GROWTH AND YIELD FORMATION OF IRRIGATED, DIRECT-SEEDED RICE AS AFFECTED BY NITROGEN-FERTILIZER

In two experiments performed in 1989 and 1990 in the South of France, the growth and yield formation of direct seeded rice as affected by fe rtilizer nitrogen (N) management were studied to gain insight in the p ossibilities of improving fertilizer N efficiency. Initial plant densi ty markedly influenced the time course of N uptake rate. An early decr ease in N uptake rates resulted in low shoot N concentrations during t he tillering and reproductive growth phases and consequently, in a dec rease of the length of the active tillering period. However, appearanc e rates of leaves and tillers were not clearly affected by N managemen t. The relative mortality rate of tillers during the reproductive phas e was linearly related to the decrease in shoot N concentration during that period. Low grain numbers per panicle were related to a low crop N status. The leaf area of the crops was determined by : the size of leaf laminae, the duration of the active tillering phase, the relative death rate of stems and the total number of leaves produced on main s tems. Despite these different processes, pooled data from both experim ents could be used to establish simple linear relationships between le af biomass and cumulative shoot N until flowering, and between leaf ar ea index and cumulative shoot N. This implied that until flowering the N use efficiency for leaf production and specific leaf weight were co nstant. Hence, crop N status and shoot growth rate were directly relat ed to the formation of crop components. However, N uptake rate per se was not a reliable variable to relate to production. Despite the latte r conclusion, we argued that the risk of a sink-limited yield might be minimized by proper N management. To subsequently reduce the risk of source-limited grain production, a moderate leaf area with high N cont ent during the post-flowering period is recommended.