Photosynthetic Decline Research Articles

Transcriptome and proteome analyses reveal high nitrate or ammonium applications alleviate photosynthetic decline of Phoebe bournei seedlings under elevated carbon dioxide by regulating glnA and rbcS.

The online version contains supplementary material available at 10.1007/s12298-024-01481-2.

The class B heat shock factor RcHsf17 from Rosa chinensis enhances basal thermotolerance in Rosa rugosa

Rosa rugosa, a species of notable economic importance, faces cultivation challenges attributed to its vulnerability to thermal stress. In this study, we report the molecular cloning and functional characterization of RcHsf17, a heat shock transcription factor (Hsf) from group B2, derived from the stress-resilient cultivar of Chinese rose, R. chinensis ‘Slater's Crimson China’. RcHsf17, which localizes to the nucleus, is responsive to heat stress and various stress-related stimuli, such as abscisic acid (ABA), jasmonic acid (JA), salicylic acid (SA), and hydrogen peroxide (H2O2). The overexpression of RcHsf17 in transgenic Arabidopsis thaliana and R. rugosa significantly enhanced their thermotolerance. In R. rugosa, overexpression of RcHsf17 led to cell wall thickening, sustained photosynthetic efficiency, and decreased levels of reactive oxygen species (ROS) and malondialdehyde (MDA). Transcriptome analysis revealed that RcHsf17 upregulated the expression of other Hsfs such as HsfA3 and HsfB1, small heat shock protein genes (sHSPs), and genes involved in cell wall biosynthesis under normal conditions. Under heat stress condition, there is a significant upregulation of HsfA6B, gene associated with photosynthesis, and genes responsible for encoding antioxidant enzymes. These findings collectively emphasize the pivotal role of RcHsf17 in conferring thermotolerance to R. rugosa, through mechanisms that include strengthening cell wall integrity, alleviating heat-induced photosynthetic decline, and enhancing the efficiency of ROS detoxification pathways.

Drought-induced photosynthetic decline and recruitment losses are mediated by light microenvironment in Rocky Mountain subalpine forest tree seedlings

Rates of tree mortality continue to rise with climate change, particularly in high elevation subalpine forests. In these systems, which depend on a narrow range of microsite conditions for successful seedling establishment, growing drought frequency and intensity may impose regeneration bottlenecks that further exacerbate population declines. However, it is unclear how regeneration processes tied to species-specific stress tolerances and microsite preferences are affected by varying magnitudes and durations of drought. To simulate moderate (50%) and severe (100% precipitation exclusion) growing-season drought, we installed precipitation exclusion shelters in shade and canopy gap light microenvironments over naturally-regenerating seedlings across a range of size-delimited establishment cohorts spanning first year emergents to large and well-established seedlings of two dominant subalpine forest species – Engelmann spruce (Picea engelmannii Parry ex Engelmann) and subalpine fir (Abies lasiocarpa (Hook) Nutt.). Seedling survival and gas exchange physiology was assessed to test the interacting effects of drought and light microenvironment on seedling performance, with contrasts to each species’ historically preferred microsite under ambient precipitation conditions. Shade partially ameliorated drought effects when precipitation reductions were moderate as illustrated by rates of seedling mortality that were statistically indistinguishable from reference rates for both species, whereas moderate drought in canopy gaps significantly or nearly-significantly increased seedling mortality in several cohorts of both spruce and fir (1.9–2.9 and 2.3–7.5 times greater mortality risk in spruce and fir, respectively). When drought was severe, however, shade induced prolonged net-negative photosynthetic carbon assimilation as low as a −1.5 μmol CO2·m−2·s−1 (growing-season average), which occurred alongside significantly increased mortality in spruce (3.2–9.4 times greater mortality risk). In contrast, mortality was significantly higher in canopy gaps under severe drought for fir (5.5–23.1 times greater mortality risk). Larger seedling size was strongly associated with lower mortality risk and more conservative stomatal behavior (e.g., predominantly <0.1 mol·H2O·m−2·s−1 growing-season averages), illustrating the sensitivity of new seedlings to microsite environments. However, persistent declines in photosynthetic carbon uptake observed across all seedling sizes in shaded microsites suggests the potential for lagged mortality and greater susceptibility to recurring drought even for larger and seemingly well-established seedlings. These results demonstrate how acute, intense droughts may alter historical patterns of spruce-fir seedling regeneration and highlight the importance of maintaining suitable microsites when implementing adaptive management strategies to avoid regeneration bottlenecks in forests that are already vulnerable to accelerating drought-induced mortality.

