Decreased CO2 Assimilation Research Articles

Diurnal High Temperatures Affect the Physiological Performance and Fruit Quality of Highbush Blueberry (Vaccinium corymbosum L.) cv. Legacy.

In this study, the physiological performance and fruit quality responses of the highbush blueberry (Vaccinium corymbosum) cultivar Legacy to high temperatures (HTs) were evaluated in a field experiment. Three-year-old V. corymbosum plants were exposed to two temperature treatments between fruit load set and harvest during the 2022/2023 season: (i) ambient temperature (AT) and (ii) high temperature (HT) (5 °C ± 1 °C above ambient temperature). A chamber covered with transparent polyethylene (100 µm thick) was used to apply the HT treatment. In our study, the diurnal temperature was maintained with a difference of 5.03 °C ± 0.12 °C between the AT and HT treatments. Our findings indicated that HT significantly decreased CO2 assimilation (Pn) by 45% and stomatal conductance (gs) by 35.2% compared to the AT treatment. By contrast, the intercellular CO2 concentration (Ci) showed higher levels (about 6%) in HT plants than in AT plants. Fruit quality analyses revealed that the fruit weight and equatorial diameter decreased by 39% and 13%, respectively, in the HT treatment compared to the AT treatment. By contrast, the firmness and total soluble solids (TSS) were higher in the HT treatment than in the AT treatment. Meanwhile, the titratable acidity showed no changes between temperature treatments. In our study, Pn reduction could be associated with stomatal and non-stomatal limitations under HT treatment. Although these findings improve our understanding of the impact of HTs on fruit growth and quality in V. corymbosum, further biochemical and molecular studies are need.

The negative impact of shade on photosynthetic efficiency in sugarcane may reflect a metabolic bottleneck

Plants with C4 metabolism normally have higher photosynthetic rates than C3 ones. As a result, several of the most productive species known are NADP-ME C4 grasses, such as sugarcane and maize. However, the advantages of the C4 cycle are most evident under high light as the CO2 concentrating mechanism comes at the expense of additional ATP. Recent works have suggested a negative impact of shading across NADP-ME C4 grasses, causing a downregulation of maximal quantum efficiency of CO2 assimilation (ϕCO2,max). The mechanisms behind the loss in photosynthetic efficiency and whether these results apply for other C4 crops species and within germplasm of a species remain unclear. We analysed the photosynthetic acclimation to shade in four sugarcane genotypes with contrasting yield. To find out whether the effects of leaf history, i.e., shading a leaf developed under full sunlight and exposed later on to shade differ from a leaf fully developed under shading, these two types of leaves were evaluated. Shaded sugarcane plants showed decreased CO2 assimilation efficiency compared to plants grown under full sunlight. Based on the fluorescence measurements, it seems that this reduction coincided with a more reduced QA redox state, which could point to a metabolic limitation downstream of the light-dependent reactions. The results were similar for all genotypes and were observed regardless of whether leaves developed under shade or under full sunlight conditions and exposed subsequently to shade, suggesting that light is the main factor affecting photosynthetic efficiency. This study reinforces the notion that this negative impact of shade could reflect a common bottleneck across NADP-ME C4 grasses.

Single and combined effects of fertilization, ectomycorrhizal inoculation, and drought on container-grown Japanese larch seedlings

Climate change can intensify drought in many regions of the world and lead to more frequent drought events or altered cycles of soil water status. Therefore, it is important to enhance the tolerance to drought and thus health, vigor, and success of transplantation seedlings used in the forestry by modifying fertilization and promoting mycorrhization. Here, we sowed seeds of Japanese larch (Larix kaempferi) in 0.2-L containers with 0.5 g (low fertilization; LF) or 2 g (high fertilization; HF) of slow-release fertilizer early in the growing season. One month later, we irrigated seedlings with non-sterilized ectomycorrhizal inoculum (ECM) or sterilized solution (non-ECM), and after about 2 months, plants were either kept well watered (WW; 500 mL water/plant/week) or subjected to drought (DR; 50 mL water per plant/week) until the end of the growing season. HF largely stimulated plant growth and above- and belowground biomass production, effects that are of practical significance, but caused a small decrease in stomatal conductance (Gs390) and transpiration rate (E390), which in practice is insignificant. ECM treatment resulted in moderate inhibition of seedling growth and biomass and largely canceled out the enhancement of biomass and foliar K content by HF. DR caused a large decrease in CO2 assimilation, and enhanced stomatal closure and induced senescence. DR also largely depleted foliar Mg and enriched foliar K. Although DR caused a large decrease in foliar P content in LF, it moderately increased P in HF. Likewise, DR increased foliar K in HF but not in LF, and decreased foliar P in ECM plants but not in non-ECM plants. Conversely, ECM plants exhibited a large enrichment in foliar P under WW and had a lower water potential under DR when grown in LF. These results indicate that the drought tolerance and health and vigor of Japanese larch seedlings can be modified by soil fertility and soil microorganisms. This study provides a basis for new multifactorial research programs aimed at producing seedlings of superior quality for forestation under climate change.

