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Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

The use of plant growth regulators on castor (Ricinus communis L.) plants may be an alternative to reduce excessive vegetative growth, which would ease harvest and even increase seed and oil yield. This study aimed to evaluate the effects of both spraying time and rate of the plant growth regulator mepiquat chloride (MC) on growth parameters, yield components, seed yield, and seed oil content of castor cultivar IAC 2028 cultivated under high plant population density conditions in a spring–summer growing season in Botucatu, southeastern Brazil. One greenhouse experiment and two field experiments were performed. Mepiquat chloride application reduced leaf growth, height, and dry matter (DM) and increased the shoot/root ratio of castor plants. When sprayed during the vegetative phase (expanded leaf at nodes 6–8 of the primary stem), MC reduced the height and DM of castor plants; however, application during the first raceme flowering or at two sequential times (expanded leaf at the eighth node of the primary stem and third expanded leaf on a secondary stem) was more effective at reducing plant height and increasing seed yield. Spray rates between 100 and 200 g a.i. ha−1 of MC resulted in lower plant height and a greater number of filled fruits per raceme (24.0–38.8%), seed weight (2.6% in one experiment), and seed yield (17.4–33.1%). The increased number of filled fruits per raceme in response to MC application influenced seed yield. Mepiquat chloride spraying did not affect castor seed oil content.

São Paulo State Univ. (UNESP) College of Agricultural Sciences

Mato Grosso Federal Institute of Education Science and Technology, Rodovia MT 235, km 12, PO Box 100

São Paulo State Univ. (UNESP) College of Agricultural Sciences Dep. of Crop Science Lageado Experimental Farm, PO Box 237

São Paulo State Univ. (UNESP) Center for Tropical Roots and Starches Lageado Experimental Farm, PO Box 237

São Paulo State Univ. (UNESP) College of Agricultural Sciences

São Paulo State Univ. (UNESP) College of Agricultural Sciences Dep. of Crop Science Lageado Experimental Farm, PO Box 237

São Paulo State Univ. (UNESP) Center for Tropical Roots and Starches Lageado Experimental Farm, PO Box 237