Sunflower oil yield responses to plant population and row spacing: vegetative and reproductive plasticity

Attempts have not been made to increase the potential yield of sunflower via the combined use of high plant populations and crowding-tolerant cultivars, an approach that has been successful in maize -another strong apical dominance crop. This may be due to the negative effects of increased disease pressure and increased susceptibility to lodging at high sunflower plant populations. Information on crop yield responses to plant populations higher than usual recommended commercial plant populations is also limited. Wide inter-row spacing could shift crop water consumption towards later developmental stages with positive effects on yield in seasons of low rainfall, but might also limit crop yields in years with good rainfall. Here we report on studies aimed at examining yield (both oil and total biomass) responses to changes in plant population at two inter-row spacings: 0.7 and 1.4 m, in crops grown without restrictions due to water or nutrients, and protected from insects and diseases. Oil yields in the 0.7 m row spacing treatments increased over the whole of the 2.0-14.3 plants m⁻² range, reaching values much greater than those obtainable at the current commercial plant population of around 5.1 plants m⁻². This increase reflected a continuous increase with increased plant population for total crop biomass, together with an unchanging harvest index. Over the 2-7 plant m⁻² range of plant populations shared between the two row spacing treatments yields were not affected by spacing. Vegetative and reproductive plasticities, as reflected in the responses of biomass and plant leaf area at mid-anthesis to ground surface area per plant (vegetative plasticity indicators) and oil yield per plant to ground surface area per plant (reproductive plasticity indicator) did not vary over the 2.0-14.3 plant m⁻² plant population range in either row spacing. In the 0.7-m row-spaced crops, reproductive plasticity was less than vegetative plasticity and a similar tendency was observed at the 1.4-m row spacing. Floret number and achene number per plant together with unit achene weight decreased over the 2.0-14.3 plant m⁻² range for both row spacings, while fertility ratio over the same range was stable. Over the same range of plant population, achene oil concentration fell slightly, while achene oil content increased slightly and then stabilised. The study of the genetic basis of the plasticities found in this work should contribute to breeding programs aimed at increasing crop tolerance to high plant population densities.