Intercropping Salt-Sensitive Lactuca sativa L. and Salt-Tolerant Salsola soda L. in a Saline Hydroponic Medium: An Agronomic and Physiological Assessment.

Competition for freshwater is increasing, with a growing population and the effects of climate change limiting its availability. In this experiment, Lactuca sativa plants were grown hydroponically with or without a 15% share of seawater (12 dS m-1) alone or intercropped with Salsola soda to demonstrate if L. sativa benefits from sodium removal by its halophyte companion. Contrary to the hypothesis, saline-grown L. sativa plants demonstrated reduced growth compared to the control plants regardless of the presence or absence of S. soda. Both limitations in CO2 supply and photosystem efficiency may have decreased CO2 assimilation rates and growth in L. sativa plants grown in the seawater-amended solutions. Surprisingly, leaf pigment concentrations increased in salt-treated L. sativa plants, and most notably among those intercropped with S. soda, suggesting that intercropping may have led to shade-induced increases in chlorophyll pigments. Furthermore, increased levels of proline indicate that salt-treated L. sativa plants were experiencing stress. In contrast, S. soda produced greater biomass in saline conditions than in control conditions. The mineral element, carbohydrate, protein, polyphenol and nitrate profiles of both species differed in their response to salinity. In particular, salt-sensitive L. sativa plants had greater accumulations of Fe, Ca, P, total phenolic compounds and nitrates under saline conditions than salt-tolerant S. soda. The obtained results suggest that intercropping salt-sensitive L. sativa with S. soda in a hydroponic system did not ameliorate the growing conditions of the salt-sensitive species as was hypothesized and may have exacerbated the abiotic stress by increasing competition for limited resources such as light. In contrast, the saline medium induced an improvement in the nutritional profile of S. soda. These results demonstrate an upper limit of the seawater share and planting density that can be used in saline agriculture when intercropping S. soda plants with other salt-sensitive crops.

The role of photosynthetic activity in the regulation of flg22-induced local and systemic defence reaction in tomato

Flagellin (flg22) induces rapid and long-lasting defence responses. It may also affect the photosynthetic activity depending on several internal and external factors, such as the phytohormone ethylene or the day/night time. Based on the results, flg22 treatment, neither in the light phase nor in the evening, caused any significant change in chlorophyll fluorescence induction parameters in the leaves of wild-type and ethylene-receptor mutant Never ripe tomato plants measured the next morning. However, flg22 in the light phase decreased the effective quantum yield and the photochemical quenching both locally and systemically in guard cells. In parallel, the production of reactive oxygen species and nitric oxide increased, which contributed to the stomatal closure and a decrease in CO2 assimilation the next day. A decrease in sugar content and elevated hexokinase activity measured after flg22 exposure can also contribute to local defence responses in intact tomato plants.

Cadmium and lead excess differently affect growth, photosynthetic activity and nutritional status of Trigonella foenum-graecum L.

