Objectives Spring drought is a major abiotic stress that severely restricts maize growth, development, and yield formation. Potassium polyaspartate, a natural water-soluble amino acid polymer containing peptide bonds, carboxyl groups, and other active groups, can enhance nutrient uptake and transport in plants and improve drought resistance. Its stress-alleviating efficacy is closely associated with its molecular weight. This study aimed to explore the regulatory effects and mechanisms of potassium polyaspartate with different molecular weights on drought stress alleviation and post drought rewatering responses in maize seedlings.

Methods A hydroponic experiment was conducted using maize seedlings as test materials, with drought conditions simulated by adding PEG-6000 to the nutrient solution. Potassium polyaspartate with molecular weights <2 kDa (low), 5−8 kDa (middle), and >10 kDa (high) were selected for foliar spraying tests. Treatments included normal water supply (CK) and drought stress (D), each combined with foliar application of low-, medium-, and high-molecular-weight potassium polyaspartate (designated as L, M, H, DL, DM, DH respectively). Samples were collected 7 days after spraying (Group G1). Additionally, water supply was restored for the D, DL, DM, and DH treatments, with samples collected 7 days post-rehydration (Group G2). Growth indicators, root activity, root antioxidant enzymes, root oxidative damage, osmotic adjustment substances, and leaf photosynthetic parameters of maize were determined. The mechanism of action was analyzed by combining these physiological data with transcriptomic analysis.

Results Under normal water supply conditions, foliar application of potassium polyaspartate had no significant effect on root growth and antioxidant enzyme activity, except that the low-molecular-weight potassium polyaspartate increased the total root length, root surface area, root volume, average root diameter, and number of root tips by 36.5%, 16.1%, 21.8%, 14.7%, and 25.3% respectively. Drought stress significantly inhibited maize growth, remarkably increased root oxidase activity, and impaired root physiological functions. However, foliar application of potassium polyaspartate effectively alleviated drought stress, with the low-molecular-weight potassium polyaspartate treatment showing the most significant effect. Compared with the drought stress treatment, the low-molecular-weight potassium polyaspartate increased root activity by 21.5%; enhanced the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) by 77.3%, 39.5%, and 50.3% respectively; significantly reduced the superoxide anion ( \rmO_\small 2^\overline \,\cdot\, ) production rate, hydrogen peroxide (H₂O₂) content, and malondialdehyde (MDA) content by 40.8%, 63.0%, and 64.4% respectively; increased the contents of soluble protein (SP) and free proline (Pro) by 25.4% and 35.9% respectively; and elevated the net photosynthetic rate (P_n), transpiration rate (T_r), and stomatal conductance (G_s) by 112.7%, 131.2%, and 200.0% respectively. After rewatering, the growth status of maize recovered significantly, and the recovery effect of the potassium polyaspartate-treated groups was better than that of the non-sprayed group. Transcriptomic analysis showed that potassium polyaspartate could significantly upregulate the expression levels of ZMPRX42, ZmNAC48, and ZmVPP1 genes during drought and drought-rewatering processes, thereby enhancing the stress response ability of maize plants.

Conclusions Potassium polyaspartate of different molecular weights can all improve the drought resistance of maize seedlings and promote post-drought compensatory growth, which is achieved through multiple pathways including optimizing root morphology, enhancing antioxidant capacity, regulating osmotic substances, and upregulating drought-resistant genes. Among them, low-molecular-weight potassium polyaspartate (<2 kDa) exhibits the best effect. This study provides support for maize drought-resistant cultivation and the development of special preparations. Subsequent studies need to be carried out on the whole growth period, in-depth molecular mechanisms, and field verification to promote large-scale application.