Straw Mulching Alleviates Drought Stress in Spring Maize at the Tasseling Stage: Regulation of Photosynthetic Gas Exchange and Photosystem II Photochemical Function

Although the tasseling stage is a pivotal window for drought stress in maize, the photosynthetic physiological mechanism by which straw mulching compensates for drought-induced impairment relative to conventional rain-fed practice remains unclear. In this study, a two-factor randomized block experiment was conducted with two straw mulching levels (full mulching, FM, 9000 kg ha−1; half mulching, HM, 4500 kg ha−1) and three drought gradients (mild, 60–70% FC; moderate, 50–60% FC; severe, 40–50% FC) during the tasseling stage, with the conventional rain-fed control (CK) being non-mulched and well-watered (80% FC). The results show that only FM under mild drought (FM + LD) maintained the net photosynthetic rate, PSII photochemical efficiency (Fv/Fm), and grain yield at levels not significantly different from the CK (p > 0.05). Under mild drought, photosynthetic inhibition was dominated by stomatal limitation, with FM maintaining effective stomatal regulation. As drought intensified, the limiting factor shifted to non-stomatal limitation; however, FM showed significantly higher Fv/Fm and lower Vj than HM, indicating better PSII preservation. At 24 h after rewatering, donor-side function largely recovered across all treatments, whereas acceptor-side parameters remained elevated except in T1 (FM + LD). Overall, FM under mild drought at the tasseling stage achieved photosynthetic performance and yield comparable to conventional rain-fed management, providing a theoretical basis for water-saving cultivation in semi-arid regions.