Bot. Bull. Acad. Sin. (1995) 36: 113-120

Oka and Lu — Competition and self-thinning in rice

Competition, density response and self-thinning observed in rice

H.I. Oka¹ and Chi-Shan Lu

Department of Agronomy, National Chung Hsing University Taichung, Taiwan 402, Republic of China

(Received July 19, 1994; Accepted January 9, 1995)

Abstract. To observe neighbor effects on plants at different planting densities, rice varieties Taichung 65 (T65) and Taichung Native 1 (TNI) were planted at a sequence of seven interplant spacings, each one half of the preceding, from 32 cm to 0.5 cm. The regression equation for single-plant biomass (W) on the number of plants per m² (N), In W = In K - b In N, fitted the data from all experiments and gave a coefficient of determination exceeding 0.97. Panicle weight per plant showed the same pattern of density response. The values of b were close to 1.0 in pure stands, being somewhat lower for plant biomass than for panicle weight. When early-sown plants and those sown l0 days later were mixed, the late-sown plants were suppressed by early ones and their b values approached 1.5. They had a high mortality (self-thinning) rate at high densities. When T65 and TN1 were mixed, TN1 was more competitive than T65, which had a higher mortality than did TN1. As density increased, the distribution of single-plant biomass and panicle weight was skewed to the right, and the interplant variance increased. The variance was strongly correlated with mortality and regression coefficient on density. These values were also correlated with skewness and the Gini coefficient, which show the inequality between individual plants.

Keywords: Biomass; Competition; Density; Neighbor effect; Regression on density; Self-thinning; Skewed distribution.

¹Corresponding authors. H.I. Oka, National Institute of Genetics, Mishima, Shizuoka-Ken 411, Japan.