Silicon Enhances Morpho–Physio–Biochemical Responses in Arsenic Stressed Spinach (Spinacia oleracea L.) by Minimizing Its Uptake

Abstract

Soil contamination with toxic heavy metals [such as arsenic (As)] is becoming a serious global problem due to rapid development of social economy. Silicon (Si), being an important fertilizer element, has been found effective in enhancing plant tolerance against biotic and abiotic stresses. For this purpose, we have designed the current experiment to explore the contribution of Si in mediating growth and eco-physiology by alleviating As stress in a leafy vegetable spinach (Spinacia oleracea L.). Fifteen days old seedlings of S. oleracea were subjected to the different concentrations of As, i.e., 0 (no As), 50, and 100 µM in the soil which were also supplied with the different exogenous levels of Si, i.e., 0 (no Si), 1.5, and 3 mM. Results from the present study revealed that the As toxicity induced a substantial decreased in shoot length, root length, number of leaves, leaf area, shoot fresh weight, root fresh weight, shoot dry weight, root dry weight, chlorophyll-a, chlorophyll-b, total chlorophyll, carotenoid content, net photosynthesis, stomatal conductance, transpiration rate, soluble sugar, reducing sugar, non-reducing sugar contents, calcium (Ca2+), magnesium (Mg2+), iron (Fe2+), and phosphorus (P) contents in the roots and shoots of the plants. In contrast, increasing levels of As in the soil significantly (P < 0.05) increased As concentration in the roots and shoots of the plants, phenolic content, malondialdehyde (MDA), hydrogen peroxide (H2O2), electrolyte leakage (EL), fumaric acid, acetic acid, citric acid, formic acid, malic acid, oxalic acid contents in the roots of the plants. Although, the activities of enzymatic antioxidants such as superoxidase dismutase, peroxidase, catalase, ascorbate peroxidase in the roots and shoots of the plants and non-enzymatic such as phenolic, flavonoid, ascorbic acid, and anthocyanin contents were initially increased with the exposure of 50 µM As, but decreased by the increasing the As concentration 100 µM in the soil. Addition of Si into the soil significantly alleviated As toxicity effects on S. oleracea by improving photosynthetic capacity and ultimately plant growth. Increased activities of antioxidant enzymes in Si-treated plants seem to play a role in capturing stress-induced reactive oxygen species as was evident from lower level of MDA, H2O2, MDA, and EL in Si-treated plants. Research findings, therefore, suggested that Si application can ameliorate As toxicity in S. oleracea seedlings and resulted in improved plant growth and composition under metal stress as depicted by balanced exudation of organic acids.