Dry matter, minerals and carbohydrates in the capitulum of sunflower ( Helianthus annuus): Effects of competition between seeds, and defoliation

Abstract

Poor seed-set, which often occurs in the central region of the sunflower capitulum, may be due to competition between developing inner and outer seeds, or to inadequate vascular connections to the inner part of the capitulum. We investigated these factors by restricting pollination, and thus seed-set, to defined regions and differently-sized areas of the capitulum of a cytoplasmic-male-sterile genotype. We also investigated the effects of defoliation on seed set. Seed characters, and contents of carbohydrates, N, P, K, Mg and Ca in capitulum parts were measured at physiological maturity. Single-seed weight was largest when pollination was allowed on only 33% of the capitulum diameter in any position on the capitulum. Seed oil concentration was highest when pollination was restricted to the outer whorls of florets and when seed numbers were greatest. Concentrations of kernel N, Mg and P, and seed-hull Mg and P were highest when pollination was restricted to inner whorls of florets. There were no significant effects on Ca. Capitulum dry matter and nutrient contents were only lowered when pollination was completely prevented. Concentrations of N, P and Mg in the receptacle were highest when pollination was completely prevented; these nutrients also accumulated in hulls of normal seeds and in the hull structures developed from unfertilized florets. Concentrations of K, Ca and fructose were highest when seeds were present. Thus, the nutritional demand of developing seeds does not appear to control the movement of N, P and Mg to the capitulum, but it may do so for K, Ca and fructose. Complete defoliation and, to a lesser extent, partial defoliation at anthesis decreased seed number, total seed weight and oil yield per plant. Only complete defoliation decreased single-seed weight and oil concentration below control values. The fructose content of receptacles at maturity was proportional to the leaf weight remaining, but the glucose content was not affected by any defoliation treatment. Seed N was decreased by defoliation but stem N was not. In the absence of leaf N, stem N does not seem to be redistributed as a compensatory N supply to developing seeds. We conclude that the vasculature to the centre of capitulum can adequately supply nutrients to central seeds, and that the outer seeds control the growth of inner seeds, probably by competition for spa???on the receptacle and by preferential sequestration of nutrients.