Context Heavy metal contamination of soils is a serious environmental problem worldwide. Cadmium (Cd) and lead (Pb) are considered among the most important types of pollutants. Aim To investigate the response of a local fenugreek cultivar against lead and cadmium. Methods Physiological changes were studied under different concentrations (0, 100, 200, 300 and 400 μM) of PbCl2 and CdCl2. Key results Fenugreek growth decreased gradually with increasing Cd and Pb supply. This decrease was accompanied by a gradual decline in shoot and root length and photosynthetic parameters. However, Cd treatments showed pronounced effects in fenugreek seedlings as compared to Pb. The tolerance index was between 0.41 and 0.81, which suggests relative tolerance of this cultivar to Pb and Cd. This species was also able to maintain stable water status. Nevertheless, in presence of high Cd concentration (400 μM), this species showed substantial decrease in CO2 assimilation (86%), transpiration rate (87%), stomatal conductance (57%), chlorophyll content (35%) and carotenoid content (53%). Shoot proline content was increased significantly under 200 and 300 μM Cd, and slightly under 100 and 200 μM Pb. Furthermore, Cd and Pb induced a decrease in shoot magnesium and phosphorus content. Conversely, shoot iron content was increased. Data showed that fenugreek accumulated Cd, and translocated to the harvestable parts (up to 20 mg/kg DW under 400 μM). However, Pb was mostly accumulated in roots. Conclusions Our results revealed that the relative tolerance of fenugreek to Pb excess was coupled to a remarkable accumulation of this element in root, which favourite the phytostabilisation process. Implications FM, FL, SJ and OTZ execute the manipulation and the culture of plants. FM, OTZ, IN and KZ analyse and examine obtained results. FM, OTZ and KZ write and preparethe manuscript. IN, HM and KZ prepare the conception and the realisation ofthis work.

The net photosynthetic rate of the cotton boll-leaf system determines boll weight under various plant densities

The yield of cotton (Gossypium hirsutum L.) can be effectively maximized through high planting density. Estimating the yields from single-leaf photosynthesis under various plant densities is difficult as the source-sink relationship of cotton is complicated. By considering the boll-leaf system (BLS: including the main-stem leaf, sympodial leaf and non-leaf organs) as the basic unit of the cotton canopy, the present study asserts that the photosynthetic function of BLS determines single boll weight. This hypothesis was tested by establishing plant densities to explore changes in photosynthesis of the cotton BLS and the consequent effects on boll weight. The results showed that high plant density increased total boll number per unit ground area, whereas single boll weight remained relatively stable, thereby increasing cotton yield. High plant density significantly limited the biomass of the BLS and reduced the distribution ratio of assimilate to sympodial leaves, whereas the dry matter proportion of cotton bolls increased. Although the leaf area and photosynthetic rate of both the main-stem and sympodial leaves of the BLS decreased with increasing plant density, the extent of decrease in the latter was greater. CO2 assimilation within the BLS decreased slightly with increasing plant density, and significant decreases in CO2 assimilation were only evident in extremely high plant densities. A significant positive linear correlation was observed between CO2 assimilation of the BLS and single boll weight. Therefore, photosynthesis of the BLS was identified as the primary factor maintaining boll weight under various plant densities. The main-stem leaf plays a key role in maintaining photosynthesis of the BLS, whereas BLS photosynthesis can be improved by plastically regulating the photosynthetic performance in sympodial leaves.

Regression models to stratify the copper toxicity responses and tolerance mechanisms of Glycine max (L.) Merr. plants.

Root antioxidant defense, restricted root-to-shoot Cu translocation, altered nutrient partition, and leaf gas exchange adjustments occurred as tolerance mechanisms of soybean plants to increasing soil Cu levels. The intensive application of copper (Cu) fungicides has been related to the accumulation of this metal in agricultural soils. This study aimed to evaluate the effects of increasing soil Cu levels on soybean (Glycine max) plants. Soybean was cultivated under greenhouse conditions in soils containing different Cu concentrations (11.2, 52.3, 79.4, 133.5, 164.0, 205.1, or 243.8mgkg-1), and biochemical and morphophysiological plant responses were analyzed through linear and nonlinear regression models. Although Cu concentrations around 50mgkg-1 promoted some positive effects on the initial development of soybean plants (e.g., increased root length and dry weight), these Cu concentrations also induced root oxidative stress and activated defense mechanisms (such as the induction of antioxidant response, N and S accumulation in the roots). At higher concentrations, Cu led to growth inhibition (mainly of the root), nutritional imbalance, and damage to the photosynthetic apparatus of soybean plants, resulting in decreased CO2 assimilation and stomatal conductance. In contrast, low translocation of Cu to the leaves, conservative water use, and increased carboxylation efficiency contributed to the partial mitigation of Cu-induced stress. These responses allowed soybean plants treated with Cu levels in the soil as high as 90mgkg-1 to maintain growth parameters higher than or similar to those of plants in the non-contaminated soil. These data provide a warning for the potentially deleterious consequences of the increasing use of Cu-based fungicides. However, it is necessary to verify how the responses to Cu contamination are affected by different types of soil and soybean cultivars.

