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Cold acclimation modifies the photosynthetic machinery and enables plants to survive at sub-zero temperatures, whereas in warm habitats, many species suffer even at non-freezing temperatures. We have measured chlorophyll a fluorescence (ChlF) and CO2 assimilation to investigate the effects of cold acclimation, and of low temperatures, on a cold-sensitive Arabidopsis thaliana accession C24. Upon excitation with low intensity (40 µmol photons m− 2 s− 1) ~ 620 nm light, slow (minute range) ChlF transients, at ~ 22 °C, showed two waves in the SMT phase (S, semi steady-state; M, maximum; T, terminal steady-state), whereas CO2 assimilation showed a linear increase with time. Low-temperature treatment (down to − 1.5 °C) strongly modulated the SMT phase and stimulated a peak in the CO2 assimilation induction curve. We show that the SMT phase, at ~ 22 °C, was abolished when measured under high actinic irradiance, or when 3-(3, 4-dichlorophenyl)-1, 1- dimethylurea (DCMU, an inhibitor of electron flow) or methyl viologen (MV, a Photosystem I (PSI) electron acceptor) was added to the system. Our data suggest that stimulation of the SMT wave, at low temperatures, has multiple reasons, which may include changes in both photochemical and biochemical reactions leading to modulations in non-photochemical quenching (NPQ) of the excited state of Chl, "state transitions," as well as changes in the rate of cyclic electron flow through PSI. Further, we suggest that cold acclimation, in accession C24, promotes "state transition" and protects photosystems by preventing high excitation pressure during low-temperature exposure. [ABSTRACT FROM AUTHOR]

Minimizing the impact of heat and drought on crop yields requires varieties with effective protective mechanisms. We tested the hypothesis that even a short-term high temperature amplifies the negative effects of reduced water availability on leaf gas-exchange, but can induce long-lasting improvement in plant water-use efficiency after the stress period. Accordingly, three common varieties of winter wheat (Triticum aestivum) were grown under field conditions. During the stem extension, the plants were exposed to distinct temperatures (daily maximum 26 vs. 38 °C), water availabilities (75% of field water capacity vs. permanent wilting point), and their combination for 14 days. All treatments reduced light-saturated rates of CO 2 assimilation and transpiration, particularly when heat and drought were combined. Drought enhanced water-use efficiency (WUE) in all varieties (31.4–36.4%), but not at high temperatures (decrease by 17–52%). Intrinsic WUE (iWUE), determined from the stable carbon isotope composition of grains, was enhanced by 7.9–37% in all treatments and varieties; however, not all changes were significant. The combination of heat and drought tended to increase total protein content in grains but reduced spike productivity. Noticeably, the strongest decline in spike productivity was observed in Elan – the variety displaying the smallest enhancement of iWUE, while it was negligible in Pannonia which shows the most pronounced improvement of iWUE. We conclude that even several hot and dry days can improve iWUE for the rest of the vegetation season. This improvement, however, does not necessarily lead to increased crop productivity possibly due to physiological trade-offs. • High temperature amplifies drought-induced impairments of gas-exchange processes. • Drought enhances water-use efficiency (WUE), but not at high temperatures. • Heat and drought during stem extension increase protein content in grains. • Grain carbon isotopes indicate enhanced intrinsic WUE after a short period of stress. [ABSTRACT FROM AUTHOR]

Background and Aims There is a conspicuous increase of poikilohydric organisms (mosses, liverworts and macrolichens) with altitude in the tropics. This study addresses the hypothesis that the lack of bryophytes in the lowlands is due to high-temperature effects on the carbon balance. In particular, it is tested experimentally whether temperature responses of CO2-exchange rates would lead to higher respiratory carbon losses at night, relative to potential daily gains, in lowland compared with lower montane forests. Methods Gas-exchange measurements were used to determine water-, light-, CO2- and temperature-response curves of net photosynthesis and dark respiration of 18 tropical bryophyte species from three altitudes (sea level, 500 m and 1200 m) in Panama. Key Results Optimum temperatures of net photosynthesis were closely related to mean temperatures in the habitats in which the species grew at the different altitudes. The ratio of dark respiration to net photosynthesis at mean ambient night and day temperatures did not, as expected, decrease with altitude. Water-, light- and CO2-responses varied between species but not systematically with altitude. Conclusions Drivers other than temperature-dependent metabolic rates must be more important in explaining the altitudinal gradient in bryophyte abundance. This does not discard near-zero carbon balances as a major problem for lowland species, but the main effect of temperature probably lies in increasing evaporation rates, thus restricting the time available for photosynthetic carbon gain, rather than in increasing nightly respiration rates. Since optimum temperatures for photosynthesis were so fine tuned to habitat temperatures we analysed published temperature responses of bryophyte species worldwide and found the same pattern on the large scale as we found along the tropical mountain slope we studied. [ABSTRACT FROM AUTHOR]

• Legacy and Bluegold cultivars were differentially affected by solar UV irradiation and seasons. • Bluegold was no able to reduce oxidative damage compared to Legacy. • Bluegold cultivar triggered antioxidant defense to cope with + UV-A/B, maintaining CO 2 assimilation. • MYBPA1 , CHS , and F 3' H genes increased concomitant with flavonoid and anthocyanin concentrations in Bluegold. Solar UV irradiation allows life on Earth, but becomes harmful at high levels due to UV-B and UV-A irradiation, generating negative effects for all organisms, including plants. We determined the effects of solar UV irradiation on photosynthetic performance, biochemical markers, and gene expression in five-year-old plants of two highbush blueberry (Vaccinium corymbosum L.) cultivars, Legacy and Bluegold. Plants growing under field conditions were subjected to three solar UV irradiation filter treatments: UV-transmitting filter (+UV-A/B), UV-B blocking filter (-UV-B), and UV-A/B blocking filter (-UV-A/B) during two seasons. Net photosynthesis did not change among solar radiation filter treatments in either cultivar. Bluegold showed a significant reduction in stomatal conductance (g s) during both seasons under +UV-A/B compared to -UV-A/B (control treatment); while effective quantum yield (ΦPSII) and electron transport rate (ETR) did not change in Legacy, Bluegold showed a reduction in ΦPSII and ETR during the 2014 season. The highest significant increase (P ≤ 0.05) in total phenol concentration was observed in Bluegold (40%) under +UV-A/B in the 2013 season compared to Legacy. A similar tendency in flavonoids and anthocyanin was observed but in both seasons. In conclusion, an interaction genotype-environment was observed, where Legacy and Bluegold cultivars were differentially affected by solar UV irradiation and seasons in field conditions. Although Bluegold maintains CO 2 assimilation, activating antioxidant defense, gene expression (MYBPA1 , CHS , and F 3' H) and metabolites (flavonoid and anthocyanins) to counteract +UV-A/B treatment, this cultivar was no able to reduce oxidative stress and to recover photochemical efficiency of PSII compared to Legacy. Our findings contribute to the understanding of blueberry responses to solar UV irradiation at physiological, molecular, and metabolite levels under field conditions. [ABSTRACT FROM AUTHOR]