Abstract: Field experiments were conducted in Nanjing in 2007 and 2008, involving two rice cultivars and three nitrogen application rates. The plant morphology and spatiotemporal distribution of PAR at different canopy heights were measured at filling stage with SunScan canopy analysis system. Nitrogen application rate had significant effects on the plant height, panicle curvature, and leaf angle of rice. The leaf area was greater in mid canopy than in upper and basal canopy. The maximum leaf area index (LAI) layer appeared at 0.60 of relative canopy height. The average PAR transmittance decreased from top to bottom in rice canopy, exhibiting a quick attenuation within the upper and mid canopy and a slow reduction within the basal canopy. In addition, the average PAR transmittance at different canopy heights decreased with increasing nitrogen application rate. The diurnal variation of average PAR transmittance was greater at noon than in the morning and afternoon. With increasing LAI accumulation, the average PAR transmittance at different canopy heights decreased in an exponential pattern. The diurnal variation of population extinction coefficient (K) was smaller at noon than in the morning and afternoon, and the K value was in the range of 0.35-0.50. The 3-dimensional distribution of PAR in rice canopy indicated that the PAR transmittance on horizontal plane was greater, and exhibited much more light flecks in upper and mid canopy than in basal canopy. The PAR density distribution at the same canopy height was non-uniform on the horizontal plane. Higher PAR transmittance, greater diurnal variation of PAR transmittance, and smaller population extinction coefficient were observed for the erect plant type rice cultivar.

Key words: photosynthetically active radiation (PAR), extinction coefficient, rice (Oryza sativa L.), light distribution, transmittance, water stress, wheat, non-structural carbohydrates.