Reduction In Photosynthetic Capacity Research Articles

Probing ozone effects on European hornbeam (Carpinus betulus L. and Ostrya carpinifolia Scop.) leaf water content through THz imaging and dynamic stomatal response

We investigated the impact of ozone exposure on Hornbeam using a novel dual approach based on Terahertz (THz) imaging in a free-air ozone exposure experiment (three ozone levels: ambient; 1.5 times ambient; twice ambient). The research aims at unraveling the physiological responses induced by elevated ozone levels on water dynamics. THz imaging unveiled dynamic changes in leaf water content, providing a non-invasive approach to leaf water monitoring. Leaf gas exchange measurements assessed stomatal responses to light variation. Our findings showcase a compelling correlation between elevated ozone levels and reduction in photosynthetic rate and impairment of stomatal function, i.e. “stomatal sluggishness”, indicative of nuanced regulatory mechanism. Stomatal sluggishness was particularly evident in Carpinus betulus (CB) compared to Ostrya carpinifolia (OC) and was linked to reduction in photosynthetic capacity. THz-based imaging techniques confirmed this result indicating a negative effect of O3 on leaf-level total water content. In addition, spatial analysis of leaf water status using these techniques also highlighted that the negative effect of O3 on water status was progressing even in less sensitive OC plants though visible foliar injury was not detected. In fact, OC showed a relative dry area of 1.6 ± 1.6 % in the control group and 3.8 ± 1.3 % under high ozone levels. THz-based imaging techniques provided a deep understanding of O3 behavior in plants and may be recommended for precision biosensing in the early detection of O3-induced damage. The integration of THz imaging and physiological analysis resulted in comprehensive understanding of Hornbeam acclimation response to ozone exposure.

How does UV-B radiation influence the photosynthesis and secondary metabolism of Schisandra chinensis leaves?

Schisandra chinensis (Trucz.) is a medicinal plant with a rich profile of physiologically active substances, including lignans. The levels of these substances can be affected by external factors such as light exposure, temperature, and humidity. The current study aimed to explore the impacts of UV-B radiation on the photosynthetic properties and biochemical parameters as well as the contents of major lignans and phenolic compounds in two-year-old S. chinensis plants. The results revealed that exposure to UV-B radiation (10.0 μW cm−2, 30 min per day) did not cause any significant changes in the photosynthetic properties of S. chinensis plants until the 7th or 15th days of treatment. However, a significant reduction in photosynthetic capacity was observed after 30 days of UV-B exposure. The contents of photosynthetic pigments including chlorophyll a, b, total chlorophyll, and carotenoid decreased under UV-B treatment. Besides, we found that the highest level of lignan synthesis, a major bioactive compound in S. chinensis, occurred after seven days of UV-B treatment. However, lignan levels gradually decreased over time, with most individual lignans either having similar or lower contents than the control at 30 days. Furthermore, a comprehensive analysis of 35 phenolic compounds present in S. chinensis leaves showed that quercetin, isoquercetin, and 4-hydroxycinnamic acid were critical phenolic compounds responsible for moderating the plant's response to ultraviolet stress. Consistent with the trend of lignan content, these phenolic compounds exhibited the highest levels after seven days of UV-B treatment and gradually decreased over time. Our findings demonstrate that an appropriate dose of UV-B exposure can stimulate the synthesis of bioactive compounds such as lignans and phenolics via activation of the phenylpropanoid pathway in S. chinensis, enhancing its medicinal value.

Post-veraison increase in source-sink ratio via manipulation of sink availability gradually reduces leaf functionality in grapevine (cv. Pinot noir)