POTASSIUM DOES NOT ATTENUATE SALT STRESS IN YELLOW PASSION FRUIT UNDER IRRIGATION MANAGEMENT STRATEGIES

ABSTRACT The objective of this study was to evaluate the gas exchange, growth and production of yellow passion fruit cv. BRS GA1, as a function of irrigation management strategies with saline water and potassium doses. The experiment was conducted under field conditions in São Domingos, PB, Brazil. A randomized block design was used in a 6 * 2 factorial scheme. The treatments consisted of six strategies of irrigation with saline waters (irrigation with low-salinity water throughout the crop cycle - SE; irrigation with high-salinity water in the vegetative stage - VE; flowering stage - FL; fruiting stage - FR; in the successive vegetative/flowering stages - VE/FL; vegetative/fruiting stages - VE/FR) and two doses of potassium (100 and 130% of the K2O recommendation), with four replicates and four plants per plot. The 100% dose corresponded to 60 g of K 2O plant-1 year-1. The effects of using high-salinity water (3.2 dS m-1) alternated with low-salinity water (1.3 dS m-1) were evaluated in different stages of the cultivation cycle. Irrigation with saline water in the fruiting stage promoted an increase in intercellular CO2 concentration and decrease in CO2 assimilation, with effects of non-stomatal origin standing out as limiting factors of photosynthetic efficiency. The highest CO 2 assimilation rate in plants subjected to water salinity of 1.3 dS m-1 throughout the cycle resulted in increments in the number of fruits and in the production per plant of the passion fruit cv. BRS GA1.

Cytochrome oxidase and alternative oxidase pathways of mitochondrial electron transport chain are important for the photosynthetic performance of pea plants under salinity stress conditions

The flexible plant mitochondrial electron transport chain with cytochrome c oxidase (COX) and alternative oxidase (AOX) pathways is known to be modulated by abiotic stress conditions. The effect of salinity stress on the mitochondrial electron transport chain and the importance of COX and AOX pathways for optimization of photosynthesis under salinity stress conditions is not clearly understood. In the current study, importance of COX and AOX pathways for photosynthetic performance of pea plants (Pisum sativum L. Pea Arkel cv) was analysed by using the mitochondrial electron transport chain inhibitors Antimycin A (AA) and salicylhydroxamic acid (SHAM) which restrict the electron flow through COX and AOX pathways respectively. Salinity stress resulted in decreased CO2 assimilation rates, leaf stomatal conductance, transpiration and leaf intercellular CO2 concentration in a stress dependent manner. Superimposition of leaves of salt stressed plants with AA and SHAM caused cellular H2O2 and O2− accumulation along with cell death. Additionally, aggravation in decrease of CO2 assimilation rates, leaf stomatal conductance, transpiration and leaf intercellular CO2 concentration upon superimposition with AA and SHAM during salinity stress suggests the importance of mitochondrial oxidative electron transport for photosynthesis. Increased expression of AOX1a and AOX2 transcripts along with AOX protein levels indicated up regulation of AOX pathway in leaves during salinity stress. Chlorophyll fluorescence measurements revealed enhanced damage to Photosystem (PS) II in the presence of AA and SHAM during salinity stress. Results suggested the beneficial role of COX and AOX pathways for optimal photosynthetic performance in pea leaves during salinity stress conditions.

