Soaking Maize Seeds in Zeatin-Type Cytokinin Biostimulators Improves Salt Tolerance by Enhancing the Antioxidant System and Photosynthetic Efficiency.

There is an urgent need for innovative strategies to raise the performance of environmentally stressed plants. The seeds of single-cross yellow Zea mays (L.) hybrid Giza-168 were soaked in Cis-(c-Z-Ck) or trans-zeatin-type cytokinin (t-Z-Ck) solutions at a concentration of 50 or 40 µM, respectively. Salinity stress was imposed at 0, 75 or 150 mM NaCl in the Hoagland nutrient solution (full strength) used for irrigation. The total carotenoids content was negatively affected by only 150 mM NaCl, while both 75 and 150 mM NaCl negatively affected the growth and yield components, relative water content, membrane stability index, photochemical activity, gas exchange, K+ and chlorophyll contents, K+/Na+ ratio, and photosynthetic efficiency. However, all of these traits were significantly improved by c-Z-Ck pretreatment and further enhanced by t-Z-Ck pretreatment compared with the corresponding controls. Furthermore, the contents of proline, soluble sugars, ascorbate, and glutathione, as well as enzymatic antioxidant activities, were significantly elevated by both salt stress concentrations and increased more by both biostimulators compared to the control. Compared to c-Z-Ck, t-Z-Ck was superior in mitigating the harmful effects of the high H2O2 levels caused by salt stress on the levels of malondialdehyde and ion leakage compared to the control. Under normal or stress conditions, t-Z-Ck pretreatment was better than c-Z-Ck pretreatment, while both positively affected maize hormonal contents. As a result, t-Z-Ck is recommended to enhance the growth and productivity of maize plants by suppressing the effects of oxidative stress caused by saline water irrigation.

Rebalancing Nutrients, Reinforcing Antioxidant and Osmoregulatory Capacity, and Improving Yield Quality in Drought-Stressed Phaseolus vulgaris by Foliar Application of a Bee-Honey Solution.

Bee-honey solution (BHS) is considered a plant growth multi-biostimulator because it is rich in osmoprotectants, antioxidants, vitamins, and mineral nutrients that can promote drought stress (DtS) resistance in common bean plants. As a novel strategy, BHS has been used in a few studies, which shows that the application of BHS can overcome the stress effects on plant productivity and can contribute significantly to bridging the gap between agricultural production and the steady increase in population under climate changes. Under sufficient watering (SW (100% of crop evapotranspiration; ETc) and DtS (60% of ETc)), the enhancing impacts of foliar application with BHS (0%, 0.5%, 1.0%, and 1.5%) on growth, productivity, yield quality, physiological-biochemical indices, antioxidative defense ingredients, and nutrient status were examined in common bean plants (cultivar Bronco). DtS considerably decreased growth and yield traits, green pod quality, and water use efficiency (WUE); however, application of BHS at all concentrations significantly increased all of these parameters under normal or DtS conditions. Membrane stability index, relative water content, nutrient contents, SPAD (chlorophyll content), and PSII efficiency (Fv/Fm, photochemical activity, and performance index) were markedly reduced under DtS; however, they increased significantly under normal or DtS conditions by foliar spraying of BHS at all concentrations. The negative impacts of DtS were due to increased oxidants [hydrogen peroxide (H2O2) and superoxide (O2•-)], electrolyte leakage (EL), and malondialdehyde (MDA). As a result, the activity of the antioxidant system (ascorbate peroxidase, glutathione reductase, catalase, superoxide dismutase, α-tocopherol, glutathione, and ascorbate) and levels of osmoprotectants (soluble protein, soluble sugars, glycine betaine, and proline) were increased. However, all BHS concentrations further increased osmoprotectant and antioxidant capacity, along with decreased MDA and EL under DtS. What is interesting in this study was that a BHS concentration of 1.0% gave the best results under SW, while a BHS concentration of 1.5% gave the best results under DtS. Therefore, a BHS concentration of 1.5% could be a viable strategy to mitigate the DtS impairment in common beans to achieve satisfactory growth, productivity, and green pod quality under DtS.