also characterised by increased stomatal density in both O3 enriched treatments relative to controls. Our results suggest that in order to improve the reliability of metrics for O3 risk assessment, the seasonal changes in the response of gs and photosynthetic machinery to O3 stress should be considered. [ABSTRACT FROM AUTHOR]

Determining climate change feedbacks from tropical rainforests requires an understanding of how carbon gain through photosynthesis and loss through respiration will be altered. One of the key changes that tropical rainforests may experience under future climate change scenarios is reduced soil moisture availability. In this study we examine if and how both leaf photosynthesis and leaf dark respiration acclimate following more than 12 years of experimental soil moisture deficit, via a through-fall exclusion experiment ( TFE) in an eastern Amazonian rainforest. We find that experimentally drought-stressed trees and taxa maintain the same maximum leaf photosynthetic capacity as trees in corresponding control forest, independent of their susceptibility to drought-induced mortality. We hypothesize that photosynthetic capacity is maintained across all treatments and taxa to take advantage of short-lived periods of high moisture availability, when stomatal conductance ( gs) and photosynthesis can increase rapidly, potentially compensating for reduced assimilate supply at other times. Average leaf dark respiration ( Rd) was elevated in the TFE-treated forest trees relative to the control by 28.2 ± 2.8% (mean ± one standard error). This mean Rd value was dominated by a 48.5 ± 3.6% increase in the Rd of drought-sensitive taxa, and likely reflects the need for additional metabolic support required for stress-related repair, and hydraulic or osmotic maintenance processes. Following soil moisture deficit that is maintained for several years, our data suggest that changes in respiration drive greater shifts in the canopy carbon balance, than changes in photosynthetic capacity. [ABSTRACT FROM AUTHOR]

Accurate evaluation of dark respiration of plants is important for estimation of the plant carbon budget. The response of leaf dark respiration of winter wheat to changes in CO concentration and temperature was studied, using an open top chamber during 2011-2012, to understand how leaf dark respiration of winter wheat will respond to climate change. The results indicated that leaf dark respiration decreased linearly with increased CO concentration. Dark respiration decreased by about 11% under 560 μmol mol CO compared with that under 390 μmol mol CO. Leaf dark respiration showed an exponential relationship with temperature, and the temperature constant ( Q) was close to 2. Moreover, the responses of leaf dark respiration to CO concentration and temperature were independent. A leaf dark respiration model based on CO concentration and temperature responses was developed. This model provides a method for estimation of the leaf dark respiration rate of winter wheat under future climate change and guidance for establishment of crop carbon countermeasures. [ABSTRACT FROM AUTHOR]

In tropical mountains, trees are the dominant life form from sea level to above 4,000-m altitude under highly variable thermal conditions (range of mean annual temperatures: <8 to >28°C). How light-saturated net photosynthesis of tropical trees adapts to variation in temperature, atmospheric CO concentration, and further environmental factors, that change along elevation gradients, is not precisely known. With gas exchange measurements in mature trees, we determined light-saturated net photosynthesis at ambient temperature (T) and [CO] ( A) of 40 tree species from 21 families in tropical mountain forests at 1000-, 2000-, and 3000-m elevation in southern Ecuador. We tested the hypothesis that stand-level averages of A and leaf dark respiration ( R) per leaf area remain constant with elevation. Stand-level means of A were 8.8, 11.3, and 7.2 μmol CO m s; those of R 0.8, 0.6, and 0.7 μmol CO m s at 1000-, 2000-, and 3000-m elevation, respectively, with no significant altitudinal trend. We obtained coefficients of among-species variation in A and R of 20-53% ( n = 10-16 tree species per stand). Examining our data in the context of a pan-tropical A data base for mature tropical trees (c. 170 species from 18 sites at variable elevation) revealed that area-based A decreases in tropical mountains by, on average, 1.3 μmol CO m s per km altitude increase (or by 0.2 μmol CO m s per K temperature decrease). The A decrease occurred despite an increase in leaf mass per area with altitude. Local geological and soil fertility conditions and related foliar N and P concentrations considerably influenced the altitudinal A patterns. We conclude that elevation is an important influencing factor of the photosynthetic activity of tropical trees. Lowered A together with a reduced stand leaf area decrease canopy C gain with elevation in tropical mountains. [ABSTRACT FROM AUTHOR]

