Physiological responses of early-season rice to simulated root damage from mechanical transplanting

Yield loss from mechanical transplanting in early-season rice is often attributed to root damage, yet the physiological mechanism remains poorly understood. A two-year field experiment was conducted in early-season rice with two varieties (Lingliangyou 211, hybrid; Zhongzao 39, conventional inbred) under three root damage treatments: no damage (CK), mild (roots trimmed to 2 cm, T1), and severe (roots trimmed to 1 cm, T2). Root regeneration, leaf water status, antioxidant enzyme activities (SOD, POD, CAT), malondialdehyde (MDA), soluble protein, and PSII maximum quantum yield (Fv/Fm) were measured at 1, 3, 5, and 7 days after transplanting in 2022, whereas grain yield and yield components were evaluated in both 2021 and 2022. Results showed that T2 reduced grain yield by 8–9% across both years, whereas T1 had no significant effect. The two varieties exhibited consistent physiological responses to root damage. Both T1 and T2 stimulated compensatory new root proliferation, with significantly higher new root numbers at days 5–7 relative to CK. However, T2 induced sustained oxidative stress, as indicated by elevated MDA in roots throughout the recovery period and in leaves from day 5 onward. In contrast, T1 caused only a transient increase in root MDA. T2 reduced Fv/Fm by 23–26%, pre-heading biomass production by 13%, and highest tiller number by 10–21%. Conversely, T1 maintained all these parameters at control levels. Severe root damage reduced PSII photochemical efficiency and biomass in association with systemic oxidative stress, whereas mild damage was effectively contained. These findings indicate that, under the tested conditions, severe root trimming that leaves approximately 1 cm of intact roots was detrimental to grain yield, whereas retaining approximately 2 cm did not significantly compromise productivity.