Estimation of physiological maturity in sunflower as a function of fruit water concentration

Visual methods are commonly used to estimate physiological maturity (PM) in sunflower crops. These methods, while simple and less laborious than following grain dry weight dynamics, are subjective and results can be affected by environmental conditions. The objective of the present work was to study the relationship between the dynamics of dry matter and water concentration (WC) of sunflower fruits, with the aim of identifying the timing of physiological maturity on the basis of fruit WC. The effects of brief periods of high temperature stress (lasting 4 or 6 days) during grain-filling were also explored. Eight different sunflower genotypes (inbred lines and hybrids) were studied in four separate experiments conducted under different growth conditions (greenhouse and field) and contrasting dates of sowing (autumn, spring and summer), generating a broad range of grain-filling durations (28-41 days) and final fruit weight (30-105 mg fruit-1). In these experiments the evolution of fruit fresh and dry weights were followed in fruit from the peripheral and intermediate positions on the capitulum from anthesis to harvest maturity. Tri-linear functions were fitted to the relative (to final) fruit dry weight (RFDW) to fruit WC relationships (R2 from 0.94 to 0.99). Across experiments, genotypes and fruit positions on the capitulum, PM in non-stressed plants was attained when fruits exhibited 38% WC. This model was validated against independent data, successfully simulating the dynamics of fruit dry weight based on fruit WC (r = 0.99; P < 0.001). Verifications against published data generated by other authors also proved satisfactory. High temperature stress that caused a shortening of grain-filling and reductions in final fruit weight >20% with respect to controls, raised fruit WC at PM to ca. 50%. Simulations were performed to explore the effects of variations in timing of anthesis that occur between positions on the capitulum and among plants in a crop on fruit WC at maximum yield. At a crop level, 38% WC in these simulations corresponded to grain yield >95% of the maximum attainable when harvest was delayed until last growing fruits reached 38% WC. Simulations also showed that in crops exposed to high temperature during grain-filling, there would be no risk of shortfall with respect to potential yield using 38% WC as an indicator of PM. We conclude that this simple, fast and non-subjective method based on fruit WC would be useful to determine PM in sunflower.