Assessing adaptation and mitigation potential of roadside trees under the influence of vehicular emissions: A case study of Grevillea robusta and Mangifera indica planted in an urban city of India

The ever-increasing vehicle counts have resulted in a significant increase in air pollution impacting human and natural ecosystems including trees, and physical properties. Roadside plantations often act as a first defense line against the vehicular emissions to mitigate the impacts of pollutants. However, they are themselves vulnerable to these pollutants with varying levels of tolerance capacity. This demands a scientific investigation to assess the role of roadside plantation for better management and planning for urban sprawl where selected trees could be grown to mitigate the impacts of harmful pollutants. The present study assesses the impacts of vehicular emissions on the adaptation and mitigation potential of two important roadside tree species i.e. Grevillea robusta and Mangifera indica planted along roadsides in the capital city of Uttarakhand. Uttarakhand is one of the Indian Western Himalayan State and its capital city is situated on the foothills of Himalaya. The adaptation and mitigation potential were evaluated by studying the response of pollutants on the functional traits which drive the physiology of the trees. The CO2 assimilation rate, transpiration rate, stomatal conductance, water use efficiency (WUE), air pollution tolerance index (APTI), copper and proline accumulation, dust removal efficiency (DRE), leaf thickness and cooling created by plantation were studied to evaluate the response of trees exposed to roadside traffics. To compare the influence of pollutants, traits of trees grown in a control site with few or absence of vehicular movement were compared with the roadside trees. The control site represented part of a reserve forest where human interference is controlled and human-induced activities are prohibited. The vehicular frequency was found to modulate tree characteristics. The tree characteristics representing WUE, APTI, proline and copper accumulation, leaf thickness, cooling impact, and DRE were enhanced significantly, while the decreased CO2 assimilation rate was observed near roadside trees compared to the control site. We found both of the species to perform well to be used as one of the potential species for roadside and urban greening. However, there is a need to assess the potential of other species in reference to the present study.

Influence of Abiotic Stress Factors on the Antioxidant Properties and Polyphenols Profile Composition of Green Barley (Hordeum vulgare L.).

The influence of stress factors on the plant can, on the one hand, lead to worse functioning of the plant and loss of its crop, but on the other, it can have a positive effect on the metabolism of compounds with documented biological activity. In this study, the effect of light and drought intensity on photosynthetic activity and physiological status of two barley varieties, as well as the antimicrobial, antioxidant properties and profile of polyphenolic compounds of green barley were analysed. It was shown that under the conditions of water shortage, the KWS Olof variety showed a smaller decrease in CO2 assimilation and transpiration and higher values of these parameters at both light intensities. Only in the KWS Olof variety increased stress as a result of increased light intensity. It has also been shown that both the intensity of radiation and drought-related stress have a significant impact on the profile of polyphenolic compounds from green barley, without a simple relationship between the impact of stress factors on the content of polyphenols. Changes in the profile of polyphenolic compounds augmented the antioxidant and antimicrobial activity of the material. This, in turn, proposes the possibility of reducing the applied doses of herbal material thanks to a greater content of active substances in extracts obtained from the plants used to produce medicinal preparations.

Magnesium Deficiency Induced Global Transcriptome Change in Citrus sinensis Leaves Revealed by RNA-Seq.

Magnesium (Mg) deficiency is one of the major constraining factors that limit the yield and quality of agricultural products. Uniform seedlings of the Citrus sinensis were irrigated with Mg deficient (0 mM MgSO4) and Mg sufficient (1 mM MgSO4) nutrient solutions for 16 weeks. CO2 assimilation, starch, soluble carbohydrates, TBARS content and H2O2 production were measured. Transcriptomic analysis of C. sinensis leaves was performed by Illumina sequencing. Our results showed that Mg deficiency decreased CO2 assimilation, but increased starch, sucrose, TBARS content and H2O2 production in C. sinensis leaves. A total of 4864 genes showed differential expression in response to Mg deficiency revealed by RNA-Seq and the transcriptomic data were further validated by real-time quantitative PCR (RT-qPCR). Gene ontology (GO) enrichment analysis indicated that the mechanisms underlying Mg deficiency tolerance in C. sinensis may be attributed to the following aspects: (a) enhanced microtubule-based movement and cell cycle regulation; (b) elevated signal transduction in response to biotic and abiotic stimuli; (c) alteration of biological processes by tightly controlling phosphorylation especially protein phosphorylation; (d) down-regulation of light harvesting and photosynthesis due to the accumulation of carbohydrates; (e) up-regulation of cell wall remodeling and antioxidant system. Our results provide a comprehensive insight into the transcriptomic profile of key components involved in the Mg deficiency tolerance in C. sinensis and enrich our understanding of the molecular mechanisms by which plants adapted to a Mg deficient condition.

