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The positive effects of high atmospheric CO₂ concentrations [CO₂] decrease over time in most C₃ plants because of down-regulation of photosynthesis. A notable exception to this trend is plants hosting N-fixing bacteria. The decrease in photosynthetic capacity associated with an extended exposure to high [CO₂] was therefore studied in non-nodulating rice that can establish endophytic interactions. Rice plants were inoculated with diazotrophic endophytes isolated from the Salicaceae and CO₂ response curves of photosynthesis were determined in the absence or presence of endophytes at the panicle initiation stage. Non-inoculated plants grown under elevated [CO₂] showed a down-regulation of photosynthesis compared to those grown under ambient [CO₂]. In contrast, the endophyteinoculated plants did not show a decrease in photosynthesis associated with high [CO₂], and they exhibited higher photosynthetic electron transport and mesophyll conductance rates than non-inoculated plants under high [CO₂]. The endophyte-dependent alleviation of decreases in photosynthesis under high [CO₂] led to an increase in water-use efficiency. These effects were most pronounced when the N supply was limited. The results suggest that inoculation with N-fixing endophytes could be an effective means of improving plant growth under high [CO₂] by alleviating N limitations.

Endophytes are microbial symbionts living inside plants and have been extensively researched in recent decades for their functions associated with plant responses to environmental stress. We conducted a meta-analysis of endophyte effects on host plants’ growth and fitness in response to three abiotic stress factors: drought, nitrogen deficiency, and excessive salinity. Ninety-four endophyte strains and 42 host plant species from the literature were evaluated in the analysis. Endophytes increased biomass accumulation of host plants under all three stress conditions. The stress mitigation effects by endophytes were similar among different plant taxa or functional groups with few exceptions; eudicots and C₄ species gained more biomass than monocots and C₃ species with endophytes, respectively, under drought conditions. Our analysis supports the effectiveness of endophytes in mitigating drought, nitrogen deficiency, and salinity stress in a wide range of host species with little evidence of plant-endophyte specificity.

To investigate the effects of short-term low temperatures, three-year-old avocado ( Persea americana cv. Hass) seedlings were treated with 1, − 2, or − 5 °C for 1 h and subsequently recovered in ambient condition for 24 h. Leaf color changes were investigated with chlorophyll, carotenoid, and phenolic contents. Photosynthetic responses were examined using gas exchange analysis. With H 2 O 2 contents as oxidative stresses, enzymatic (ascorbate peroxidase, APX; glutathione reductase, GR; catalase, CAT; peroxidase, POD) and non-enzymatic antioxidant activities were determined using spectrophotometry. Leaves in the avocado seedlings started to be discolored with changes in the contents of chlorophyll a , carotenoids, and phenolics when treated with − 5 °C. However, the H 2 O 2 content was not different in leaves treated with low temperatures. Photosynthetic activities decreased in leaves in the seedlings treated with − 5 °C. Of antioxidant enzymes, APX and GR have high activities in leaves in the seedlings treated with 1 and − 2 °C. In leaves in the seedlings treated with − 5 °C, the activities of all enzymes decreased. Non-enzymatic antioxidant activity was not different among leaves treated with low temperatures. These results indicated that APX and GR would play a critical role in withstanding chilling stress in ‘Hass’ avocado seedlings. However, under lethal temperature, even for a short time, the plants suffered irreversible damage with the breakdown of photosystem and antioxidant system.

Bacterial and yeast endophytes isolated from the Salicaceae family have been shown to promote growth and alleviate stress in plants from different taxa. To determine the physiological pathways through which endophytes affect plant water relations, we investigated leaf water potential, whole-plant water use, and stomatal responses of rice plants to Salicaceae endophyte inoculation under CO enrichment and water deficit. Daytime stomatal conductance and stomatal density were lower in inoculated plants compared to controls. Leaf ABA concentrations increased with endophyte inoculation. As a result, transpirational water use decreased significantly with endophyte inoculation while biomass did not change or slightly increased. This response led to a significant increase in cumulative water use efficiency at harvest. Different endophyte strains produced the same results in host plant water relations and stomatal responses. These stomatal responses were also observed under elevated CO conditions, and the increase in water use efficiency was more pronounced under water deficit conditions. The effect on water use efficiency was positively correlated with daily light integrals across different experiments. Our results provide insights on the physiological mechanisms of plant-endophyte interactions involving plant water relations and stomatal functions.

Endophytes are fungi, bacteria, or yeast symbionts that live in the intercellular spaces or vascular tissues of host plants. Investigations indicate that endophytes isolated from the Salicaceae family (Populus and Salix) hosts provide several benefits that promote plant growth, including but not limited to di-nitrogen fixation, plant hormone production, nutrient acquisition, stress tolerance, and defense against phytopathogens. In exchange, the microorganisms receive domicile and photosynthates. Considering the known characteristics of nitrogen fixation and plant hormone production, we hypothesized that apple trees grown under nitrogen-limited conditions would show improved biometrics with endophyte inoculation. Our research objectives were to investigate the endophyte effects on plant physiology and fruiting. We examined these effects through ecophysiology metrics involving rates of photosynthesis, stomatal conductance and density, transpiration, biomass accretion, chlorophyll content and fluorescence, and fruit soluble sugar content and biomass. Our results showed evidence of the endophytes’ colonization in apple trees, decreased stomatal density, delayed leaf senescence, and increased lateral root biomass with endophytes. A highlight of the findings was a significant increase in both fruit soluble sugar content and biomass. Future research into the mechanistic underpinnings of this phenomenon stands to offer novel insights on how microbiota may alter carbohydrate metabolism under nitrogen-deficient conditions.