The vegetative-reproductive balance is critical in determining grapevine quality and productivity. Many agricultural practices (bunch thinning, defoliation, green pruning) can modify the source-sink relationships leading to altered vineyard efficiency and berry quality. Field experiments aimed at understanding how a decrease in sink availability influences leaf physiology of the reproductive shoot. In Pinot noir trained to guyot, control shoots were compared to shoots subjected to basal girdling (G), bunch removal (BR) or a combination of both treatments (G + BR). Morpho-physiological traits including leaf water status, gas-exchange, leaf temperature, chlorophyll fluorescence, leaf starch and soluble sugars, petiole and leaf dry weight were collected at different time of the day and over several days to evaluate the dynamics of leaf functionality following treatments application. The gradual reduction in sink availability induced significant reductions in leaf gas-exchange and PSII efficiency from the second day after the treatments’ application. Nonetheless, while reductions in photosynthetic capacity and increases in leaf temperature associated with a compromised stomatal physiology were present in the G + BR shoots, no differences were observed between bunch removal and control treatments. Girdling and G + BR showed a significant reduction in leaf chlorophyll a and b as well as carotenoids content when compared to control shoots. The increase in the source-sink ratio subsequently produced a marked increase in leaf starch, hexoses and sucrose accompanied by a reduction in the specific leaf area while the girdling did not affect the weight and brix degree of the bunches. This work shows how the progressive reduction in sink availability compromises leaf physiological functionality putatively owing to a reduction in assimilates translocation capacity from the leaf and hence starch storage and accumulation. However, limited effects were observed for shoots in which only the bunch was removed suggesting that assimilates allocation to other vegetative and/or reproductive sinks can maintain leaf functionality even in days characterized by unfavourable environmental conditions. • Sink availability greatly influences source dynamics • Reducing sink availability in grapevine resulted in higher diurnal susceptibility of the leaf to environmental stresses such as high VPD and air temperature • Source-sink relationships can indirectly govern water status via stomatal regulation of transpiration • Source-sink dynamics should be considered as a potential mechanism to exploit for reducing the impact of climate change in grapevine.

Growth status and physiological changes of sugar beet seedlings in response to acidic pH environments

Sugar beet (Beta vulgaris L.) is an important sugar crop that is popularly cultivated in a variety of agriculture conditions. Here, we studied sugar beet growth in different pH soils (pH 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, and 9.0) and analyzed their growth status and physiology. Sugar beet growth was best at pH 9.0 and worst at pH 5.0. As the soil pH decreased from 9.0 to 5.0, the osmoregulatory substances, antioxidant enzyme activity, and elemental contents in leaves and roots showed increasing trends, while photosynthesis and macronutrient contents showed decreasing trends. To explore the physiological mechanisms sugar beet use to respond to different pH environments, we analyzed the correlations between leaf net photosynthesis rate and physiological changes and nutrient contents of sugar beet. One of the factors inhibiting sugar beet growth in low pH soils was a reduction in photosynthetic capacity. The accumulation of osmoregulatory substances and increased peroxidative damage may have led to the decrease in leaf net photosynthesis rate. Furthermore, the decrease in nutrient content and accumulation of metal elements were correlated with the decrease in leaf photosynthetic rate. QRT-PCR analysis showed higher expression levels of antioxidant enzyme genes in the leaves and roots of sugar beet grown in low pH environments compared to those in high pH environments. Correspondingly, antioxidant enzyme activity was significantly higher in beets in low pH environments than in beets in high pH environments. These results provide important insight into the physiological responses by which sugar beet can adapt to different pH soils.

The response system of the growth, physiological and uptake characteristics of Pinus bungeana under ozone stress