Exogenous monoterpenes mitigate H2O2-induced lipid damage but do not attenuate photosynthetic decline during water deficit in tomato.

Although monoterpenes are suggested to mediate oxidative status, their role in abiotic stress responses is currently unclear. A foliar spray of monoterpenes increased antioxidant capacity and decreased oxidative stress of Solanum lycopersicum under water deficit stress. The foliar content of monoterpenes increased with spray concentration indicating foliar uptake of exogenous monoterpenes. Exogenous monoterpene application substantially decreased foliar accumulation of hydrogen peroxide (H2O2) and lipid peroxidation (malondialdehyde, MDA). However, it appears that monoterpenes prevent the accumulation of reactive oxygen species rather than mitigating subsequent ROS-induced damage. Low spray concentration (1.25mM) proved most effective in decreasing oxidative stress but did not up-regulate the activity of key antioxidant enzymes (superoxide dismutase and ascorbate peroxidase) even though higher (2.5 and 5mM) spray concentrations did, suggesting a complex role for monoterpenes in mediating antioxidant processes. Furthermore, soil drying caused similar photosynthetic limitations in all plants irrespective of monoterpene treatments, apparently driven by strong reductions in stomatal conductance as Photosystem II efficiency only decreased in very dry soil. We suggest that exogenous monoterpenes may mitigate drought-induced oxidative stress by direct quenching and/or upregulating endogenous antioxidative processes. The protective properties of specific monoterpenes and endogenous antioxidants require further investigation.

Drought susceptibility of southern African C4 grasses: Phylogenetically and photosynthetically determined?

Abstract Factors that determine C4 grass distributions have been well documented, with evidence in the literature for C4 photosynthetic subtypes displaying varying levels of drought susceptibility. However, the interactions between C4 photosynthetic subtype and phylogeny add complexity and are relatively under studied. We use species distribution modelling to determine the influence of rainfall on distribution patterns of representative C4 grass families and subtypes. Select C4 grass species, representing different photosynthetic subtypes (NADP‐Me and NAD‐Me) and lineages (Panicoideae and Aristidoideae), were subjected to a progressive 58‐day drought period and recovery phase, to explore drought responses through leaf water relations, gas exchange and chlorophyll fluorescence. We show Panicoideae NADP‐Me species to be more susceptible to drought than both Panicoideae NAD‐Me and Aristidoideae NADP‐Me species due to apparent greater metabolic impairment. The differences between groups were related to how rapidly photosynthesis declines with exposure to drought and the rate of recovery postdrought, rather than the maximum extent of photosynthetic decline. The mechanisms for the relative maintenance of plant water status differed between the Panicoideae NAD‐Me species, which utilized greater stomatal control, and the Aristidoideae NADP‐Me species, which maintained water uptake through osmotic adjustment. Synthesis. We show here that drought susceptibility differs both phylogenetically and according to photosynthetic subtype, but that the role of phylogeny may outweigh physiological control. This research adds novel insight into the physiological differences behind observed rainfall‐related differences in C4 grass distribution patterns. Read the free Plain Language Summary for this article on the Journal blog.

Seasonal decline in leaf photosynthesis in perennial switchgrass explained by sink limitations and water deficit.

