Is the tolerance of sunflower floret differentiation to crop density associated with the stem growth and with the oil yield response to density?

Crop floret number per unit area and, to a lesser extent grain set, are the determinants of yield that are associated with the increase in sunflower oil yield at crop densities greater than the most frequently used in Argentina of 3–5.1 pl.m⁻². A better understanding of the responses of these two yield determinants to crop density and their intra-specific variability could contribute to the development of higher yielding cultivars suited to higher crop densities. In an initial experiment, we measured the aboveground biomass partitioning to leaves and stems between emergence and the end of floret differentiation, the weight and length of the basal internodes whose growth overlapped with the floret differentiation phase, plant height at the star visible (R1) and anthesis (R5.5) stages, and florets per plant in response to crop density across a 0.7–14.3 pl.m⁻² range. In Exps. 2 and 3, designed to explore intra-specific variability for responses to crop density we measured, in five hybrids, the last four variables plus oil yield in crops grown at 5.1, 10.2 and 14.3 pl.m⁻². Lastly, in Exp. 4, we measured the responses of the basal internode of four of the hybrids used in Exps. 2 and 3 to three levels of red/far red ratio (R/FR) in the light incident on spaced plants. Plant height at the R1 stage and above-ground biomass partitioning to the stem increased significantly with crop density, contrasting with a fall in floret number per plant in Exp. 1. The length and weight of the fourth internode, the internode whose growth period overlapped the most with the floret differentiation phase, increased with crop density, while floret number per plant showed a significant decrease with increasing weight of the fourth internode. The hybrids examined in Exps. 2 and 3 exhibited intraspecific variability in their responses to crop density for plant height at R1 stage, fourth internode length, floret number per plant, in the relationships between floret number per plant with fourth internode length and plant height at R1 stage, and in oil yield. The length of the first internode of the four hybrids examined in Exp. 4 increased consistently with reductions in the R/FR ratio of the light incident on the plants, suggesting that perception of the R/FR ratio by phytochrome may play an important part in the responses of floret number per plant to crop density and its intraspecific variability. The lack of association in some hybrids of the responses of floret number and oil yield per square meter to crop density showed that grain set and unit grain weight are additional yield determinants that require attention when considering intraspecific variability in yield responses to high density.