Objective: To study the changes of growth, physiological and absorption characteristics of Pinus bungeana under ozone (O3) stress, to elucidate the correlations among the indicators, and to determine its degree of response to O3. Methods: The growth, physiological characteristics and O3 uptake capacity of Pinus bungeana seedlings were measured in an open-top O3 fumigation manual control experiment with three concentration gradients (NF: normal atmospheric O3 concentration, NF40: normal atmospheric O3 concentration plus 40 nmlol/mol; NF80: normal atmospheric O3 concentration plus 80 nmol/mol), and the relationships between the characteristics of Pinus bungeana under different O3 concentrations were investigated with correlation analysis, redundancy analysis and analysis of variance. Results: (1) Plant height growth (ΔH), diameter growth at 50 cm (ΔDBH), stomatal size (S), stomatal density (M), stomatal opening (K), stomatal conductance (Gs), net photosynthetic rate (Pn), transpiration rate (Et), water use efficiency (WUE), maximum photochemical efficiency (Fv/Fm), chlorophyll content (CHL), whole tree water consumption (W), and O3 uptake rate () all decreased with the increase of O3 concentration; while intercellular CO2 concentration () and relative conductivity (L) increased with the increase of O3 concentration; (2) growth indicators of Pinus bungeana under O3 stress (ΔH, ΔDBH) were the most correlated with O3 uptake status (, W), followed by photosynthetic indicators (, WUE, ,, ) and growth indicators (ΔH, ΔDBH) and stomatal characteristics (K, M, S) under O3 stress, some physiological indicators (L, ) were relatively weakly correlated with photosynthesis (, WUE,,, ) and stomatal (K, M, S); (3) all the indicators of Pinus bungeana were significantly different under O3 treatments of NF and NF80 (P < 0.05), ΔH, ΔDBH, M, CHL, , , W and were most significantly different under NF and NF40 treatments, and K, S, WUE, , , , L were more significantly different under NF40 and NF80 treatments. Conclusion: The experiment proved that the growth of Pinus bungeana was slowed, photosynthetic capacity was reduced, and the absorption capacity of O3 was further reduced by long-term exposure to high concentration of O3. The growth of Pinus bungeana was most correlated with the changes of O3 absorption characteristics, and the stomatal characteristics were most correlated with photosynthetic physiological characteristics, and the reduction of photosynthetic capacity etc. further led to the curtailment of its growth.

Physiological and multi-omics responses of Neoporphyra haitanensis to dehydration-rehydration cycles

BackgroundSeaweeds in the upper intertidal zone experience extreme desiccation during low tide, followed by rapid rehydration during high tide. Porphyra sensu lato are typical upper intertidal seaweeds. Therefore, it is valuable to investigate the adaptive mechanisms of seaweed in response to dehydration-rehydration stress.ResultsA reduction in photosynthetic capacity and cell shrinkage were observed when N. haitanensis was dehydrated, and such changes were ameliorated once rehydrated. And the rate and extent of rehydration were affected by the air flow speed, water content before rehydration, and storage temperature and time. Rapid dehydration at high air-flow speed and storage at − 20 °C with water content of 10% caused less damage to N. haitanensis and better-protected cell activity. Moreover, proteomic and metabolomic analyses revealed the abundance members of the differentially expressed proteins (DEPs) and differentially abundant metabolites (DAMs) mainly involved in antioxidant system and osmotic regulation. The ascorbic acid-glutathione coupled with polyamine antioxidant system was enhanced in the dehydration response of N. haitanensis. The increased soluble sugar content, the accumulated polyols, but hardly changed (iso)floridoside and insignificant amount of sucrose during dehydration indicated that polyols as energetically cheaper organic osmolytes might help resist desiccation. Interestingly, the recovery of DAMs and DEPs upon rehydration was fast.ConclusionsOur research results revealed that rapid dehydration and storage at − 20 °C were beneficial for recovery of N. haitanensis. And the strategy to resist dehydration was strongly directed toward antioxidant activation and osmotic regulation. This work provided valuable insights into physiological changes and adaptative mechanism in desiccation, which can be applied for seaweed farming.

Acclimation to low light modifies nitrogen uptake in Halophila ovalis (R.Brown) J.D. Hooker

