Quantifying the effects of nitrogen on fruit growth and yield of cucumber crop in greenhouses

Nitrogen supply can improve crop growth and yield. An over-use of nitrogen fertilizer in greenhouse crop productions, however, causes many environmental problems. The aim of this study was to quantify the effects of nitrogen on fruit growth and yield so as to facilitate the optimization of nitrogen management for cucumber (Cucumis sativus) crop in greenhouses. Four experiments with different levels of nitrogen treatments, substrates and planting dates on cucumber (cv. Deltastar) were conducted in greenhouses located at Shanghai during 2005 and 2007. Using data of one experiment, seasonal time courses of leaf nitrogen content (N_L), leaf area per plant (LA), and the number of fruits growing per plant (n_FG), as well as time course of the length of individual fruit growing on the plant (L_F(i)) under different levels of nitrogen supply conditions were, respectively, determined as functions of a photo-thermal index (PTI). The impact of N_L on LA was determined by curve fitting to the experimental data. The source/sink ratio (LA/n_FG), an indicative of the source size per fruit, was then derived from the seasonal time courses of LA and n_FG. The impact of N_L on L_F(i) was indirectly quantified by the relationship between source/sink ratio (LA/n_FG) and the elongation rate of individual fruit (R_FL(i)). Both the harvest date and fresh weight (W_F(i)) of individual fruit growing at different node, and number of harvested fruits (n_FH) were then calculated as functions of the fruit length. These quantitative relationships were assembled to form a model for predicting the effects of nitrogen on fruit growth and yield (fruit fresh weight per plant). Independent data from other experiments were used to validate the model. Our model gives satisfactory predictions of cucumber fruit growth and yield under different levels of nitrogen supply and growing season conditions. The coefficient of determination (r²) and the relative root mean squared error (rRMSE) between the predicted and measured values are, respectively, 0.92 and 0.22 (r², rRMSE) for leaf area per plant, 0.90 and 0.24 for the number of fruits growing on the plant, 0.91 and 0.22, 0.90 and 0.23, and 0.92 and 0.21, respectively, for the length, harvest date and fresh weight of individual fruit growing on the plant, 0.94 and 0.20 for yield. The model may be used for the optimization of nitrogen management for cucumber production in greenhouses. Further model calibration and test would be needed when applying this model to a wider range of conditions.