Spatial and Temporal Variation in Photon Flux Density on Rice (Oryza sativa L.) Leaf Surface

Spatial and temporal variations of photosynthetic photon flux density (PFD) measured using small photodiodes (Hamamatsu, model G1118) attached on the leaf surface of rice (Oryza sativa L.) differed with the orientation and inclination even at the same height of the canopy. Under sunny conditions, the fluctuation pattern of PFD was mainly determined by the orientation and inclination of the leaf surface, and not by the daily change of PFD in the open, e. g., high peaks of PFD were observed in the morning on east-oriented leaves, but only low peaks on north-oriented leaves. Under overcast conditions, however, the PFD on the leaf surface depended highly on the PFD in the open irrespective of the leaf orientation and inclination. The present study suggests that the orientation and inclination should be considered as the major factors influencing the PFD regime (spatial variation and daily total PFD) within the rice canopy. To determine the effects of spatial and temporal variations of PFD on crop productivity, the daily net assimilation of each leaf was estimated from the obtained data and the light-photosynthesis curve.