Seagrass meadows can improve water quality through their nutrient removal capacity. However, this key ecosystem function might be affected by reduced light availability in the water column through impacts on seagrass photosynthesis. Photosynthesis provides energy and carbon skeletons for nitrogen uptake and assimilation in plants – a reduction in photosynthetic capacity should alter nitrogen metabolism. In our study, we explored how nitrogen metabolism in a common tropical seagrass, Halophila ovalis, responded as the plant photo-acclimated to reduced light levels. H. ovalis was exposed to various shading levels (0% control, 25% and 50%) in the field at Chek Jawa, Singapore for 28 days. Photo-acclimation was tracked by measuring light use characteristics through PAM fluorometry (rapid light curves) and protein expression pertaining to photosynthetic processes (RbcL, PsbA and PsbS). Nitrate and ammonium uptake rates were assessed in leaves and root-rhizome complexes, and plant tissue nutrients (carbon and nitrogen) were quantified at the end of the experiment. Shaded plants displayed significant photoacclimation responses, such as lower maximum electron transport rates (ETRmax) and saturating irradiance (Ek), as well as a down-regulation of PsbA and PsbS. Several parameters (ETRmax, Ek and PsbS protein levels) were identified to be promising candidates for monitoring impacts of light reduction on seagrass health and warrant further investigations. As the seagrass acclimated to reduced light levels, a shift in nitrogen use was observed through a reduction in relative nitrate uptake rates in the leaves and root-rhizome complexes, and an increase in relative ammonium uptake rates in the leaves. This signaled potential modifications in nutrient removal capacity, via nitrogen uptake, for seagrasses in low light conditions. Carbon-to‑nitrogen ratio in plant tissue and the expression of RbcL did not exhibit significant change with light reduction. Our findings highlight that inter-related processes of photosynthesis and nitrogen assimilation should be considered when evaluating the potential impacts of light reduction on seagrass meadows and the ecosystem functions they provide.

In situ photosynthetic performance of Porites lutea inhabiting contrasting habitats of the Northern Straits of Malacca (NSoM), Malaysia

ABSTRACT Coral reefs in the Northern Straits of Malacca (NSoM), Malaysia, are frequently exposed to high concentrations of total suspended solids (TSS), thus reducing the light availability for photosynthesis. This study describes the photosynthetic performances of Porites lutea inhabiting contrasting habitats of Pulau Kendi, Pulau Songsong, and Pulau Payar in the NSoM. The light attenuation (Kd (PAR)) was significantly different between all sites, whereby highly turbid water of Pulau Kendi has the highest Kd (PAR) (m−1) = 0.8 ± 0.0 and TSS (mg/L) = 95.7 ± 2.5 in comparison to the protected reef in Pulau Payar, Kd (PAR) (m−1) = 0.5 ± 0.0 and TSS (mg/L) = 36.7 ± 0.4. Here, Pulse-Amplitude-Modulated fluorometry (PAM) and Rapid Light Curves (RLCs) indicate that P. lutea exhibits a different trend of photosynthetic performances to cope with in situ light availability. Turbid waters of Pulau Kendi were observed to provide some protection from light-induced photoinhibition whereby the maximum photosynthetic yield (F v /F m = 0.8 ± 0.0) was significantly higher than those in Pulau Payar and Pulau Songsong. This observation suggested that they could survive near darkness with low light availability for photosynthesis, but a significant reduction in photosynthetic capacity (rETRmax = 77.5 ± 7.4) was also observed. In contrast, greater photosynthetic capacities were observed in P. lutea inhabiting the high-light environment of Pulau Payar. This study emphasized that P. lutea can photoacclimate by maximizing light availability for photosynthesis to ensure survival in turbid nearshore environments.

Potassium modulates central carbon metabolism to participate in regulating CO2 transport and assimilation in Brassica napus leaves

Potassium (K) regulates plant metabolism and enhances plant’s ability to adapt to adversity. However, under different K deficiency stress, the net photosynthetic rate (An) was reduced, influenced by CO2 conductance or biochemical capacities. The interplay between metabolome and photosynthetic characteristics under K deficiency stress was analyzed to explore the mechanisms by which K regulates photosynthetic capacity. With increasing K deficiency stress, dominations limiting An varied from CO2 conductance to biochemical limitations. Multivariate analyses indicated that organic acids, amino acids and sedoheptulose-7-bisphosphate were significantly related to An, CO2 conductance and carboxylation rate. Under moderate K deficiency, organic acids were up-regulated. Acidification of subcellular compartments reduced sedoheptulose-1,7-bisphosphatase activity, inducing downregulation of sedoheptulose-7-bisphosphate and hindrance of ribulose bisphosphate regeneration. Moreover, increased CO2 shortage with increasing K deficiency induced a shift of increased citric acid to amino acid synthesis, causing excessive accumulation of amino acids. In addition, the reduced serine level indicated impaired photorespiration. These two changes triggered more serious reduction in photosynthetic capacity. The intimate, changes in photosynthetic capacities were tightly coupled with shifts in central C metabolism, which provides insights into the methods used to enhance An and plant’s adaptability to abiotic stresses, through the regulation of C metabolites using molecular technology.

