Phenology–soil moisture synchronisation and foliar boron regulate physiological resilience and yield stability of field pea (Pisum sativum L.) in rainfed rice-fallow systems

Rice-fallow agroecosystems across South and Southeast Asia represent a major opportunity for sustainable intensification, yet productivity of post-rice pulse crops remains limited by narrow sowing windows, rapid depletion of residual soil moisture, and micronutrient deficiencies. Field pea ( Pisum sativum L.), a short-duration legume adapted to cool-season conditions, is well suited to these systems; however, yield stability under rainfed rice fallows is frequently constrained by terminal moisture stress and boron (B) deficiency. Here, we tested whether interactions between sowing phenology and foliar boron application regulate soil moisture dynamics, plant physiological responses, reproductive processes, and yield formation under rainfed conditions. A two-year field experiment (2017–2018 and 2018–2019) was conducted using a split-plot design with four sowing dates (10 November, 20 November, 30 November, and 10 December) and four foliar boric acid concentrations (0, 0.1, 0.2, and 0.3%), applied at pre-flowering and pod initiation stages. Early sowing (10–20 November) maintained root-zone soil moisture above critical thresholds during flowering, extended crop duration by 14–17 days, and increased grain yield by 22–47% compared with late sowing. Delayed sowing accelerated soil moisture depletion, reduced plant water status (relative leaf water content and canopy temperature depression), and impaired photosynthetic capacity, leading to lower chlorophyll content, photosynthetic rate, leaf area index, and stomatal conductance. These physiological constraints were accompanied by reduced plant B concentration, pollen viability, and pollen germination, collectively contributing to yield decline. Foliar boron application at 0.2% enhanced relative leaf water content (3–6%), photosynthetic rate (8–15%), pollen viability (15–16%), and grain yield (~ 25%) relative to untreated controls, while responses plateaued at B application at 0.3%. Principal component analysis indicated that treatment responses were primarily structured along a physiological axis integrating soil moisture availability, plant water status, photosynthetic traits, and reproductive performance. Overall, sowing of field pea during 10–20 November combined with two foliar sprays of 0.2% boric acid is a practical and economically viable strategy for improving productivity of field pea in rainfed rice-fallow systems. Our findings demonstrate that synchronisation between crop phenology and residual soil moisture availability, combined with boron-mediated regulation of physiological and reproductive traits, governs yield stability of field pea in rainfed rice-fallow systems. These results provide mechanistic insights into crop adaptation strategies for water-limited environments and highlight management interactions that may enhance resilience across rainfed post-rice agroecosystems.