Plant production under climate changes requires unique treatments to overcome the detrimental effects of abiotic stress, such as water deficiency stress. Silicon (Si) has many beneficial effects, especially in plants subjected to different types of stress. Hence, Si foliar application was used to study its potential effects on boosting osmolytes content, activity of antioxidant enzymes, growth of vegetative and flowering organs, and yield of squash plants (Cucurbita pepo L.) under deficit irrigation. Two field experiments were conducted during the winter seasons of 2019 and 2020 at the Experimental Farm, Faculty of Agriculture, Ain Shams University, Egypt. Three concentrations of Si at 0, 2000 and 4000 ppm in the form of monosilicic acid (H4SiO4) under two levels of water irrigation at 80% of water holding capacity (WHC) as control and 50% of WHC as drought treatment. Drought treatment significantly reduced the growth parameters: shoot height, shoot fresh and dry weights, leaves number/plant, average leaf area, fruit setting and yield traits, and the physiological attributes: leaf relative water content (LRWC), total chlorophylls (SPAD), total soluble proteins (TSP) and catalase (CAT) activity. Meanwhile, significant increases in the concentrations of stress indicators (total free amino acids and proline) were observed compared to the control plants (80% WHC). Both rates of Si achieved marked increases in the vegetative growth, flowering, chlorophylls, LRWC, TSP, superoxide dismutase (SOD), CAT, peroxidase (POD) and polyphenol oxidase (PPO) activity, which in turn reflects on improving fruit setting %, total fruits, and yield of squash plants under well or deficit irrigation. Spraying the high level of Si (4000 ppm) was the most effective treatment that emended the adverse effects of drought.
