Source-Sink Dynamics and Grain Yield Formation of Rice Under Single and Dual Salinity-Drought Stresses

Rice production faces serious threats from salinity and drought, which individually or in combination disrupt source-sink relationships and grain yield formation. While the effects of each stress have been widely studied, their interactive impact from a source-sink perspective remains unclear. This study investigated source-sink dynamics and grain yield formation in rice under single and dual salinity-drought stresses. A completely randomized design was conducted in a screen house using two rice varieties (Inpara 8 and IR64) grown under control, salinity (4 dS m^-1), drought (40% field capacity), and dual salinity-drought stress. Key source traits (leaf area index, net assimilation rate) and sink traits (pollen viability, panicle length, panicle number per clump, grain number per panicle, filled grain percentage, grain weight, productivity, and harvest index) were assessed, followed by correlation and structural equation modeling - partial least square (SEM-PLS) analysis. Results revealed that salinity and drought, especially when combined, drastically reduced reproductive success, with pollen viability declining by up to 24% and filled grain percentage decreasing by nearly 58%. Yield-related traits, including panicle number, grain number, and grain weight per clump, decreased sharply, reducing productivity and harvest index by over 80% under dual stress. Varietal effects were minor, indicating that stress severity outweighed genotypic differences. Correlation and SEM-PLS analyses highlighted sink-related traits as the primary determinants of yield, whereas source traits played a secondary role. These findings demonstrate that reproductive sink strength is the critical bottleneck under dual salinity-drought stress, emphasizing the need for breeding strategies that enhance sink capacity and resilience, supported by integrated management to sustain rice productivity.