One of the goals of rice breeding programs in Southern Brazil during the last 40 years was to modify the developmental cycle of the genotypes. In general, the developmental cycle was shortened in order to increase rice competitons with red rice, a main weed in rice pads. The objective of this study was to determine the relationship between the duration of rice developmental cycle with main stem final leaf number and with main stem leaf appearance rate. A field experiment was carried out in Santa Maria, RS, in five sowing dates in two growing seasons (2003/2004 and 2004/2005). Seven rice genotypes varying from very early to late maturation were used: IRGA 421, IRGA 420, IRGA 417, IRGA 416, BRS 7 (TAIM), BR-IRGA 409, EPAGRI 109. Plants were grown in 12 liter pots buried in the soil. In each cultivar and sowing date, the duration, in °C day (Tb=11°C) of emergence (EM) - flag leaf colar appearance (R2) and R2 - physiological maturity (R9) phases, the main stem final leaf number and the phyllochron (time interval between the appearance of two successive leaves) were measured. Each 100°C day increase in the duration of the EM-R2 phase were due to a 0,5 - 0,7 main stem final leaf number increase and each 100°C day increase in the duration of the EM-R2 phase resulted in a 100°C day increase in total cycle EM-R9. These results indicate that the shortening of total developmental cycle (EM-R9) of rice is due to a decrease in main stem final leaf number and not due to change in leaf appearance rate.
thermal time; Oryza sativa L.; phyllocron; final leaf number; plant development
