CONICET | Buscador de Institutos y Recursos Humanos

ABSTRACT Plants in the rows of high crop population density sunflower crops exhibit a self-organizing process that commences shortly after seedling emergence: a given stem inclines towards one inter-row and the neighbouring stem inclines towards the opposite inter-row. To date, the causes and consequences of this process have not been explored in sunflower or in other grain crops. The aim of this work was to describe the dynamics of sunflower self-organization, focusing on the causes that elicit this crop response. Five experiments were conducted at the Facultad de Agronomía, Universidad de Buenos Aires. Factors affecting crop responses that were examined included: crop population density, quality of light incident on upper leaves of the plants, and the mechanical restriction of the self-organization process. Dynamics of the process were followed at four-day intervals or, in a more detailed fashion, using time-lapse photography to document the organization process and the degree of shading between neighbouring plants at 10-minute intervals. Effects of self-organization on grain yield and grain number were determined in some experiments. The self organization process (as exhibited by the proportion, of all plants in a row, of inclined stems) was greater and occurred earlier in the high population density plots. Thus, 70% of plants deviated from the vertical position at 14 pl. m-2 (equally distributed among the two inter-row positions) at floral initiation as against 0% at 5 pl. m2 . Low red/far red ratio light (but not low blue light) incident on the upper portion of otherwise unshaded plants induced a high incidence of stem inclination (82%) and greater inclination angles (30º) than in the controls (0%, 0º). Analysis of time-lapse photography records for individual plants showed that the initiation of stem inclination took place shortly after the plant began to be shaded by its neighbours. Grain yield was significantly lower in plots in which the self-organization process was mechanically restricted. It is concluded that a) shading of the target plant by the leaves of neighbouring plants reduced the red/far red ratio of the light incident on the target plant, inducing stem inclination; b) stem inclination diminished the negative effects of the increased crop density on grain number and grain yield per plant.