Visual definition of physiological maturity in sunflower (Helianthus annuus L.) is associated with receptacle quantitative color parameters

Identifying physiological maturity (PM) in sunflower (Helianthus annuus L.) by visual methods is subjective. The present study was conducted during two years in two short season sunflower hybrids (Macón and MG60) to determine the relationship between quantitative color parameters in the receptacle and physiological markers such as fruit dry weight (FDW) and fruit water content (FWC) from flowering to PM. Fruits from the external 25% of the capitulum radius were sampled at 3-5-day intervals from first anthesis until harvest maturity. Fruit and receptacle fresh and dry weight were calculated, and color changes of the receptacle base were followed over time using a spectrophotometer. Comparison of colorimetric coordinates a* and b*, defined by the CIELAB color space enabled quantitative correlation of color changes in the receptacle with the maturation stage of the fruits and their moisture content. In both hybrids and years, fruits attained maximum dry weight when the receptacle color turned from dark green to buttery-yellow. Strong correlations were found between FWC and a* for Macón the first (r=-0.877) and second year (r=-0.934) and for MG60, (r=-0.912 and r=-0.891) the first and second year, respectively. The same results were found for b* for Macón (r=-0.901 and r=-0.829) and for MG60 (r=-0.898 and r= -0.863) for the first and second year respectively. Maximum b* at FWC between 40 to 41% had the highest correlation with maximum fruit dry weight for both hybrids and years, and was a good indicator for identifying the attainment of PM. This work represents an original contribution and a first step towards the development of a model for predicting PM in sunflower by using colorimetric measurements.