1. The Amazon region may experience increasing moisture limitation over this century. Leaf dark respiration ( R) is a key component of the Amazon rain forest carbon (C) cycle, but relatively little is known about its sensitivity to drought. 2. Here, we present measurements of R standardized to 25 °C and leaf morphology from different canopy heights over 5 years at a rain forest subject to a large-scale through-fall reduction (TFR) experiment, and nearby, unmodified Control forest, at the Caxiuanã reserve in the eastern Amazon. 3. In all five post-treatment measurement campaigns, mean R at 25 °C was elevated in the TFR forest compared to the Control forest experiencing normal rainfall. After 5 years of the TFR treatment, R per unit leaf area and mass had increased by 65% and 42%, respectively, relative to pre-treatment means. In contrast, leaf area index ( L) in the TFR forest was consistently lower than the Control, falling by 23% compared to the pre-treatment mean, largely because of a decline in specific leaf area ( S). 4. The consistent and significant effects of the TFR treatment on R, L and S suggest that severe drought events in the Amazon, of the kind that may occur more frequently in future, could cause a substantial increase in canopy carbon dioxide emissions from this ecosystem to the atmosphere. [ABSTRACT FROM AUTHOR]

Contrasting results regarding elevated CO2 effects on leafdark respiration (Rd) have hampered efforts to incorporate this importantcomponent of the plant carbon budget into long-term predictions of ecologicalresponses to rising atmospheric CO2. To help resolve some of theseinconsistencies in the literature, we used meta-analysis to quantitativelysummarize 45 area-based leaf Rd (Rda) and 44 mass-based leaf Rd(Rdm) observations from independent studies on 33 species. Ouranalysis showed that across all studies, leaf Rdm was significantlyreduced (−18%, P < 0.05), while leaf Rda was marginallyincreased (+8%, P < 0.15), under elevated CO2. There weresignificant differences among categorical groups in CO2 effects onleaf Rda and Rdm. For example, leaf Rda ofherbaceous species increased 28%, but leaf Rda of woody speciesremained unchanged under elevated CO2. Plants exposed to elevatedCO2 for < 60 days had significantly higher leaf Rda atelevated compared to ambient CO2, while plants exposed to elevatedCO2 for longer period of time showed no response. The magnitude ofreduction in leaf Rdm for plants exposed to elevated CO2for > 100 days was significantly greater than that for plants exposed toelevated CO2 for < 100 days. Our meta-analysis of publishedresults suggest that the amount of carbon loss through leaf Rd will likelyincrease in a higher CO2 environment because of higher leafRda and a proportionally greater leaf biomass increase than leafRdm reduction at elevated CO2. Our results alsodemonstrated the strong dependency of Rd responses to elevated CO2onexperimental conditions. [ABSTRACT FROM AUTHOR]

Summary • Dioecious species represent an important component of terrestrial ecosystems, but little is known about gender-specific responses to elevated atmospheric CO2. • In an open-top chamber experiment carried out in Michigan, USA, the physiological and growth responses were studied of male and female Populus tremuloides to elevated CO2 and soil nitrogen concentrations. • Male trees had a higher net photosynthetic rate than female trees, but the difference was greater at elevated (25%) than at ambient (13%) CO2. Leaf dark respiration, averaged across the growing season, tended to be higher in males than in females, and increased significantly in male and female trees with CO2 enrichment. Female trees had higher total biomass than male trees grown in low-nitrogen soil and at ambient CO2, but not in other treatments. Elevated CO2 increased the total biomass of males by 58–66% and of females by 22–70%. • Differing physiological and growth responses to CO2 enrichment by male and female trees should be taken into consideration when predicting the effects of global environmental changes on forest ecosystem structure and functioning. [ABSTRACT FROM AUTHOR]