Quantifying the effects of leaf nitrogen content on leaf photosynthesis rate of greenhouse cucumber under different par and temperature conditions.

摘要

Leaf nitrogen content is the most commonly used information for crop nitrogen management since it strongly affects leaf photosynthesis rate. Leaf N content, however, is affected by photosynthetically active radiation (PAR) and temperature conditions. This results in that the relationship between the leaf N content and the leaf photosynthesis rate obtained under specific site and season cannot be applied to other sites and seasons. The aim of this study was to develop a model for estimating the effects of leaf N content on leaf photosynthesis rate of greenhouse cucumber under different PAR and temperature conditions through an experimental study. Experiments with 4 N treatments and sowing dates were conducted in greenhouses in Shanghai during 2003 and 2005. Light saturated leaf photosynthesis rate (Pmax) of fully expanded cucumber leaves was measured using the Li-6400 system after the nitrogen treatments started at anthesis. The seasonal time course of the optimal leaf N concentration (Nopt) for photosynthesis was found to be an exponential function of the physiological development time (PDT). The parameters of this function were then related to PAR and temperature conditions using the seasonal average value of the 'product of normalized thermal time and PAR (PTTP)'. The ratio of Pmax at an actual leaf N concentration (N) to that at the optimal leaf N concentration (Nopt) was also found to be an exponential function of the ratio of N/Nopt. Based on these quantitative relationships, a model for estimating leaf photosynthesis rate of greenhouse cucumber using leaf N concentration under different PAR and temperature conditions was developed. Independent experimental data were used to validate the model. The coefficient of determination (R2) and the root mean square error (RMSE) between the estimated and the measured leaf photosynthesis rate based on the 1:1 line are 0.83 and 1.56 micro mol CO₂ m-2 s-1, respectively. The model developed in this study gives satisfactory estimation of leaf photosynthesis rate of greenhouse cucumber growing during different seasons. Experimental data from different sites and cultivars are, however, needed to further validate and test the model before applying it to new sites and cultivars.