Leaf photosynthesis of perennial grasses usually decreases markedly from early to late summer, even when the canopy remains green and environmental conditions are favorable for photosynthesis. Understanding the physiological basis of this photosynthetic decline reveals the potential for yield improvement. We tested the association of seasonal photosynthetic decline in switchgrass (Panicum virgatum L.) with water availability by comparing plants experiencing ambient rainfall with plants in a rainfall exclusion experiment in Michigan, USA. For switchgrass exposed to ambient rainfall, daily net CO2 assimilation ( ) declined from 0.9 mol CO2 m-2 day-1 in early summer to 0.43 mol CO2 m-2 day-1 in late summer (53% reduction; P<0.0001). Under rainfall exclusion shelters, soil water content was 73% lower and was 12% and 26% lower in July and September, respectively, compared to those of the rainfed plants. Despite these differences, the seasonal photosynthetic decline was similar in the season-long rainfall exclusion compared to the rainfed plants; in switchgrass under the shelters declined from 0.85 mol CO2 m-2 day-1 in early summer to 0.39 mol CO2 m-2 day-1 (54% reduction; P<0.0001) in late summer. These results suggest that while water deficit limited late in the season, abundant late-season rainfalls were not enough to restore in the rainfed plants to early-summer values suggesting water deficit was not the sole driver of the decline. Alongside change in photosynthesis, starch in the rhizomes increased 4-fold (P<0.0001) and stabilized when leaf photosynthesis reached constant low values. Additionally, water limitation under shelters had no negative effects on the timing of rhizome starch accumulation, and rhizome starch content increased ~ 6-fold. These results showed that rhizomes also affect leaf photosynthesis during the growing season. Towards the end of the growing season, when vegetative growth is completed and rhizome reserves are filled, diminishing rhizome sink activity likely explained the observed photosynthetic declines in plants under both ambient and reduced water availability.

Landscape gradient of autumn photosynthetic decline in Abies sachalinensis seedlings

Understanding what environmental factors are genetically linked to a phenological event is critical for predicting responses to climate change. Photosynthetic phenology often varies among a species of evergreen conifers due to local adaptation. However, few empirical studies have revealed relevant relationships between climatic factors in provenance environments and photosynthetic phenology. This study evaluated the effects of environmental conditions of the growing site and seed source provenance on the seasonal changes in maximal photochemical quantum yield of photosystem II (Fv/Fm) in a common garden experiment with 2-year-old seedlings of Sakhalin fir (Abies sachalinensis), a representative species with local adaptation, from four seed source provenances. A logistic model was constructed to explain the seasonal variation of Fv/Fm from July to October and the relationships between the estimated model parameters and representative factors featuring provenance environments were evaluated. The landscape gradient of the detected model parameters responsible for the provenance environments was visualized in a map of the distribution area. The lowest temperature was the most plausible factor in the growing environment to explain the seasonal changes of Fv/Fm. Among the representative meteorological factors of provenance environments, the lowest temperatures in July showed significant relationships with two model parameters, explaining the lower limit of Fv/Fm and the higher sensitivity of autumn Fv/Fm decline. The estimated spatial maps of model parameters consistently showed that the higher the lowest temperature in July in the provenance environment, the lower the Fv/Fm in October and the greater the decrease in the autumn Fv/Fm decline. Therefore, the lowest summer temperature could be associated with the local adaptation of autumn photosynthetic phenology in A. sachalinensis.

Alleviation of salinity induced growth and photosynthetic decline in wheat due to biochar and jasmonic acid application involves up-regulation of ascorbate-glutathione pathway, glyoxylase system and secondary metabolite accumulation

Combined application of biochar and jasmonic acid for inducing salt stress tolerance in wheat. A 100 mM salt stress was applied fifteen days after germination. biochar (BC, 5%) and jasmonic acid (JA, 5 μM) was applied in individual and in joint applications. The results revealed that growth traits, contents of δ-Amino levulinic acid, glutamate 1-semialdehyde, total chlorophylls and carotenoids, nutrients, nitrate reductase and leaf gas exchange traits were reduced under salt stress. Salt stress increased oxidative stress biomarkers and enzymes of glyoxylase system considerably. It was found that the single and joint application of biochar or jasmonic acid alleviated the decline with maximal alleviation observed in biochar + jasmonic acid treated plants by preventing the oxidative damage and boosting the activities of antioxidant enzymes and enzymes of glyoxylase system. The accumulation of osmolytes and secondary metabolites was more evident under joint biochar and jasmonic acid treatments than individual under NaCl stress. The contents of reducing agents (ascorbate and glutathione) were orchestrated under joint biochar and jasmonic acid application. Nevertheless, the decline in the activity of nitrate reductase and nutrient contents was maximally alleviated under joint application of BC and JA. Therefore, it can be concluded that combined application of biochar and jasmonic acid can be exploited for improved growth and photosynthesis and salinity stress tolerance in wheat and perhaps in other cereal crops.