Retardation of the Calvin Cycle Contributes to the Reduced CO2 Assimilation Ability of Rice Stripe Virus-Infected N. benthamiana and Suppresses Viral Infection.

Rice stripe virus (RSV) is naturally transmitted by the small brown planthopper and infects plants of the family Poaceae. Under laboratory conditions, RSV can infect Nicotiana benthamiana by mechanical inoculation, providing a useful system to study RSV–plant interactions. Measurements of CO2 assimilation ability and PSII photochemical efficiency showed that these were both reduced in N. benthamiana plants infected by RSV. These plants also had decreased expression of the N. benthamiana Phosphoribulokinases (NbPRKs), the key gene in the Calvin cycle. When the NbPRKs were silenced using the TRV-Virus Induced Gene Silencing system, the plants had decreased CO2 assimilation ability, indicating that the downregulated expression of NbPRKs contributes to the reduced CO2 assimilation ability of RSV-infected plants. Additionally, NbPRKs-silenced plants were more resistant to RSV. Similarly, resistance was enhanced by silencing of either N. benthamiana Rubisco small subunit (NbRbCS) or Phosphoglycerate kinase (NbPGK), two other key genes in the Calvin cycle. Conversely, transgenic plants overexpressing NbPRK1 were more susceptible to RSV infection. The results suggest that a normally functional Calvin cycle may be necessary for RSV infection of N. benthamiana.

Aluminum-responsive genes revealed by RNA-Seq and related physiological responses in leaves of two Citrus species with contrasting aluminum-tolerance

Little is known about the physiological and molecular responses of leaves to aluminum (Al)-toxicity. Seedlings of Al-intolerant Citrus grandis and Al-tolerant Citrus sinensis were supplied daily with nutrient solution containing 0 mM (control) and 1.0 mM (Al-toxicity) AlCl3·6H2O for 18 weeks. We found that Al-treatment only decreased CO2 assimilation in C. grandis leaves, and that the Al-induced alterations of gene expression profiles were less in C. sinensis leaves than those in C. grandis leaves, indicating that C. sinensis seedlings were more tolerant to Al-toxicity than C. grandis ones. Al concentration was similar between Al-treated C. sinensis and C. grandis roots, but it was higher in Al-treated C. grandis stems and leaves than that in Al-treated C. sinensis stems and leaves. Al-treated C. sinensis seedlings accumulated relatively more Al in roots and transported relatively little Al to shoots. This might be responsible for the higher Al-tolerance of C. sinensis. Further analysis showed that the following several aspects might account for the higher Al-tolerance of C. sinensis, including: (a) Al-treated C. sinensis leaves had higher capacity to maintain the homeostasis of energy and phosphate, the stability of lipid composition and the integrity of cell wall than did Al-treated C. grandis leaves; (b) Al-triggered production of reactive oxygen species (ROS) and the other cytotoxic compounds was less in Al-treated C. sinensis leaves than that in Al-treated C. grandis leaves, because Al-toxicity decreased CO2 assimilation only in C. grandis leaves; accordingly, more upregulated genes involved in the detoxifications of ROS, aldehydes and methylglyoxal were identified in Al-treated C. grandis leaves; in addition, flavonoid concentration was increased only in Al-treated C. grandis leaves; (c) Al-treated C. sinensis leaves could keep a better balance between protein phosphorylation and dephosphorylation than did Al-treated C. grandis leaves; and (d) both the equilibrium of hormones and hormone-mediated signal transduction were greatly disrupted in Al-treated C. grandis leaves, but less altered in Al-treated C. sinensis leaves. Finally, we discussed the differences in Al-responsive genes between Citrus roots and leaves.

RAPTOR Controls Developmental Growth Transitions by Altering the Hormonal and Metabolic Balance.

