Quantifying The Effects Of Nitrogen On Leaf Photosynthesis And Chlorophyll Fluorescence Parameters Of Greenhouse Cucumber After Flowering

Photosynthesis and chlorophyll fluorescence parameters analyzed have been important tools for estimating crop biochemical contents (nitrogen, chlorophyll), and is becoming increasingly popular in recent years. In this study, a series of field experiments with greenhouse cucumber including different nitrogen treatments were carried out in a Venlo-type greenhouse, a multi-span PVC greenhouse, and a multi-span plastic greenhouse in Shanghai (E121.5°, N31.2°) from 2003 to 2005 to collect data for model development and validation. Leaf photosynthesis rate and leaf chlorophyll fluorescence parameters were measured with Li-6400 portable photosynthesis system and a modulated fluorometer (FMS-2 Hansatech, Norfolk, UK) using the saturation pulse method after the nitrogen treatments started, respectively. The objectives of this research were to quantify the effects of leaf nitrogen concentration on leaf photosynthesis rate of greenhouse cucumber under different radiation and temperature conditions, to determine the relationship between leaf nitrogen concentration and leaf chlorophyll fluorescence parameters, and the relationship between leaf photosynthesis rate and leaf chlorophyll fluorescence parameters, and to develop models for monitoring nitrogen status in greenhouse cucumber. This word would provide theoretical basis and key techniques for non-destructive monitoring of nitrogen fertilization management in greenhouse cucumber.The effects of leaf nitrogen concentration on leaf photosynthesis rate of greenhouse cucumber were investigated under different radiation and temperature conditions. The results showed that the optimal leaf nitrogen concentration for photosynthesis was found to be an exponential function of the physiological development time (PDT) and the product of thermal effectiveness and PAR (TEP). The leaf optimal nitrogen concentration was calculated by formula as follows: N%_opt(t) is leaf optimal nitrogen concentration of anytime in whole growing season; a is initial leaf nitrogen concentration of seedings when transplanting, determined as 2.5% from the result of our experiment data; N%_max is the maximum leaf nitrogen concentration of the whole growing season; b is the increasing rate of leaf nitrogen concentration from planting date to the time when leaf nitrogen concentration reaches N%_max; Tpdt is physiological development time for the duration from transplanting to the date when the N%_max obtained, determined as 20 days from the result of our experiments data; N%_min is the minimum leaf nitrogen concentration from the time when leaf nitrogen concentration reaches N%_max to the end of the growing season; TC is the time coefficient for the leaf nitrogen concentration changing N%_max to N%_min.y=y_max*sin (0.5Ï€*ATEP/ATEP_opt)y is referred to parameters of N%_max, b, N%_min, TC; y_max is defined as the maximum of these parameters, which appeared in the experiments (from August to November 2005) as our experiments, and determined as 4.8%,0.155,3.8%å’Œ9.5, respectively; ATEP is the average daily value of TEP over the period from planting date to fruit setting (for N%_max and b) and from fruit setting to fruit harvest (for N%_min and TC), MJ·m^-2; ATEP_opt is the average daily value of TEP over the period from planting date to fruit setting and from fruit setting to fruit harvest when the maximum values of N%_max, b, N%_min, TC attained, determined as 4.01å’Œ2.89 MJ·m^-2 according to the data from the experiments (from August to November 2005), respectively.The relationship between the ratio of the actual maximum leaf gross photosynthesis rate (Pgmax_i) to that under conditions without nitrogen deficiency (Pgmax₀) (PR= Pgmax_i / Pgmax₀) and the ratio of the actual leaf nitrogen concentration (N%) to the optimal leaf nitrogen concentration (N%_opt) (NR= N%/ N%_opt) was also found to be an exponential function.Based on these quantitative relationships, a general model was developed to estimate the effects of leaf nitrogen concentration on the maximum leaf gross photosynthesis rate of greenhouse cucumber under different PAR and temperature conditions.PR=1.05*(1-exp(-5.6*(NR-0.42)/1.05)) R²=0.80 SE=0.10PR is the ratio of the actual maximum leaf gross photosynthesis rate to that under conditions without nitrogen deficiency; NR is the ratio of the actual leaf nitrogen concentration to the optimal leaf nitrogen concentration.Independent experimental data were used to validate the model. The coefficient of determination (R²) and the root mean squared error (RMSE) between the estimated and the measured maximum leaf gross photosynthesis rate based on the 1:1 line are 0.83 and 1.56umol CO₂·m^-2-s^-1, respectively.The actual photochemical efficiency of PSII reaction (ΦP5II) under conditions without nitrogen deficiency was found to be an exponential function of air temperature at 1.5m above the ground inside the greenhouse. The optimal leafΦPSII under different temperature was calculated as follow:ΦPSII(T) is the optimal leafΦPSII under conditions without nitrogen deficiency when the temperature is T;ΦPSII_max is the maximumΦPSII of the whole growing season; a is the increasing rate ofΦPSII from the base temperature to the base optimum temperature whenΦPSII_max obtained; b is the decreasing rate ofΦPSII from the upper optimum temperature to the maximum temperature after theΦPSII_max obtained; T'_b, T'_ob, T'_ou, T'_m are the base temperature, the base optimum temperature, the upper optimum temperature and the maximum temperature, determined as 5, 22, 28,45℃, respectively, for greenhouse cucumber photosynthesis. The parameters ofΦPSII_max and b are determined as 0.65 and 0.13 from experiments data, respectively. Farther experiments of low temperature growing seasons are needed to determine the parameters of a.Based on this quantitative relationship, a general equation was developed to estimate the relationship between leaf nitrogen concentration and leafΦPSII under different temperature conditions.ΦPSII R =0.95*(1-exp(-6*(NR-0.4)/0.95)) R²=0.680, SE=0.103NR is the ratio of the actual to the optimal leaf nitrogen concentration;ΦPSII R is the ratio of the actual to the optimal leafΦPSII.Independent experimental data were used to validate the model. The coefficient of determination (R²) and the root mean squared error (RMSE) between the predicted and the measured leafΦPSII based on the 1:1 line are 0.86 and 0.05, respectively.The relationship between leafΦPSII and the maximum leaf gross photosynthesis rate of greenhouse cucumber under different temperature conditions was estimated as follow, based on the relationship between leafΦPSII and air temperature at 1.5m above the ground inside the greenhouse.PR =1.05*(1-exp (-4.5*(ΦPSIIR -0.3)/ 1.05)) R²= 0.68, SE = 0.10PR is is the ratio of the actual maximum leaf gross photosynthesis rate to that under conditions without nitrogen deficiency;ΦPSII R is the ratio of the actual to the optimal leafΦPSII.A nitrogen monitored model based on quantified relationship between chlorophyll fluorescence parameters and leaf photosynthesis rate was developed. Independent experimental data were used to validate the model. The coefficient of determination (R²) and the root mean squared error (RMSE) between the predicted and the measured leaf nitrogen concentration based on the 1:1 line are 0.66 and 0.175%, respectively, showing that the model is good for nitrogen fertilization management in greenhouse cucumber.