Integrating Protein and Residual Dynamics: Understanding Low Oil Concentration at Elevated Source/Sink Ratios in Sunflower.

INTRODUCTION/OBJECTIVE: To evaluate the effect of the source/sink (S/S) ratio on oil and protein accumulation dynamics in sunflower and its impact on final grain composition. METHODS: Field experiments (2012-2014) in Balcarce used three hybrids with S/S ratios modified through defoliation and grain thinning. Oil (NMR) and protein (NIR) contents were measured. Accumulation rates and durations were analyzed using linear-plateau models based on thermal time. RESULTS: High S/S ratios increased protein concentration (from 12.6% to 20.4%) but reduced oil concentration. While oil accumulation ceased around 500°Cd, protein accumulation continued until 800°Cd. Protein accumulation rates were significantly higher with increased assimilate availability than in the control and defoliated treatments. DISCUSSION: Protein synthesis was less sensitive to defoliation than oil, suggesting independent regulatory mechanisms. The protein increases at high S/S ratios, resulting from both higher accumulation rates and extended periods, likely influenced by sucrose signaling. The data refute a simple 1:1 trade-off between oil and protein. CONCLUSION: Sunflower grain composition involves variations in "residual" components like fiber and carbohydrates. Breeding to reduce this residual fraction could simultaneously enhance oil and protein content, optimizing the industrial quality of both oil and protein meals.