Morphological and physiological responses of two willow species from different habitats to salt stress

Plant salt tolerance is a complex mechanism, and different plant species have different strategies for surviving salt stress. In the present study, we analyzed and compared the morphological and physiological responses of two willow species (Salix linearistipularis and Salix matsudana) from different habitats to salt stress. S. linearistipularis exhibited higher seed germination rates and seedling root Na+ efflux than S. matsudana under salt stress. After salt treatment, S. linearistipularis leaves exhibited less Na+ accumulation, loss of water and chlorophyll, reduction in photosynthetic capacity, and damage to leaf cell structure than leaves of S. matsudana. Scanning electron microscopy combined with gas chromatography mass spectrometry showed that S. linearistipularis leaves had higher cuticular wax loads than S. matsudana leaves. Overall, our results showed that S. linearistipularis had higher salt tolerance than S. matsudana, which was associated with different morphological and physiological responses to salt stress. Furthermore, our study suggested that S. linearistipularis could be a promising tree species for saline-alkali land greening and improvement.

Isotopic and Water Relation Responses to Ozone and Water Stress in Seedlings of Three Oak Species with Different Adaptation Strategies

The impact of global changes on forest ecosystem processes is based on the species-specific responses of trees to the combined effect of multiple stressors and the capacity of each species to acclimate and cope with the environment modification. Combined environmental constraints can severely affect plant and ecological processes involved in plant functionality. This study provides novel insights into the impact of a simultaneous pairing of abiotic stresses (i.e., water and ozone (O3) stress) on the responses of oak species. Water stress (using 40 and 100% of soil water content at field capacity—WS and WW treatments, respectively) and O3 exposure (1.0, 1.2, and 1.4 times the ambient concentration—AA, 1.2AA, and 1.4AA, respectively) were carried out on Quercus robur L., Quercus ilex L., and Quercus pubescens Willd. seedlings, to study physiological traits (1. isotope signature [δ13C, δ18O and δ15N], 2. water relation [leaf water potential, leaf water content], 3. leaf gas exchange [light-saturated net photosynthesis, Asat, and stomatal conductance, gs]) for adaptation strategies in a Free-Air Controlled Exposure (FACE) experiment. Ozone decreased Asat in Q. robur and Q. pubescens while water stress decreased it in all three oak species. Ozone did not affect δ13C, whereas δ18O was influenced by O3 especially in Q. robur. This may reflect a reduction of gs with the concomitant reduction in photosynthetic capacity. However, the effect of elevated O3 on leaf gas exchange as indicated by the combined analysis of stable isotopes was much lower than that of water stress. Water stress was detectable by δ13C and by δ18O in all three oak species, while δ15N did not define plant response to stress conditions in any species. The δ13C signal was correlated to leaf water content (LWC) in Q. robur and Q. ilex, showing isohydric and anisohydric strategy, respectively, at increasing stress intensity (low value of LWC). No interactive effect of water stress and O3 exposure on the isotopic responses was found, suggesting no cross-protection on seasonal carbon assimilation independently on the species adaptation strategy.

Impairment of photosynthetic capacity and hydrogen peroxide removal in castor bean affected by bacterial leaf spot