Photosynthetic decline in aging perennial grass is not fully explained by leaf nitrogen.

Aging in perennial plants is traditionally observed in terms of changes in end-of-season biomass; however, the driving phenological and physiological changes are poorly understood. We found that 3-year-old (mature) stands of the perennial grass Miscanthus×giganteus had 19-30% lower Anet than 1-year-old M.×giganteus (juvenile) stands; 10-34% lower maximum carboxylation rates of Rubisco and 34% lower light-saturated Anet (Asat). These changes could be related to nitrogen (N) limitations, as mature plants were larger and had 14-34% lower leaf N on an area basis (Na) than juveniles. However, N fertilization restored Na to juvenile levels but compensated only 50% of the observed decline in leaf photosynthesis with age. Comparison of leaf photosynthesis per unit of leaf N (PNUE) showed that mature stands had at least 26% lower PNUE than juvenile stands across all N fertilization rates, suggesting that other factors, besides N, may be limiting photosynthesis in mature stands. We hypothesize that sink limitations in mature stands could be causing feedback inhibition of photosynthesis which is associated with the age-related decline in photosynthesis.

Carbon Source Reduction Postpones Autumn Leaf Senescence in a Widespread Deciduous Tree.

The growing-season length of temperate and boreal trees has a strong effect on the global carbon cycle. Yet, a poor understanding of the drivers of phenological processes, such as autumn leaf senescence in deciduous trees, limits our capacity to estimate growing-season lengths under climate change. While temperature has been shown to be an important driver of autumn leaf senescence, carbon source–sink dynamics have been proposed as a mechanism that could help explain variation of this important process. According to the carbon sink limitation hypothesis, senescence is regulated by the interplay between plant carbon source and sink dynamics, so that senescence occurs later upon low carbon inputs (source) and earlier upon low carbon demand (sink). Here, we manipulated carbon source–sink dynamics in birch saplings (Betula pendula) to test the relevance of carbon sink limitation for autumn leaf senescence and photosynthetic decline in a widespread deciduous tree. Specifically, we conducted a gradient of leaf and bud removal treatments and monitored the effects on autumnal declines in net photosynthesis and the timing of leaf senescence. In line with the carbon sink limitation hypothesis, we observed that leaf removal tended to increase total leaf-level autumn photosynthesis and delayed the timing of senescence. Conversely, we did not observe an effect of bud removal on either photosynthesis or senescence, which was likely caused by the fact that our bud removal treatment did not considerably affect the plant carbon sink. While we cannot fully rule out that the observed effect of leaf removal was influenced by possible treatment-level differences in leaf age or soil resource availability, our results provide support for the hypothesis of carbon sink limitation as a driver of growing-season length and move the scientific field closer to narrowing the uncertainty in climate change predictions.

Pathogen-induced hydraulic decline limits photosynthesis and starch storage in grapevines (Vitis sp.).

Xylella fastidiosa (Xf) is the bacterial pathogen responsible for Pierce's Disease (PD) in grapevine (Vitis vinifera L.) and numerous diseases in agriculturally and ecologically important species. Current theory suggests that localized inoculations via insect feeding lead to bacterial spread through the xylem, reducing water transport capacity, leading to declines in productivity, and ultimately death. Yet, the underlying mechanisms of Xf-induced mortality are not fully understood. In this study, we documented the development of PD symptoms over 12-13 weeks postinoculation. Subsequently assessed photosynthetic capacity, starch storage, and stem hydraulics in four grapevine genotypes (two PD-resistant and two PD-susceptible), comparing those physiological changes to control plants. PD-susceptible genotypes showed a coordinated decline in photosynthesis, starch storage, and stem hydraulics, whereas Xf-inoculation led to no change in starch and stem hydraulics in the PD-resistant genotypes. Together these data support the idea of a link between loss of hydraulic conductivity due to tylosis production with a downstream photosynthetic decline and starch depletion in the PD-susceptible genotypes. Our data support the theory that hydraulic failure and carbon starvation underlie plant mortality resulting from PD.

