The applications of ZnO nanoparticles in agriculture have largely contributed to crop growth regulation, quality enhancement, and induction of stress tolerance, while the underlying mechanisms remain elusive. Herein, the involvement of melatonin synthesis and metabolism in the process of nano-ZnO induced drought tolerance was investigated in maize. Drought stress resulted in the changes of subcellular ultrastructure, the accumulation of malondialdehyde and osmolytes in leaf. The nano-ZnO (100 mg L⁻¹) application promoted the melatonin synthesis and activated the antioxidant enzyme system, which alleviated drought-induced damage to mitochondria and chloroplast. These changes were associated with upregulation of the relative transcript abundance of Fe/Mn SOD, Cu/Zn SOD, APX, CAT, TDC, SNAT, COMT, and ASMT induced by nano-ZnO application. It was suggested that modifications in endogenous melatonin synthesis were involved in the nano-ZnO induced drought tolerance in maize.

The release of nanoparticles into the environment is inevitable, which has raised global environmental concern. Melatonin is involved in various stress responses in plants. The present study investigated the effects of melatonin on photosynthetic carbon (C) assimilation and plant growth in nano-ZnO stressed plants. It was found that melatonin improved the photosynthetic C assimilation in nano-ZnO stressed wheat plants, mainly due to the enhanced photosynthetic energy transport efficiency, higher chlorophyll concentration and higher activities of Rubisco and ATPases. In addition, melatonin enhanced the activities of antioxidant enzymes to protect the photosynthetic electron transport system in wheat leaves against the oxidative burst caused by nano-ZnO stress. These results suggest that melatonin could improve the tolerance of wheat plants to nano-ZnO stress.

Cold priming can alleviate the effects of subsequent cold stress on wheat plant growth. Melatonin plays a key role in cold stress response in plants. In this study, the effects of foliar melatonin application during recovery on the cold tolerance of cold primed wheat plants were investigated. It was found that both melatonin and cold priming increased the photosynthetic rate and stomatal conductance, enhanced the activities of antioxidant enzymes, and altered the related gene expressions in wheat under cold stress. Melatonin application is helpful for the photosynthetic carbon assimilation and membrane stability of the cold primed plants under cold stress. These results suggested that foliar melatonin application during recovery enhanced the cold priming induced tolerance to subsequent low temperature stress in wheat.

Stomata play a critical role in the regulation of gas exchange between the interior of the leaf and the exterior environment and are affected by environmental and endogenous stimuli. This study aimed to evaluate the effect of the arbuscular mycorrhizal (AM) fungus, Rhizophagus irregularis, on the stomatal behavior of wheat (Triticum aestivum L.) plants under combination with elevated CO₂ and NaCl stress. Wheat seedlings were exposed to ambient (400 ppm) or elevated (700 ppm) CO₂ concentrations and 0, 1, and 2 g kg⁻¹ dry soil NaCl treatments for 10 weeks. AM symbiosis increased the leaf area and stomatal density (SD) of the abaxial surface. Stomatal size and the aperture of adaxial and abaxial leaf surfaces were higher in the AM than non-AM plants under elevated CO₂ and salinity stress. AM plants showed higher stomatal conductance (g_s) and maximum rate of g_s to water vapor (g_smax) compared with non-AM plants. Moreover, leaf water potential (Ψ) was increased and carbon isotope discrimination (Δ¹³C) was decreased by AM colonization, and both were significantly associated with stomatal conductance. The results suggest that AM symbiosis alters stomatal morphology by changing SD and the size of the guard cells and stomatal pores, thereby improving the stomatal conductance and water relations of wheat leaves under combined elevated CO₂ and salinity stress.

As an important signal molecule, salicylic acid (SA) improves plant tolerance to aluminum (Al) stress. The objective of this study was to investigate the effects of exogenous SA application on the dynamics of endogenous SA and reactive oxygen species in soybean (Glycine max L.) exposed to Al stress. The roots of soybean seedlings were exposed to a combination of AlCl3 (30 μM) and SA (10 μM)/PAC (100 μM, paclobutrazol, SA biosynthesis inhibitor) for 3, 6, 9 and 12 h. Al stress induced an increase in endogenous SA concentration in a time-dependent manner, also verified by the up-regulated expression of GmNPR1, an SA-responsive gene. Al stress increased the activities of phenylalanine ammonia-lyase (PAL) and benzoic acid 2-hydroxylase (BA2H), and the contents of SA, O2- and malondialdehyde (MDA) in the root apex. The application of exogenous SA increased PAL and BA2H, and reduced O2- and MDA contents in soybean roots under Al stress. PAC inhibited the SA induced increase in BA2H activity. In addition, the SA application resulted in a rapid increase in hydrogen peroxide (H2O2) concentration under Al stress, followed by a sharp decrease. Compared with the plants exposed to Al alone, Al+SA plants possessed higher activities of superoxide dismutase, peroxidase and ascorbate peroxidase, and lower catalase activity, indicating that SA alleviated Al-induced oxidative damage. These results suggested that PAL and BA2H were involved in Al-induced SA production and showed that SA alleviated the adverse effects of Al toxicity by modulating the cellular H2O2 level and the antioxidant enzyme activities in the soybean root apex.