Bacterial leaf spot caused by Xanthomonas axonopodis pv. ricini (Xar) is the only important disease of bacterial etiology reported to threaten castor bean production. No cultivars expressing resistance to the pathogen are known. Hence, knowledge of the physiological and biochemical alterations caused by the bacterium to castor bean plants could shed light into alternative approaches to obtain plant resistance. Leaf gas exchange parameters were measured with a portable infrared gas analyzer; Chlorophyll a fluorescence parameters were determined with an Imaging-PAM chlorophyll fluorometer; photosynthetic pigments, malondialdehyde (MDA) and activities of superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), peroxidase (POX) and glutathione reductase (GR) were quantified using standard biochemical assays; and production of hydrogen peroxide (H2O2) and superoxide anion (O2•-) was detected by histochemistry. Inoculated leaves exhibited significant reduction in photosynthetic capacity, accumulation of H2O2, absence of O2•- accumulation, higher concentrations of MDA and higher activities of SOD, APX and POX compared to non-inoculated plants. Bacterial infection of castor bean triggered severe alterations, mainly related to stomatal closure that consequently compromised the CO2 influx into infected tissue. Small increases in SOD activity seemed to be sufficient to remove O2•-, but detoxification of the H2O2 generated during the O2•- dismutation process appeared to be inefficient. Higher concentrations of MDA in inoculated compared to non-inoculated plants indicated membrane damage due to lipid peroxidation. Some of the physiological and biochemical alterations in castor bean plants resulting from Xar infection have not previously been reported for other plants infected by bacteria.

Growth responses and physiological and biochemical changes in five ornamental plants grown in urban lead-contaminated soils.

An increasing concentration of lead (Pb) in urban contaminated soil due to anthropogenic activities has been a global issue threatening human health. The use of urban ornamental plants as phytoremediation of Pb-contaminated soil is a new choice. In the present experiment, the physiological and biochemical response of five ornamental plants to increase in concentrations of C4H6O4Pb·H2O in the soil were measured to investigate these plans' Pb tolerance strategies and abilities. Our results showed that Pb stress significantly inhibited the growth and the biomass of all the plants. The root activity (RA), net photosynthetic rate (P n), and chlorophyll (Chl) content in Pb-stressed leaves were significantly decreased, whereas the leaf proline (Pro), soluble sugar (SS), and membrane stability index (MSI) were remarkable increased compared with those in the control group. By application of all-subsets regression and linear regression, the reduction in photosynthetic capacity in the five plants is mainly due to the decrease in the leaf Chl content caused by Pb stress. The bioconcentration factor (BCF) in Canna generalis was greater than 1, while in the other plants were lower than 1, suggesting that Canna generalis had the highest Pb accumulation ability. The translocation factor (TF) in all the plants were lower than 1, suggesting that Pb preferentially accumulated in the external part of roots. By calculating the comprehensive evaluation value (CEV), Iris germanica L. was found to be the most sensitive species, and Canna generalis was the most tolerant species, to Pb stress among the five ornamental plants.

Stress Memory in Seagrasses: First Insight Into the Effects of Thermal Priming and the Role of Epigenetic Modifications.

While thermal priming and the relative role of epigenetic modifications have been widely studied in terrestrial plants, their roles remain unexplored in seagrasses so far. Here, we experimentally compared the ability of two different functional types of seagrass species, dominant in the Southern hemisphere, climax species Posidonia australis and pioneer species Zostera muelleri, to acquire thermal-stress memory to better survive successive stressful thermal events. To this end, a two-heatwave experimental design was conducted in a mesocosm setup. Findings across levels of biological organization including the molecular (gene expression), physiological (photosynthetic performances and pigments content) and organismal (growth) levels provided the first evidence of thermal priming in seagrasses. Non-preheated plants suffered a significant reduction in photosynthetic capacity, leaf growth and chlorophyll a content, while preheated plants were able to cope better with the recurrent stressful event. Gene expression results demonstrated significant regulation of methylation-related genes in response to thermal stress, suggesting that epigenetic modifications could play a central role in seagrass thermal stress memory. In addition, we revealed some interspecific differences in thermal responses between the two different functional types of seagrass species. These results provide the first insights into thermal priming and relative epigenetic modifications in seagrasses paving the way for more comprehensive forecasting and management of thermal stress in these marine foundation species in an era of rapid environmental change.