Is foliar Cl- concentration the cause of photosynthetic decline in grapevine during mild salinity?

Moderate levels of Cl- have been associated with grapevine salt tolerance. The hypothesis to be tested in this work is: photosynthesis in grapevine is negatively correlated with foliar Cl- concentration. To further test this hypothesis, multiple mild salinity experiments on four different Vitis genotypes (Cabernet-Sauvignon, Riparia Gloire, Ramsey and SC2) were conducted and photosynthesis, ion concentrations and gene expression responses were quantified. The salt-tolerant rootstock Ramsey had greater Cl- exclusion capabilities than V. vinifera cultivars both during rooted cutting greenhouse experiments and three years of field-grafted experiments; SC2 also excluded Cl-. Differential gene expression indicated that salinity affected transcript abundance more in salt-sensitive genotypes (97.7 % of DEGs in the dataset), especially chloroplast-related transcripts. The transcript abundances of known anion transporters were determined and a family of putative B transporters was associated with the Cl- exclusion phenotype. Photosynthesis and growth were maintained in Ramsey and SC2 under mild salinity. However, photosynthesis declined in Cabernet-Sauvignon with isosmotic 20 mM salt concentrations of NaCl, KCl or NaNO3, independent of the salt type. While foliar Cl- concentrations did correlate with salt tolerance during control and NaCl conditions, it was not found to be the cause of photosynthetic decline in Vitis during mild salinity.

Summer temperatures reach the thermal tolerance threshold of photosynthetic decline in temperate conifers.

Climate change-related environmental stress has been recognized as a driving force in accelerating forest mortality over the last decades in Central Europe. Here, we aim to elucidate the thermal sensitivity of three native conifer species, namely Norway spruce (Picea abies), Scots pine (Pinus sylvestris) and silver fir (Abies alba), and three non-native species, namely Austrian pine (Pinus nigra), Douglas fir (Pseudotsuga menziesii) and Atlas cedar (Cedrus atlantica). Thermal sensitivity, defined here as a decline of the maximum quantum yield of photosystem II (Fv /Fm ) with increasing temperature, was measured under varying levels of heat stress and compared with the turgor loss point (πtlp ) as a drought resistance trait. We calculated three different leaf thermotolerance traits: the temperature at the onset (5%) of the Fv /Fm decline (T5), the temperature at which Fv /Fm was half the maximum value (T50) and the temperature at which only 5% Fv /Fm remained (T95). T5 ranged from 38.5 ± 0.8 °C to 43.1 ± 0.6 °C across all species, while T50 values were at least 9 to 11 degrees above the maximum air temperatures on record for all species. Only Austrian pine had a notably higher T5 value than recorded maximum air temperatures. Species with higher T5 values were characterized by a less negative πtlp compared to species with lower T5. The six species could be divided into 'drought-tolerant heat-sensitive' and 'drought-sensitive heat-tolerant' groups. Exposure to short-term high temperatures thus exhibits a considerable threat to conifer species in Central European forest production systems.

Down-Regulation of Photosynthetic Electron Transport and Decline in CO2 Assimilation under Low Frequencies of Pulsed Lights.