Vegetative growth requires the systemic coordination of numerous cellular processes, which are controlled by regulatory proteins that monitor extracellular and intracellular cues and translate them into growth decisions. In eukaryotes, one of the central factors regulating growth is the serine/threonine protein kinase Target of Rapamycin (TOR), which forms complexes with regulatory proteins. To understand the function of one such regulatory protein, Regulatory-Associated Protein of TOR 1B (RAPTOR1B), in plants, we analyzed the effect of raptor1b mutations on growth and physiology in Arabidopsis (Arabidopsis thaliana) by detailed phenotyping, metabolomic, lipidomic, and proteomic analyses. Mutation of RAPTOR1B resulted in a strong reduction of TOR kinase activity, leading to massive changes in central carbon and nitrogen metabolism, accumulation of excess starch, and induction of autophagy. These shifts led to a significant reduction of plant growth that occurred nonlinearly during developmental stage transitions. This phenotype was accompanied by changes in cell morphology and tissue anatomy. In contrast to previous studies in rice (Oryza sativa), we found that the Arabidopsis raptor1b mutation did not affect chloroplast development or photosynthetic electron transport efficiency; however, it resulted in decreased CO2 assimilation rate and increased stomatal conductance. The raptor1b mutants also had reduced abscisic acid levels. Surprisingly, abscisic acid feeding experiments resulted in partial complementation of the growth phenotypes, indicating the tight interaction between TOR function and hormone synthesis and signaling in plants.

Effects of drought stress on photosynthesis and photosynthetic electron transport chain in young apple tree leaves.

ABSTRACTIn our study, the effects of water stress on photosynthesis and photosynthetic electron transport chain (PETC) were studied in several ways, including monitoring the change of gas exchange parameters, modulated chlorophyll fluorescence, rapid fluorescence induction kinetics, reactive oxygen species (ROS), antioxidant enzyme activities and D1 protein levels in apple leaves. Our results show that when leaf water potential (ψw) is above –1.5 MPa, the stomatal limitation should be the main reason for a drop of photosynthesis. In this period, photosynthetic rate (PN), stomatal conductance (Gs), transpiration rate (E) and intercellular CO2 concentration (Ci) all showed a strong positive correlation with ψw. Modulated chlorophyll fluorescence parameters related to photosynthetic biochemistry activity including maximum photochemical efficiency (Fv/Fm), actual photochemical efficiency of PSII (ΦPSII), photochemical quenching coefficient (qP) and coefficient of photochemical fluorescence quenching assuming interconnected PSII antennae (qL) also showed a strong positive correlation as ψw gradually decreased. On the other hand, in this period, Stern-Volmer type non-photochemical quenching coefficient (NPQ) and quantum yield of light-induced non-photochemical fluorescence quenching [Y(NPQ)] kept going up, which shows an attempt to dissipate excess energy to avoid damage to plants. When ψw was below –1.5 MPa, PN continued to decrease linearly, while Ci increased and a ‘V’ model presents the correlation between Ci and ψw by polynomial regression. This implies that, in this period, the drop in photosynthesis activity might be caused by non-stomatal limitation. Fv/Fm, ΦPSII, qP and qL in apple leaves treated with water stress were much lower than in control, while NPQ and Y(NPQ) started to go down. This demonstrates that excess energy might exceed the tolerance ability of apple leaves. Consistent with changes of these parameters, excess energy led to an increase in the production of ROS including H2O2 and O2•−. Although the activities of antioxidant enzymes like catalase (CAT), superoxide dismutase (SOD) and peroxidase (POD) increased dramatically and ascorbate peroxidase (APX) decreased in apple leaves with drought stress, it was still not sufficient to scavenge ROS. Consequently, the accumulation of ROS triggered a reduction of net D1 protein content, a core protein in the PSII reaction center. As D1 is responsible for the photosynthetic electron transport from plastoquinone A (QA) to plastoquinone B (QB), the capacity of PETC between QA and QB was considerably downregulated. The decline of photosynthesis and activity of PETC may result in the shortage of adenosine triphosphate (ATP) and limitation the regeneration of RuBP (Jmax), a key enzyme in CO2 assimilation. These are all non-stomatal factors and together contributed to decreased CO2 assimilation under severe water stress.