Metabolism regulation during salt exposure in the halophyte Cakile maritima

The halophyte Cakile maritima is a Brassicacea that has developed numerous mechanisms for managing salt. In the present study, we analyze the metabolic responses of C. maritima to increasing salt exposure in parallel with growth and photosynthetic parameters. At 10 days, 100 mM NaCl treatment has no effect, whereas 400 mM treatment decreases both growth and photosynthetic capacity. Accordingly, the metabolism was weakly impacted at 100 mM NaCl with an increase in only a few amino acids and sugars, whereas 400 mM treated plants shows noticeable changes: an increase in amino acid abundance, sugars decrease and an organic acid depletion. At 20 days, 400 mM treatment leads to more severe effects on growth and photosynthesis, whereas plant growth remains unaffected by the 100 mM NaCl treatment, despite a reduction in photosynthetic capacity. Plants treated with 400 mM NaCl present an amplified metabolic response with additional metabolites reflecting salt stress as GABA, proline and glycine. One noticeable feature of halophily in C. maritima is the increase in sugar content at low stress whereas longer or higher stress lead to a decrease in sugar content. In high salt conditions the stimulation of amino acid biosynthesis is the main strategy for osmoprotection to cope for salt stress.

Exogenous Melatonin Alleviates Cold Stress by Promoting Antioxidant Defense and Redox Homeostasis in Camellia sinensis L.

The unprecedented early spring frost that appears as a cold stress adversely affects growth and productivity in tea (Camellia sinensis L.); therefore, it is indispensable to develop approaches to improve the cold tolerance of tea. Here, we investigated the effect of pretreatment with exogenous melatonin on the net photosynthetic rate, the maximum photochemical efficiency of PSII, chlorophyll content, lipid peroxidation, reactive oxygen species (ROS) accumulation, antioxidant potential, and redox homeostasis in leaves of tea plants following cold stress. Our results revealed that cold treatment induced oxidative stress by increasing ROS accumulation, which in turn affected the photosynthetic process in tea leaves. However, treatment with melatonin mitigated cold-induced reductions in photosynthetic capacity by reducing oxidative stress through enhanced antioxidant potential and redox homeostasis. This study provides strong evidence that melatonin could alleviate cold-induced adverse effects in tea plants.

Reversible colour change in leaves enhances pollinator attraction and reproductive success in Saururus chinensis (Saururaceae).

Although there has been much experimental work on leaf colour change associated with selection generated by abiotic environmental factors and antagonists, the role of leaf colour change in pollinator attraction has been largely ignored. We tested whether whitening of the apical leaves subtending the inflorescences of Saururus chinensis during flowering enhances pollinator attraction, and whether re-greening of the white leaves after flowering increases carbon assimilation and promotes seed development. White leaves were removed or covered, and the effects of these manipulations on pollinator visitation and subsequent reproductive success were assessed. The net photosynthetic rates of leaves of different colour were measured and their photosynthetic contributions to seed development were evaluated. Saururus chinensis is able to self-pollinate autonomously, but depends largely on flies for pollination. White leaves had different reflectance spectra from green leaves, and white leaves attracted significantly more pollinators and led to significantly higher fruit and seed set. Although leaf whitening resulted in a reduction in photosynthetic capacity, it translated into only a small decrease in seed mass. When leaves had turned back from white to green after flowering their photosynthetic capacity was similar to that of 'normal' green leaves and promoted seed development. The reversible leaf colour change in S. chinensis appears to be adaptive because it enhances pollination success during flowering, with a small photosynthetic cost, while re-greening of these leaves after flowering helps to meet the carbon requirements for seed development.

Nitrogen Availability Dampens the Positive Impacts of CO2 Fertilization on Terrestrial Ecosystem Carbon and Water Cycles