The decline in CO2 assimilation in leaves exposed to decreasing frequencies of pulsed light is well characterized, in contrast to the regulation of photosynthetic electron transport under these conditions. Thus, we exposed sunflower leaves to pulsed lights of different frequencies but with the same duty ratio (25%) and averaged light intensity (575 μmoles photons m−2 s−1). The rates of net photosynthesis Pn were constant from 125 to 10 Hz, and declined by 70% from 10 to 0.1 Hz. This decline coincided with (1) a marked increase in nonphotochemical quenching (NPQ), and (2) the completion after 25 ms of illumination of the first phase of P700 photooxidation, the primary electron donor of PSI. Under longer light pulses (<5 Hz), there was a slower and larger P700 photooxidation phase that could be attributed to the larger NPQ and to a resistance of electron flow on the PSI donor side indicated by 44% slower kinetics of a P700+ dark reduction. In addition, at low frequencies, the decrease in quantum yield of photochemistry was 2.3-times larger for PSII than for PSI. Globally, our results indicate that the decline in CO2 assimilation at 10 Hz and lower frequencies coincide with the formation of NPQ and a restriction of electron flows toward PSI, favoring the accumulation of harmless P700+.

Measurement of photosynthesis in excised leaves of ornamental trees: a novel method to estimate leaf level drought tolerance and increase experimental sample size

Measuring gas exchange on excised ornamental tree leaves may be a rapid and accurate method to study leaf-level drought tolerance, and greatly increase experiment sample size. Drought is a common phenomenon in arid and semi-arid urban environments. Because water is valuable and limited in these environments, it is crucial to study drought tolerance of ornamental tree species used in urban forests. In a first of its kind study, we observed leaf-level drought tolerance using photosynthetic decline curves in excised leaves of seven ornamental urban tree species: Shantung maple (Acer truncatum), Mexican redbud (Cercis canadensis mexicana), Texas redbud (C. canadensis texensis), White redbud (C. canadensis texensis ‘Alba’), Oklahoma redbud (C. canadensis texensis ‘Oklahoma’), Chinquapin oak (Quercus muehlenbergii), and English oak (Q. robur). For both excised and non-excised treatments, two leaves from the same tree were simultaneously clamped into cuvettes of two Li-Cor LI 6400 XT portable photosynthetic systems. For the excision treatment, the leaf was excised from the branch at 120 s. By comparing photosynthetic decline curves from excised leaves with their non-excised counterparts, we found Mexican redbud and White redbud leaves take a longer period of time (450 s. and 360 s., respectively) compared to English oak leaves (90 s.) to show a significant decline in photosynthetic gas exchange. This technique also proposes a solution to a common problem faced by many plant researchers. While using the leaf excision technique on large trees, we were able to significantly increase sample size with rapidity and accuracy. Thus, we suggest measuring gas exchange on excised leaves of select ornamental tree species as a rapid and accurate method to estimate leaf-level drought tolerance, and greatly increase the number of samples measured in a given time period.

AMF inoculation and phosphorus supplementation alleviates drought induced growth and photosynthetic decline in Nicotiana tabacum by up-regulating antioxidant metabolism and osmolyte accumulation

Arbuscular mycorrhizal fungi (AMF) improves phosphorus (P) uptake in plants and regulates growth under environmental stresses. Present trials were undertaken to study the effect of phosphorus (P, 40 mg kg−1 soil) supplementation and AMF (Glomus versiforme) inoculation on the growth performance of tobacco (Nicotiana tabacum L. variety Yunyan 87) under drought stress. Supplementation of P and inoculation of AMF significantly enhanced growth, biomass accumulation and root activity of tobacco plants under normal conditions and mitigated the drought induced decline. Chlorophyll and carotenoid contents, photosynthesis and PSII efficiency increased due to AMF and P treatment attaining maximal values due to their combined treatment. Drought resulted in generation of reactive oxygen species (ROS) like superoxide (O2−) and hydrogen peroxide (H2O2) resulting in increased lipid peroxidation hence reducing membrane stability. Plants inoculated with AMF and/or supplemented with P exhibited reduced accumulation of ROS and lipid peroxidation. Content of osmolytes including proline, sugars and free amino acids were enhanced due to P and AMF treatment under normal as well as drought conditions that influenced the leaf relative water content. Reduction in oxidative damage in AMF and P treated plants was apparently linked with the stimulation of antioxidant system in them. Activities of SOD, CAT, APX, POD and GR, and the content of AsA and GSH improved significantly due to P and AMF treatment both under normal and drought conditions thereby ameliorating the oxidative effects. Further, accumulation of phenols increased more obviously due to P and/or AMF inoculation than in drought. Increased osmolyte and phenol content due to P and/or AMF helped tobacco plants to counteract the drought mediated damage by strengthening the non-enzymatic antioxidant system. P and/or AMF ameliorated the decline in contents of nitrogen, potassium and phosphorous, and activity of nitrate reductase more due to their combined treatments. Drought induced decline in nicotine content was mitigated because of P and/or AMF treatment. Additionally, mycorrhizal inoculation along with P supplementation increased indole-3-acetic acid (IAA) and abscisic acid (ABA) concentrations both in roots and leaves under drought stress. Hence P supplementation along with AMF inoculation mitigates the photosynthetic alteration and growth in tobacco plants by up-regulating antioxidant system and osmolyte accumulation.

