Salt Free Side Determines the Maize (Zea mays L.) Seedling Growth Performances Under a Heterogeneous Salinity Environment

To understand the impact of salinity heterogeneity on maize growth, a split-root experiment involving both homogeneous and heterogeneous salinity environments was designed. Four homogeneous salinity levels (2, 4, 6 and 8 g L⁻¹), four heterogeneous treatments (0/2, 0/4, 0/6, and 0/8 g L⁻¹) and a control (CK) with 0 g L⁻¹ NaCl were applied to respective sides of split-root pots. Findings revealed that while heterogeneous salinity treatments up to 8 g L⁻¹ did not significantly alter seedling morphology, homogeneous salinity levels above 2 g L⁻¹ markedly inhibited growth. Both salinity stress scenarios enhanced physiological responses in maize leaves, peaking at 6 and 8 g L⁻¹ salinities. Stress-related indexes, including proline, malondialdehyde (MDA) and soluble sugar contents, increased by 105%, 189% and 95%, respectively, under heterogeneous salinity, versus 229%, 370% and 231% under homogeneous conditions, relative to the CK. Interestingly, the partial salinity stress of heterogeneous treatments stimulated root growth on the salt-free side, leading to an 11.7% average increase in root length compared to the control, thereby enhancing water uptake and biomass more effectively than homogeneous treatments. Principal component analysis (PCA) further indicated that heterogeneous salt stress could concurrently bolster morphological and physiological indicators in crops. These results highlight the critical role of salt-free zones in facilitating maize seedling growth and mitigating the adverse effects of salt stress under spatially variable salinity conditions.