Contrasting Yield Responses of Early- and Late-Season Rice to Root Damage: From Agronomic Traits to Path-Based Mechanisms

Mechanical transplanting often causes root damage to rice seedlings, but its yield impacts in double-cropping systems remain unclear. A two-year field experiment was conducted in early- and late-season rice under three root damage treatments: no damage (CK), mild root damage (T1, seedling roots pruned to 2 cm), and severe root damage (T2, seedling roots pruned to 1 cm). Results showed that T2 reduced early-season rice yield by 8% but had no significant effect on late-season rice, while T1 did not affect yield in either season. In early-season rice, the yield loss was associated with reduced pre-heading biomass (BMPre) and total biomass (both 13% lower than CK), primarily due to decreases of 5–32% in SPAD, leaf area index, specific leaf weight, pre-heading crop growth rate, and leaf area duration, with no post-heading compensation. In contrast, late-season rice sustained yield despite a 9% reduction in BMPre (attributed to similar leaf trait reductions of 4–26%) by enhancing post-heading biomass (16% increase), driven by a 17% increase in post-heading crop growth rate and an 11% decrease in the rate of leaf area decline. Partial least squares path modeling confirmed that biomass dynamics, rather than yield components, constituted the primary pathway linking root damage to yield. These results demonstrate that the impact of root damage is season-dependent, highlighting the need for root protection in early-season rice and the exploitation of compensatory capacity in late-season rice to optimize mechanical transplanting practices.