Eco-Physiological Screening of Different Tomato Genotypes in Response to High Temperatures: A Combined Field-to-Laboratory Approach.

High temperatures represent a limitation for growth and development of many crop species. Several studies have demonstrated that the yield reduction of tomato under high temperatures and drought is mainly due to a photosynthetic decline. In this paper, a set of 15 tomato genotypes were screened for tolerance to elevated temperatures by cultivating plants under plastic walk-in tunnels. To assess the potential tolerance of tomato genotypes to high temperatures, measurements of chlorophyll fluorescence, pigments content and leaf functional traits have been carried out together with the evaluation of the final yields. Based on the greenhouse trials, a group of eight putative heat-sensitive and heat-tolerant tomato genotypes was selected for laboratory experiments aimed at investigating the effects of short-term high temperatures treatments in controlled conditions. The chlorophyll fluorescence induction kinetics were recorded on detached leaves treated for 60 min at 35 °C or at 45 °C. The last treatment significantly affected the photosystem II (PSII) photochemical efficiency (namely maximum PSII quantum efficiency, Fv/Fm, and quantum yield of PSII electron transport, ΦPSII) and the non-photochemical quenching (NPQ) in the majority of genotypes. The short-term heat shock treatments also led to significant differences in the shape of the slow Kautsky kinetics and its significant time points (chlorophyll fluorescence levels minimum O, peak P, semi-steady state S, maximum M, terminal steady state T) compared to the control, demonstrating heat shock-induced changes in PSII functionality. Genotypes potentially tolerant to high temperatures have been identified. Our findings support the idea that chlorophyll fluorescence parameters (i.e., ΦPSII or NPQ) and some leaf functional traits may be used as a tool to detect high temperatures-tolerant tomato cultivars.

High Nitrate or Ammonium Applications Alleviated Photosynthetic Decline of Phoebe bournei Seedlings under Elevated Carbon Dioxide

Phoebe bournei is a precioustimber species and is listed as a national secondary protection plant in China. However, seedlings show obvious photosynthetic declinewhen grown long-term under an elevated CO2 concentration (eCO2). The global CO2 concentration is predicted to reach 700 μmol·mol−1 by the end of this century; however, little is known about what causes the photosynthetic decline of P. bournei seedlings under eCO2 or whether this photosynthetic decline could be controlled by fertilization measures. To explore this problem, one-year-old P. bournei seedlings were grown in an open-top air chamber under either an ambient CO2 (aCO2) concentration (350 ± 70 μmol·mol−1) or an eCO2 concentration (700 ± 10 μmol·mol−1) from June 12th to September 8th and cultivated in soil treated with either moderate (0.8 g per seedling) or high applications (1.2 g per seedling) of nitrate or ammonium. Under eCO2, the net photosynthetic rate (Pn) of P. bournei seedlings treated with a moderate nitrate application was 27.0% lower than that of seedlings grown under an aCO2 concentration (p &lt; 0.05), and photosynthetic declineappeared to be accompanied by a reduction of the electron transport rate (ETR), actual photochemical efficiency, chlorophyll content, ribulose-1,5-bisphosphate carboxylase/oxygenase (rubisco), rubisco activase (RCA) content, leaf thickness, and stomatal density. The Pn of seedlings treated with a high application of nitrate under eCO2 was 5.0% lower than that of seedlings grown under aCO2 (p &gt; 0.05), and photosynthetic declineoccurred more slowly, accompanied by a significant increase in rubisco content, RCA content, and stomatal density. The Pn of P. bournei seedlings treated with either a moderate or a high application of ammonium and grown under eCO2 was not significantly differentto that of seedlings grown under aCO2—there was no photosynthetic decline—and the ETR, chlorophyll content, rubisco content, RCA content, and leaf thickness values were all increased. Increasing the application of nitrate or the supply of ammonium could slow down or prevent the photosynthetic declineof P. bournei seedlings under eCO2 by changing the leaf structure and photosynthetic physiological characteristics.

