The relation between irradiance and grain yield of irrigated rice in the tropics, as influenced by cultivar, nitrogen fertilizer application and month of planting

Abstract

Irradiance at various stages of crop growth was correlated with grain yields of monthly plantings for a period of 10 years at IRRI with four varieties of rice, each grown at four levels of nitrogenous fertilizer application. Yields of all varieties were most significantly correlated with irradiance (over 20- or 30-day intervals) during both the reproductive and the ripening stages of the crops. The correlations were highest for crops grown under high irradiance, and were lowest for crops grown during the wet season, probably because of pest and disease problems. Regardless of whether irradiance was rising or falling progressively, high irradiance at any stage after panicle initiation was associated with higher yields in both older and modern varieties. Responsiveness to irradiance was greater at higher levels of N fertilizer application. Irradiance influenced yield components in the order in which they were determined, the earliest being the number of panicles/m 2 , followed in turn by spikelets/m 2 and grains/m 2 . Weight per grain was not significantly influenced by irradiance at any stage, but the number of grains/m 2 was influenced by irradiance right up until the late stages of crop growth.