Source-sink reduction and improvement in rapeseed (Brassica napus L.) during the exponential grain filling phase and responses of grain yield, its components and grain quality traits

Abstract

Rapeseed (Brassica napus L.) final grain weight and in turn grain yield, results from the interaction between assimilate supply (source) and sink capacity; however, the extent to which source limitation constrains yield formation during grain filling remains under debate. Understanding how the manipulation of the source-sink ratio (S-S ratio) affects yield and grain traits is critical for elucidating the physiological mechanisms behind yield stability in high-yield environments. This study aimed to evaluate how variations in the S-S ratio during the grain-filling phase influence grain weight and yield, biomass allocation, grain-filling dynamics, and grain quality traits in rapeseed. A field experiment was conducted during two seasons in Valdivia, Chile. One high-yield potential and adapted hybrid (Click CL) was evaluated under three radiation regimes in a randomized complete block design: control, -50% incident radiation (shading), and +50 % incident radiation (reflected radiation pannels, PET). S-S ratio treatments were applied from the beginning of grain filling (BBCH 71) to physiological maturity (BBCH 89) aimed at modify the S-S ratio during the actual grain filling period. The reduced S-S ratio increased thousand-grain weight (TGW), particularly in basal siliques, resulting in yield compensation and demonstrating a strong structural and physiological buffering capacity. Conversely, increasing the S-S ratio enhanced grain number and grain yield, while TGW remained stable. Grain quality traits responded asymmetrically: under reduced S-S, oil concentration slightly declined whereas protein concentration increased. The increased S-S ratio, had no effect on grain oil and protein concentrations, remaining similar to the control. Sieving analyses revealed a shift toward larger grain size classes under reduced S- S, whereas the distribution under increased S-S resembled the control. Overall, these findings indicate that rapeseed maintains yield stability through compensatory adjustments in grain weight and size distribution under contrasting assimilate availabilities. Under high-radiation temperate conditions, rapeseed productivity during grain filling is predominantly governed by sink capacity, highlighting its physiological plasticity and resilience to variations in source-sink balance HighlightsO_LIIn high-yield conditions without structural changes, grain filling depends on sink capacity. C_LIO_LIA 50% reduction in radiation increases grain weight and maintains grain yield. C_LIO_LIA 50% increase in radiation raises grain number and yield via more grains per plant. C_LIO_LISource reduction shifts grains to larger sizes; source increase maintains stability. C_LIO_LIOil in grain is stable with increased radiation, declines when it is reduced. C_LI