Effects of elevated atmospheric CO2 on leaf gas exchange response to progressive drought in barley and tomato plants with different endogenous ABA levels

ABA plays an important role in modulating stomatal response to drought and elevated atmospheric CO2 (e [CO2]). This study aimed to investigate the effect of e[CO2] on the response of leaf gas exchange and plant water relations of barley and tomato plants with different endogenous ABA levels to progressive soil drying. Barley and tomato plants were grown in ambient (a[CO2], 400 ppm) and e[CO2] (800 ppm) and subjected to progressive drought stress. Wild type (WT) genotypes (Steptoe barley and AC tomato) and their ABA-deficient mutants (Az34 barley and flacca) were examined. e[CO2] sensitized the photosynthetic decline with soil drying. Soil-drying induced stomatal closure was affected by [CO2] in WT genotypes, where e[CO2] sensitized stomatal closure in barley but retarded it in tomato, whereas such effects were absent in mutants. Compared to a[CO2], e[CO2] maintained leaf water potential and improved turgor pressure except in the flacca mutant. For the WT genotypes, the stomata became less sensitive to an increase in leaf ABA concentration ([ABA]leaf) under e[CO2] than a[CO2]; while for both mutants, the stomata was predominately controlled by leaf turgor and not an increase in [ABA]leaf during soil drying. Endogenous ABA level played an important role in modulating the effect of e[CO2] on stomatal response to soil drying. These findings improve our understanding of the mechanisms of stomatal control in monocot and dicot species responding to a future drier and CO2-enriched environment.

Within-Canopy Experimental Leaf Warming Induces Photosynthetic Decline Instead of Acclimation in Two Northern Hardwood Species

Northern hardwood forests are experiencing higher temperatures and more extreme heat waves, potentially altering plant physiological processes. We implemented in-situ leaf-level warming along a vertical gradient within a mature forest canopy to investigate photosynthetic acclimation potential of two northern hardwood species, Acer saccharum and Tilia americana. After seven days of +3 °C warming, photosynthetic acclimation was assessed by measuring differences between heated and control photosynthetic rates (Aopt) at leaf optimum temperatures (Topt). We also measured the effects of warming and height on maximum rate of Rubisco carboxylation, stomatal conductance, transpiration, and leaf traits: leaf area, leaf mass per area, leaf nitrogen, and leaf water content. We found no evidence of photosynthetic acclimation for either species, but rather Aopt declined with warming overall. We found slight shifts in LMA and Narea, leaf traits associated with photosynthetic capacity, after one week of experimental warming. T. americana LMA and Narea was lower in the upper canopy heated leaves than in the control leaves, contributing a shift in Narea height distribution in the heated leaves. T. americana showed evidence of greater resiliency to warming, with greater thermoregulation, physiological plasticity, and evapotranspiration. As expected, Aopt of A. saccharum increased with height, but Aopt of T. americana was highest in the sub canopy, possibly due to constraints on leaf water balance and photosynthetic capacity in the upper canopy. Thus, models relying on canopy height or light environment may incorrectly estimate vertical variation of photosynthetic capacity. If these species are not able to acclimate to warmer temperatures, we could see alteration of plant carbon balance of these two key northern hardwood species.