Plant density does not influence every castor plant equally

Castor (Ricinus communis) is an oilseed crop, which oil is used in the chemical industry for several products. There are attempts to adjust the plant density of castor aiming to increase its seed yield. This study discusses how plant density influences seed yield of individual castor plants. It explores the concept that neighboring plants establish an interplant hierarchy, and that the hierarchy changes along the cropping season. Two field experiments were performed under contrasting environmental conditions. One experiment was run under irrigation in a temperate region (Lubbock, TX, USA), with plant density varying randomly. The other experiment was run without irrigation in a tropical and semiarid region (Balsas, MA, Brazil), with plants placed in previously defined in-row distance. Data from individual plants was taken on leaf area and plant height at two-week intervals and on seed yield in 48 plants in the temperate and 84 plants in the tropical location. Plant hierarchy was calculated as the leaf area or plant height of a given plant divided by the average of the two neighboring plants in the same row. It was observed that in the temperate environment, as early as 10 days after emergence, the plant hierarchy in leaf area was correlated with the final seed yield (r = 0.36). Plant density did not influence seed yield in the temperate location, but in the tropical conditions, higher productivity was found under higher plant density. In both environments, the interplant hierarchy changed considerably along the cropping season, and the plants that dominated around flowering time had higher seed yield than the plants that became dominating later. Seed yield had the highest correlation with the plant leaf area and height at the beginning of reproductive phase.