Radiation conversion efficiency and the regulation of photosynthetic development in tropical field grown rice

Abstract

Following full expansion of rice leaves, a decline in light saturated photosynthetic rate (Pmax) can occur before shading by the subsequent expanding leaf. The duration of expression of Pmax is affected by such factors as irradiance and nitrogen nutrition. The rationale for the decline in Pmax is believed to be the breakdown and re-mobilisation of existing leaf components to expanding and rapidly growing tissue. During the expansion of the rice canopy under tropical, high irradiance conditions, this can potentially affect the radiation conversion efficiency of the crop (RCE: biomass produced per unit radiation intercepted). We have investigated the relationship between Pmax, total leaf nitrogen, protein and chlorophyll content in flag leaves and first leaves of rice plants growing in a tropical environment. Rubisco and total protein content declined before the reduction in Pmax. In flag leaves of 5 rice varieties during grain filling a sharp decline (typically over 3-6 days) in Rubisco content and chlorophyll a: b ratio occurred with no fall in Pmax or total chlorophyll content. This reduction was initiated at the same number of days after flowering in each variety although there was no consistent relationship between the timing of the rapid phase of grain filling and the decline in photosynthetic components in flag leaves. We conclude that in field-grown rice, certain photosynthetic components are produced in greater amounts than needed to maintain in situ rates of leaf photosynthesis. Results are discussed in terms of the impact upon RCE of rice crops in high irradiance conditions.