Alkalinity Inhibits Maize Low-Temperature Resistance Through Nitrogen and ROS Metabolism

Maize, a pivotal cereal crop, undergoes growth impediments due to alkaline stress or low-temperature conditions. Particularly under the compounded duress of alkalinity (elevated pH stress) and low temperature, the underlying responsive mechanisms remain inadequately understood. This study investigated the effects of varying alkaline treatments (pH 7.2, 7.9, and 8.9) on the growth and tolerance of maize seedlings under low-temperature conditions (4°C). The results demonstrated that alkaline stress exacerbated the degradation of photosynthetic pigments, impaired photosynthetic electron transport and carbon assimilation, and consequently inhibited plant biomass accumulation under low-temperature conditions. Compared to low-temperature stress alone, alkaline treatment further suppressed the glutamine synthetase-glutamate synthase (GS-GOGAT) cycle under low-temperature stress, as evidenced by reduced activities of nitrate reductase (NR), glutamine synthetase (GS), and glutamate dehydrogenase (GDH), along with downregulated expression of related genes. Simultaneously, high-alkaline treatment markedly down-regulated the expression of asparagine synthetase genes (ZmAS3 and ZmAS4), as well as aspartase 2 (ZmASN2), by 54.5%, 51.5%, and 76.4%, respectively, while simultaneously suppressing amino-acid biosynthesis and disrupting ionic homeostasis. Furthermore, high-pH stress elicited a significant accumulation of superoxide anion (O₂·^-) by 35.7% in leaves and 30.4% in roots, and of hydrogen peroxide (H₂O₂) by 37% in leaves and 7.3% in roots. This may be due to alkaline stress inhibiting the expression of key low-temperature stress-responsive genes in maize, including the c-repeat binding factor (CBF) expression inducer (ZmICE1) and transcription factors dehydration-responsive element binding protein 1.3 and 1.5 (ZmDREB1.3 and ZmDREB1.5). Therefore, alkaline stress diminished the low-temperature tolerance of maize plants, highlighting the detrimental effects of combined alkalinity and low-temperature stress on maize growth and stress resistance, with significant implications for agricultural productivity and crop yield.