Regulation of the photosynthetic electron transport and specific photoprotective mechanisms in Ricinus communis under drought and recovery

Ricinus communis is one of the major commercial non-edible oilseed crops grown in semiarid and arid environments worldwide and is reported as a drought tolerant species. Surprisingly, little is known about the mechanisms achieving this tolerance, especially in relation to photoprotection. The aim of this study was to analyze the association of the regulation of the photosynthetic electron transport and photoprotective mechanisms with drought tolerance in R. communis. Drought induced decreases in the relative water content, water potential and growth in R. communis exposed to 9 days of drought. After 6 days of rehydration, these parameters were completely recovered, demonstrating a potential of drought tolerance in this species. In addition, drought inhibited photosynthesis by stomatal and metabolic limitations (V cmax, J max, and Rubisco activity), with partial recovery after rehydration. Leaves displayed transient photoinhibition after 6 days of drought, which was completely recovered after 6 days of dehydration. The effective quantum yields and the electron transport rates of PSII and PSI were modulated to face drought avoiding the excess energy produced by decreases in CO2 assimilation. NPQ was increased during drought, and it was maintained higher than control after the recovery treatment. In addition, the estimated cyclic electron flow was induced under drought and decreased after recovery. Photorespiration was also increased under drought and maintained at higher levels after the recovery treatment. Furthermore, antioxidative enzymes activities (SOD, APX, and CAT) were increased under drought to avoid ROS harmful effects. Altogether, we clearly showed that the modulation of photoprotective mechanisms and antioxidant enzymes are crucial to this species under drought. The implication of these strikingly strategies to drought tolerance is discussed in relation to agricultural and natural systems.

Physiological and biochemical responses of Machilus ichangensis Rehd. et Wils and Taxus chinensis (Pilger) Rehd. to elevated O3 in subtropical China

Considerable researches have documented the negative effects of ozone on woody species in North America and Europe; however, little is known about how woody tree species respond to elevated O3 in subtropical China, and most of the previous studies were conducted using pot experiment. In the present study, Machilus ichangensis Rehd. et Wils (M. ichangensis) and Taxus chinensis (Pilger) Rehd. (T. chinensis), evergreen tree species in subtropical China, were exposed to non-filtered air (NF), 100nmolmol-1 O3 (E1) and 150nmolmol-1 O3 (E2), in open-top chambers under field conditions from 21st March to 2nd November 2015. In this study, O3 fumigation significantly reduced net photosynthesis rate (Pn) in M. ichangensis in the three measurements and in T. chinensis in the last measurement. Also, non-stomatal factors should be primarily responsible for the decreased Pn. O3 fumigation-induced increase in malondialdehyde, superoxide dismutase, and reduced ascorbic acid levels indicated that antioxidant defense mechanism had been stimulated to prevent O3 stress and repair the oxidative damage. Yet, the increase of antioxidant ability was not enough to counteract the harm of O3 fumigation. Because of the decrease in CO2 assimilation, the growth of the two tree species was restrained ultimately. The sensitivity of the two tree species to O3 can be determined: M. ichangensis>T. chinensis. It suggests a close link between the rising O3 concentrations and the health risk of some tree species in subtropics in the near future.

Physiological evaluation of drought tolerance in Poplar (Populus deltoids L.) for different drought levels

Experiments were conducted to evaluate drought tolerance under different levels of water stress in poplar trees. The cuttings of Populus deltoides L. (clone Kranti) were exposed to four different watering regimes (100, 75, 50 and 25% of the field capacity) and changes in physiological parameters related with drought tolerance were recorded. Drought treatments (75%, 50% and 25% FC) decreased net photosynthetic rate (Pn), transpiration rate (E), chlorophyll fluorescence (Fv/Fmax), plant height, number of leaves, specific leaf area (SLA), leaf area index (LAI) and total biomass content in all the three watering regimes compared to control (100% FC). Cuttings were showed poor performance with increasing levels of drought stress. Severity were observed in Pn, E, Fv/Fmax, plant height, stem diameter, leaf area and number of leaves, SLA, LAI and total biomass content with increasing levels of water stress. Decreased CO2 assimilation and transpiration rate due to instantaneous closure of stomata to protect the plants against hazardous effects of water stress leads to overall decrease in biomass of cuttings with 60 days water stress treatments. By visualizing the results, we can say that Scarcity of water is a severe environmental constraint to plant productivity. Drought-induced loss in plant productivity, since both the severity and duration of the stress are critical. Secondly, we can emphasise with our experiment that poplar plants can maintain their better growth and biomass only up to 75-50% of FC after that stress shows its severity so much that the aim of plants is only to survive and biomass maintenance become vague.