Growth and photosynthetic characteristics, inducibility of the CAM pathway and the functioning of the antioxidant defense system were investigated in Rosularia elymaitica (Crassulaceae) under drought and UV stresses. Drought did not substantially affect the growth of the plants, but it significantly reduced leaf thickness as well as osmotic potential, water potential and relative water content. In contrast, UV radiation treatment affected neither growth nor the water relations of leaves. Water limitation for 12 days caused a significant increase in nighttime PEPC and NAD-MDH activity and an increase in Δtitratable acidity relative to well-watered plants. The nighttime CO2 net assimilation rate increased significantly in drought-stressed plants but was still negative, resembling a C3-like pattern of gas exchange. Twenty days of UV treatment, increased Δtitratable acidity slightly and increased only daytime PEPC activity, and did not affect other parameters of carbon metabolism. As judged by maintenance of membrane integrity and stable amounts of H2O2 under UV stress, the antioxidant defense system effectively protected the plants against UV radiation. In contrast, oxidative stress occurred under severe drought stress (20 days of withholding water). Except for higher daytime APX activity in the UV-treated plants, enzyme activity in the control and in the drought- and UV-stressed plants did not show any diurnal fluctuation during 24 h. Temporal changes in Δtitratable acidity and ΔPEPC activity coincided closely with those of antioxidant enzymes; both started to increase after 12 days of drought stress. These results indicate that drought stress but not UV radiation induced the CAM-cycling pathway in R. elymaitica.
Since plant responses to selenium nanoparticles (nSe) had not been clarified, this study was carried out to evaluate the effects of nSe (10 and 100 μM) on photosynthesis performance, ion homeostasis, antioxidant system, and phenylpropanoids in strawberry exposed to salt stress. Inductively Coupled Plasma-Mass Spectroscopy analyses indicated that foliar-applied nSe can be taken up by leaves and trans-located to roots. Salinity led to an increase in Na concentration and reductions in Ca and K contents which were relieved by the nSe applications. Moreover, the nSe treatment at 10 μM alleviated the NaCl-induced lesion to PSII functioning, contributing to improvement in water-splitting complex (Fv/Fo) under salinity. The exposure to nSe at a concentration of 100 µM exhibited a moderate stress, determined by the increases in hydrogen peroxide (H2O2) and lipid peroxidation rate (membrane integrity index). The nSe10 treatment increased catalase activity and phenylpropanoid derivatives contents (salicylic acid, catechin, and caffeic acid) and decreased the content of oxidants under salinity condition. Consequently, nSe utilization at a suitable dose can be an effective method to alleviate signs of salt stress via improvements in photosynthesis, ion hemostasis, photosynthesis performance, salicylic acid (a vital signaling defensive hormone), and antioxidant machinery.