AbstractThe magnitude and variability of the terrestrial CO2 sink remain uncertain, partly due to limited global information on ecosystem nitrogen (N) and its cycle. Without N constraint in ecosystem models, the simulated benefits from CO2 fertilization and CO2‐induced increases in water use efficiency (WUE) may be overestimated. In this study, satellite observations of a relative measure of chlorophyll content are used as a proxy for leaf photosynthetic N content globally for 2003–2011. Global gross primary productivity (GPP) and evapotranspiration are estimated under elevated CO2 and N‐constrained model scenarios. Results suggest that the rate of global GPP increase is overestimated by 85% during 2000–2015 without N limitation. This limitation is found to occur in many tropical and boreal forests, where a negative leaf N trend indicates a reduction in photosynthetic capacity, thereby suppressing the positive vegetation response to enhanced CO2 fertilization. Based on our carbon‐water coupled simulations, enhanced CO2 concentration decreased stomatal conductance and hence increased WUE by 10% globally over the 1982 to 2015 time frame. Due to increased anthropogenic N application, GPP in croplands continues to grow and offset the weak negative trend in forests due to N limitation. Our results also show that the improved WUE is unlikely to ease regional droughts in croplands because of increases in evapotranspiration, which are associated with the enhanced GPP. Although the N limitation on GPP increase is large, its associated confidence interval is still wide, suggesting an urgent need for better understanding and quantification of N limitation from satellite observations.

Stomatal and non-stomatal limitations of photosynthesis for four tree species under drought: A comparison of model formulations

Drought strongly influences terrestrial C cycling via its effects on plant H2O and CO2 exchange. However, the treatment of photosynthetic physiology under drought by many ecosystem and earth system models remains poorly constrained by data. We measured the drought response of four tree species and evaluated alternative model formulations for drought effects on photosynthesis (A). We implemented a series of soil drying and rewetting events (i.e. multiple droughts) with four contrasting tree species in large pots (75L) placed in the field under rainout shelters. We measured leaf-level gas exchange, predawn and midday leaf water potential (Ψpd and Ψmd), and leaf isotopic composition (δ13C) and calculated discrimination relative to the atmosphere (Δ). We then evaluated eight modeling frameworks that simulate the effects of drought in different ways. With moderate reductions in volumetric soil water content (θ), all species reduced stomatal conductance (gs), leading to an equivalent increase in water use efficiency across species inferred from both leaf gas exchange and Δ, despite a small reduction in photosynthetic capacity. With severe reductions in θ, all species strongly reduced gs along with a coincident reduction in photosynthetic capacity, illustrating the joint importance of stomatal and non-stomatal limitations of photosynthesis under strong drought conditions. Simple empirical models as well as complex mechanistic model formulations were equally successful at capturing the measured variation in A and gs, as long as the predictor variables were available from direct measurements (θ, Ψpd, and Ψmd). However, models based on leaf water potential face an additional challenge, as we found that Ψpd was substantially different from Ψsoil predicted by standard approaches based on θ. Modeling frameworks that combine gas exchange and hydraulic traits have the advantage of mechanistic realism, but sacrificed parsimony without an improvement in predictive power in this comparison. Model choice depends on the desired balance between simple empiricism and mechanistic realism. We suggest that empirical models implementing stomatal and non-stomatal limitations based on θ are highly predictive simple models. Mechanistic models that incorporate hydraulic traits have excellent potential, but several challenges currently limit their widespread implementation.

Reduced growth due to belowground sink limitation is not fully explained by reduced photosynthesis.

Sink limitation is known to reduce plant growth, but it is not known how plant carbon (C) balance is affected, limiting our ability to predict growth under sink-limited conditions. We manipulated soil volume to impose sink limitation of growth in Eucalyptus tereticornis Sm. seedlings. Seedlings were grown in the field in containers of different sizes and planted flush to the soil alongside freely rooted (Free) seedlings. Container volume negatively affected aboveground growth throughout the experiment, and light saturated rates of leaf photosynthesis were consistently lower in seedlings in containers (-26%) compared with Free seedlings. Significant reductions in photosynthetic capacity in containerized seedlings were related to both reduced leaf nitrogen content and starch accumulation, indicating direct effects of sink limitation on photosynthetic downregulation. After 120 days, harvested biomass of Free seedlings was on average 84% higher than seedlings in containers, but biomass distribution in leaves, stems and roots was not different. However, the reduction in net leaf photosynthesis over the growth period was insufficient to explain the reduction in growth, so that we also observed an apparent reduction in whole-plant C-use efficiency (CUE) between Free seedlings and seedlings in containers. Our results show that sink limitation affects plant growth through feedbacks to both photosynthesis and CUE. Mass balance approaches to predicting plant growth under sink-limited conditions need to incorporate both of these feedbacks.