The effects of drought and the diurnal changes in photosynthetic electron transport were studied in non-nodulated plants of Casuarina equisetifolia. The induction of fluorescence showed a slightly higher I step in water-stressed than control plants, and the time from the start of irradiation to the P step of induction was significantly shortened by drought. The quantum efficiency of photosystem 2 (PS2) in the dark-adapted state (Fv/Fm) was generally not affected by drought, whereas it decreased during the central hours of the day. The decrease in quantum yield of PS2 electron transport (Φ2) in water-stressed plants was associated with decreases in the photochemical efficiency of open (oxidised) PS2 centres (Fv'/Fm') and increases in non-photochemical quenching (qN) rather than with increased closure of PS2 centres (lowered photochemical quenching, qP). In contrast, the changes in quantum yield of electron transport during the day were related to changes in qP rather than in Fv'/Fm'. When chlorophyll fluorescence was measured at the same irradiance during the day, a greater qN was observed at the end of the drying cycle than after watering, and early and late in the photoperiod than in the central hours of the day. The greater qN at the beginning and end of the day did not prevent an increase in energy not used photochemically nor dissipated non-photochemically. Drought did not affect this excess of photon energy. and R. Martínez-Carrasco, J. Sánchez-Rodriguez, P. Pérez.
Bean (Phaseolus vulgaris L. cv. Berbukskaya) seedlings were pre-treated with choline compounds, 19 mM 2-ethyltrimethylammonium chloride (Ch) or 1.6 mM 2-chloroethyltrimethylammonium chloride (CCh), during 24 h, then after 6 d the excised primary leaves were exposed to UV-B and high temperature stress. Chlorophyll (Chl) fluorescence, delayed light emission, accumulation of photosynthetic pigments, contents of thiobarbituric acid reactive substances, and activities of the active oxygen detoxifying enzymes (superoxide dismutase, ascorbate peroxidase, and glutathione reductase) were examined. Pre-treatment of plants with Ch or CCh enhanced the resistance of photosystem 2 (PS2) photochemistry to UV-B and heat injuries. The higher stress resistance can be explained by the increased activity of the detoxifying enzymes. The increased content of UV-B-absorbing pigments may also contribute to the enhanced resistance of choline-treated plants to UV-B radiation. and V. D. Kreslavski ... [et al.].