Exogenous naphthaleneacetic acid (NAA) mitigated the alkaline stress changes in the growth and physio-biochemical performance of maize (Zea mays L.) seedlings

Abstract

Naphthaleneacetic acid (NAA) is a synthetic plant hormone, considered to promote plant growth under optimal and stressful conditions. However, its role in regulating the response of crop species, including maize, to alkaline stress (AS) remains largely unknown. In this study, we investigated the effects of exogenous NAA application (160 mg/L) on growth, antioxidant potential, nitrogen assimilation, and ion regulation in maize seedlings subjected to different levels of alkaline stress (0, 30, 60, and 90 mM [1:1 ratio of Na₂CO₃ and NaHCO₃]). We observed that increased alkaline stress) levels significantly increased the concentration of Na⁺, Cl⁻, H₂O₂, and malondialdehyde (MDA), but reduced K⁺ and Mg²⁺ ions compared to control (CK) maize seedlings. Furthermore, glycine betaine, proteins, proline, sugar, and antioxidant enzymes (superoxide dismutase, glutathione reductase, peroxidase, dehydroascorbate reductase, and catalase) were significantly increased, while NO₃⁻ and NH₄⁺ metabolizing enzymes (nitrate reductase, glutamine synthetase, and glutamine oxoglutarate aminotransferase) were significantly reduced. AS induced a negative impact on maize seedlings, which was highly dependent on the concentration of exogenous NAA. Where NAA application reduced Na⁺, Cl⁻, H₂O₂, and MDA levels, but increased K⁺, enriched organic solutes, antioxidant enzyme potential, and N metabolism, and ameliorated stress damage under AS conditions. Conclusively, the NAA application mitigated the adverse effects of AS on maize seedlings. Consequently, it might be an effective strategy for improving morpho-physiological performance in alkalinity-prone areas. However, future molecular studies should examine whether NAA can positively alter maize nutrient composition and thus help resolve nutritional problems in developing countries suffering from alkaline soil.