Quantifying The Effects Of Nitrogen On Fruit Growth And Economic Yield Of Fruit Cucumber In Greenhouses

Cucumber (Cucumis sativus) is one of main greenhouse crops in the world; the economic yield of cucumber mainly depends on the fruit size, fresh weight and harvest date of individual fruit and the total fresh weight per plant. Nitrogen is a kind of important nutritive elements affecting cucumber fruit growth and economic yield, and crop model is a powerful tool of optimizing of environment management for greenhouse crop production; therefore, it is a important method to develop a model of for predicting the effects of nitrogen on fruit growth and economic yield so as to facilitate the optimization of environment management and promote economic yield for cucumber crop production in greenhouses. Four experiments with different nitrogen supply rates and planting dates on cucumber(Cucumis Sativus cv. Deltastar) were, respectively, conducted in Dutch Venlo automatic greenhouse and 2,4-th solar greenhouse located at Shanghai (32°N,121°N) during 2005 and 2007. Based on the data of experiment 2, firstly, quantitatively studying the effect of nitrogen leaf area per plant which affected the rate of fruit abortion and harvest using the index of radiation and temperature (photo-thermal index, PTI), we established a model of predicting the effect of nitrogen on the source/sink ratio in greenhouse fruit cucumber. Secondly, we developed a model of predicting the effect of nitrogen on individual fruit growth based on this model. Finally, we can predict the effect of nitrogen on the economic yield per plant of greenhouse fruit cucumber. Independent experimental data from other experiments were used to validate the model. The results show that our model gives satisfactory predictions of cucumber fruit growth and economic yield under different nitrogen supply and growing season conditions.(1) Quantifying the effects of nitrogen on leaf area per plant, the number of fruits growing on a plant and the source/sink ratio of greenhouse fruit cucumber. Based on the experimental data, seasonal time courses of leaf nitrogen content (NL), leaf area per plant (LA) and the number of fruits growing per plant (NFG) under different nitrogen supply rates were, respectively, determined as functions of a photo-thermal index (PTI). By researching the impact of NL on LA, the result showed that the maximum leaf area per plant (LAmax) increased with the increasing nitrogen supply rate. The number of fruits growing on a plant (NFG) is calculated as a function of the rates of fruits appearance, fruit abortion and fruit harvest. Our experimental data demonstrated that the number of fruits appeared was not influenced by nitrogen supply rate while the number of fruits aborted and harvested, respectively, decreased and increased with increasing nitrogen supply rate. Rate of fruit abortion was found to be a negative exponential function of LAmax whereas rate of fruit harvest a positive exponential function of LAmax. For fruit vegetables, leaf and fruit is, respectively, the main source and sink organs. Since LAmax is dependent on leaf nitrogen content, nitrogen has an indirect impact on number of fruit aborted and harvested on a plant, hence, on the sink/source ratio (LA/NFG) through its effect on LA. Based on these quantitative relationships, we can predict the source/sink ratio of greenhouse fruit cucumber under different nitrogen supply rates in everyday.(2) Quantifying the effects of nitrogen on fruit growth and economic yield of greenhouse fruit cucumber. Fruit growth was affected by nitrogen indirectly based on the effect of nitrogen on the source/sink ratio. Our experiment data confirmed that the elongation rate of cucumber fruit had a positive correlation with LA/NFG·Based on the effect on nitrogen on the source/sink ratio of greenhouse fruit cucumber, we can predict individual fruit length on the cucumber plant on day j.The economic yield of cucumber is mainly depends on the fruit size, fresh weight and harvest date of individual fruit and the total fresh weight per plant. Cucumber fruit fresh weight has a positive relationship with cubic meter of its length; and harvest date of individual fruit growing on a plant can then be determined as the date when its length on day j reaches LF(H); and then the number of fruits harvested on a plant on day j (NFH(j)) can be estimated based on harvest date of individual fruit growing on the plant; at last, the total fresh weight per plant can be calculated based on these quantitative relationships. Since fruit growth was affected by nitrogen indirectly based on the effect of nitrogen on the source/sink ratio, and the fruit with the slower growth rate required a longer time to reach LF(H), nitrogen impacted the economic yield per plant of greenhouse fruit cucumber.The coefficient of determination (R2) and the relative root mean square error (rRMSE) between the predicted and measured values are, respectively,0.92 and 0.22 for leaf area per plant,0.90 and 0.24 for the number of fruits growing on a plant,0.90 and 0.23 for the source/sink ratio per 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 0.19 for the total fresh weight per plant. The results show that our model gives satisfactory predictions of cucumber fruit growth and economic yield under different nitrogen supply and growing season conditions.Our model can predict both individual fruit growth and economic yield of cucumber grown in Shanghai based on planting date, nitrogen supply rate and radiation and temperature in greenhouse. The model can be not only used to determine an optimal nitrogen supply rate for greenhouse fruit cucumber, but also used to control individual fruit growth by changing leaf area per plant and number of fruit growing on the plant to get more economic yield and benefit for greenhouse managers.