A greenhouse study was conducted in order to evaluate the interactions of vermicompost and salinity effects on morphology and physiology of pot marigold. The experiment was conducted with vermicompost treatments at five levels (0%, 5%, 10%, 15% and 20%) and salinity treatments at five levels (0, 50, 100, 150 and 200 mM NaCl) in a completely randomized factorial design arrangement with four replications. Results showed that increasing levels of salinity led to decline in leaf area, fresh and dry weights of flower, shoot, and root, N, P, K, Fe, Mg and Zn concentrations, chlorophyll and carotenoid contents, while proline content increased in the plants. APX, SOD, POD and CTA enzyme activities significantly increased with increasing salinity from 0 to 150 mM NaCl, then declined in 200 mM treatment in the plants. Application of vermicompost increased the morpho-physiological indices and mineral nutrient uptake in the plants and could increase the plant yield by alleviating the harmful effects of salinity.

Endophytes are bacteria, fungi, and yeast that live inside plants. Endophytes provide fitness benefits to the host plant while receiving carbohydrates in return. A wide range of bacteria and yeast endophyte strains was isolated from native Salicaceae trees growing in a riparian area. Previous studies on these Salicaceae endophyte isolates have shown their symbiotic traits in various host crops across taxa. Biological di-nitrogen fixation, phytohormone production, and an enhancement of drought stress tolerance in plants were important beneficial functions of these endophyte isolates. However, their impacts on the physiology of the host plant had not been examined in-depth. The focus of the present study was on water relations, photosynthesis, and respiration of the host plant to better understand symbiotic associations in the plant eco-physiological context. Select Salicaceae diazotrophic endophytes were inoculated into rice as a model C3 plant, and the effects on the physiology were assessed in a series of greenhouse experiments. The inoculated plants showed reduced stomatal conductance in the afternoon than control plants. Stomatal density of the inoculated plants was also lower than that of control plants. The accumulation of leaf ABA during the afternoon was facilitated in the inoculated plants. The stomatal responses of the inoculated plants led to decreases in transpiration, and further to increases in water use efficiency of the plants. The endophyte inoculation alleviated down-regulation of the host plant photosynthesis to elevated CO2. Moreover, the inoculated plants showed the improvements in photosynthesis compared to control plants. The improvements featured increases in electron transport rate of the photosynthetic light reactions and increases in internal CO2 conductance of the CO2 diffusion pathways in leaves. The inoculated plants showed increases in respiration rates. In vitro respiration rates of the microbes were positively correlated to the concentrations of carbohydrate supply and the number of the microbes on growing media. The in planta and the in vitro assay results provided an estimation of microbial respiratory CO2 release in the host plant. The estimate was approximately 15% of total assimilated CO2 through photosynthesis. This suggests microbial respiratory CO2 could be a significant amount and possibly reenter the photosynthetic CO2 assimilatory pathways. The stomatal closure, the photosynthetic improvements, and the respiration responses together imply the possibility of the re-assimilation and partially explain the increases in water use efficiency of the plant. Further investigation will be required to confirm the re-assimilation hypothesis with convincing empirical evidence. The key to uncover future significant findings will be an understanding of source-sink relations and carbon-nitrogen relations in plants with endophytes – the resource exchanges between the host plant and endophytes.

The levels of stomatal, mesophyll and biochemical limitations in CO₂ assimilation of 'Bluecrop' highbush blueberry leaves were compared at two different levels of leaf water potential. The leaf water potentials were -1.49 and -1.94 MPa in daily-irrigated (DI) and non-irrigated (NI) shrubs, respectively. The NI shrubs represented plants under moderate water stress. Mesophyll conductance (g∧m) and chloroplastic CO₂ concentration (C∧c) were estimated by combined measurements of gas exchange and chlorophyll fluorescence under various intercellular CO₂ concentrations (C∧i). Net CO₂ assimilation rates (A∧n) as a function of C∧c were used for calculating maximum carboxylation efficiency (α∧cmax) at the real sites of CO₂ assimilation. Maximum A∧n (A∧nmax) from the light response curves at 400 μmol mol-¹ air of ambient CO₂ concentration (C∧a) were lower in the leaves of NI shrubs than in those of DI ones. However, electron transport rates were higher in the leaves of NI shrubs than in those of DI ones. The decrease in CO₂ assimilation following water stress may be caused by a decrease in g∧m rather than a decrease in stomatal conductance (g∧s) according to limitation analysis. Limitation rates by g∧s, calculated at 400 μmol mol-¹ air of C∧a in A∧n-C∧i curves, were not significantly different between the leaves of DI and NI shrubs. However, limitation rates by g∧m from A∧n-C∧c curves were significantly higher in the leaves of NI shrubs than in those of DI ones. Maximum carboxylation efficiency (α∧cmax) values calculated from the A∧n-C∧c curve, contrary to those calculated from the A∧n-C∧i curve, were higher in the leaves of NI shrubs than in those of DI ones. Consequently, mesophyll limitation than stomatal and biochemical limitations mainly down-regulated the photosynthesis in the leaves of 'Bluecrop' blueberry shrubs during moderate water stress.