Vertical profile of photosynthetic light response within rice canopy

Measured leaf photosynthetic light response (PLR) curves at different positions were fitted by non-rectangular hyperbola (NRH) equation to characterize vertical profile of parameters in NRH equation, namely maximum net photosynthetic rate P_nmax, initial quantum yield of assimilation φ, dark respiration rate R_d, and convexity of the curve k, at both jointing and heading stages within rice canopy. And leaf-position-specific and canopy average NRH equations were constructed respectively based on measured PLR curves at each specific leaf position and all measured PLR curves within rice canopy. The results showed that the P_nmax, φ, and R_d reached the maximum at the top second leaf and then decreased at jointing stage and decreased in downward leaves at heading stage. The k increased with lowering leaf position at both stages. The leaf-position-specific NRH equation performed well in estimating net photosynthetic rate P_n for all leaves at different positions and stages, while the canopy average NRH equation underestimated leaf P_n at upper canopy and overestimated P_n at lower canopy. The top fourth leaf was suitable for estimating photosynthetic parameters at canopy scale, as the P_nmax, φ, R_d, and k of the top fourth leaf were near to these parameters of rice canopy, and the canopy average NRH equation performed well in estimating leaf P_n for the top fourth leaf. The results will provide basic information for upscaling leaf photosynthesis to canopy scale.