Impacts of Priming with Silicon on the Growth and Tolerance of Maize Plants to Alkaline Stress

Abstract

Silicon (Si) has been known to augment plant defense against biotic and abiotic pressures. Maize (Zea maize L.) is classified as a Si accumulator and is relatively susceptible to alkaline stress. In this study, seeds of maize were grown in pots and exposed to various concentrations of Na2CO3 (0, 25, 50, and 75 mM) with or without 1.5 mM Si in the form of sodium metasilicate Na2O3Si.5H2O for 25 days. Alkaline-stressed plants showed a decrease in growth parameters, leaf relative water content (LRWC), and the contents of photosynthetic pigments, soluble sugars, total phenols and potassium ion (K(+)), as well as potassium/sodium ion (K(+)/Na(+)) ratio. By contrast, alkaline stress increased the contents of soluble proteins, total free amino acids, proline, Na(+) and malondialdehyde (MDA), as well as the activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) in stressed plants. On the other hand, application of Si by seed-priming improved growth of stressed plants, which was accompanied by the enhancement in LRWC, and levels of photosynthetic pigments, soluble sugars, soluble proteins, total free amino acids and K(+), as well as activities of SOD, CAT, and POD enzymes. Furthermore, Si supplement resulted in a decrease in the contents of proline, MDA and Na(+), which together with enhanced K(+) level led to a favorable adjustment of K(+)/Na(+) ratio, in stressed plants relative to plants treated with alkaline stress alone. Taken together, these results indicate that Si plays a pivotal role in alleviating the negative effects of alkaline stress on maize growth by improving water status, enhancing photosynthetic pigments, accumulating osmoprotectants rather than proline, activating the antioxidant machinery, and maintaining the balance of K(+)/Na(+) ratio. Thus, our findings demonstrate that seed-priming with Si is an efficient strategy that can be used to boost tolerance of maize plants to alkaline stress.

Keywords: alkaline stress; antioxidant enzymes; leaf pigments; maize; osmoprotectants; seed-priming with Si; stress mitigation.

FIGURE 1

Effects of different levels of alkaline stress with and without seed-priming with silicon (Si) on the contents of (A) chlorophyll a, (B) chlorophyll b and (C) carotenoids in leaves of 25-day-old maize plants. Bars represent standard deviation (SD) of the means (n = 3). Different letters indicate significant differences among the treatments at P < 0.05, according to Duncan’s multiple range test. FW, fresh weight.

FIGURE 2

Effects of different levels of alkaline stress with and without seed-priming with silicon (Si) on (A) Na⁺ content, (B) K⁺ content and (C) K⁺/Na⁺ ratio in 25-day-old maize plants. Bars represent standard deviation (SD) of the means (n = 3). Different letters indicate significant differences among the treatments at P < 0.05, according to Duncan’s multiple range test. DW, dry weight.

FIGURE 3

Effects of different levels of alkaline stress with and without seed-priming with silicon (Si) on (A) the content of malondialdehyde (MDA) and the activities of (B) superoxide dismutase (SOD), (C) catalase (CAT) and (D) peroxidase (POD) in leaves of 25-day-old maize plants. Bars represent standard deviation (SD) of the means (n = 3). Different letters indicate significant differences among the treatments at P < 0.05, according to Duncan’s multiple range test. FW, fresh weight.

FIGURE 4

A schematic diagram representing effects of alkaline stress (e.g., Na₂CO₃ stress) and defensive system induced by seed-priming with Si, underlying maize alkaline stress tolerance. Alkaline stress causes negative impacts by repressing photosynthesis, disrupting osmoprotectant status and negatively influencing water status and ionic balance. Excessive alkalinity can promote production of reactive oxygen species (ROS). ROS contribute to lipid peroxidation, resulting in oxidative stress that causes growth inhibition, biomass loss, and subsequently extreme yield reduction. Conversely, seed-priming with Si shows protective mechanism against alkaline stress by improving water balance and photosynthesis, accumulating osmoprotectants and maintaining ionic balance. Si also enhances the detoxification of ROS by stimulating the activities of antioxidant enzymes, such as SOD, CAT, and POD, leading to oxidative stress mitigation. As a result, Si improves alkaline stress tolerance by retaining better growth of alkaline-stressed plants. ${\mathrm{O}}_{\mathrm{2}}^{•\mathrm{-}}$, superoxide; H₂O₂, hydrogen peroxide.