Water content dynamics of achene, pericarp and embryo in sunflower: Associations with achene potential size and dry-down

The post-anthesis dynamics of the water content of whole sunflower achene and its major parts (pericarp, embryo) were examined for seven genotypes that spanned a broad range of final achene size (30-100mgachene⁻¹). Objectives were: (i) to establish the relative contributions of pericarp and embryo to whole-achene water content dynamics, (ii) to determine the relationship between maximum water content of the pericarp and final achene size, and (iii) to examine the effect of final achene size (as affected by genotype and environment) on achene dry-down dynamics after physiological maturity (=maximum achene weight). Four experiments were conducted over 2 years under field and glasshouse conditions. Across genotypes and growth conditions, whole-achene and pericarp water contents peaked earlier and more sharply during grain filling (ca. 35% of grain filling duration, or 30% of final achene weight), maximum embryo water content was achieved somewhat later and declined less sharply. Although the pericarp was a minor (17-35%) component of final achene dry weight, it contained 65-70% of achene maximum water content. Absolute pericarp water content did not fall to values close to those of the embryo until after physiological maturity. Final achene and embryo dry weights were closely (r ² 0.90 and 0.85, respectively) associated with maximum pericarp water content. After maximum achene water content, rates of whole-achene dry-down were linear (ca. 1.35%d⁻¹), and absolute rates of water loss per achene (range=1.1-3.7mgH₂Oachene⁻¹ d⁻¹) were strongly associated with achene maximum water content and final achene dry weight (r ² 0.86 and 0.75, respectively). Excluding the inbred line HA89, the remaining genotypes achieved harvest (17%) and storage (11%) achene water concentrations at about 15 and 20d, respectively, after physiological maturity, largely because absolute rates of achene water loss increased with achene size. We conclude that the pericarp is the dominant component of whole-achene water content dynamics, and that pericarp and achene maximum water contents are good indicators of potential final achene and embryo sizes and achene dry-down rates. Present results also provide a first approximation to the quantification of post-physiological maturity dry-down in this crop.