The Diagnosis Of Rice N Status Based On Leaf Physioecological Characteristics

Nitrogen is a vital element for plant growth, and its importance is second only to water. Nitrogen application is an important way to increase rice yield per unit area. However, huge loss of nitrogen to environment causes serious pollution. In order to improve the nitrogen utilization efficiency, accurate diagnosis of nitrogen status needs being conducted. In the present research, field and open-air experiment were conducted to study the effect of nitrogen application to rice leaf physiological and morphological characteristics, and the effect of shade to plant growth was also studied. The main conclusions emerging from the study are as follows:The diagnosis of leaf chlorophyll or nitrogen content based on per unit weight by SPAD was influenced by leaf thickness. Leaf thickness was measured using a specially developed displacement sensor. After that, leaf N was estimated using SPAD-502 and directly determined by Dumas combustion method. For pooled data across 3 growths of the field trial, the degree of linear fit between Nw and SPAD values was poor (R=0.557), but was improved (R=0.729) if leaf thickness was introduced as an independent variable. For another pooled data in the open-air trial, the predication of Nw was also improved when leaf thickness became the secondary independent variable, with determination coefficients increasing from 0.0114 (not significant) to 0.513. However, if the leaf N concentration was expressed based on per unit area (Na), the leaf thickness didn't influence the estimate of it by chlorophyll meter for both trials.With the increase of N application, leaf thickness became thinner and chlorophyll a/b ratios decreased. Moreover, the sensitivity of the SPAD readings of the same leaf at different leaf ages to N rates was assessed through coefficients of variation (CV). CV of SPAD readings increased from 8.8% to 21.6% during leaf lifetime, which indicates that SPAD readings became more and more sensitive to nitrogen rates as leaf aged. Therefore, SPAD readings of the lower leaves, which were physiologically older than the upper ones, were more sensitive to nitrogen rates. Moreover, the ratio of chlorophyll a/b of upper leaf to lower leaf could be used to evaluate canopy quality.Since the canopy structures during the period when SPAD readings were taken were unchanged, the light gradient in the canopy kept constant. Temporal dynamics of leaf SPAD reading, which is an estimate of leaf N concentration, was fitted by a piecewise function. During leaf senescence, the rate of decrease in SPAD readings would increase with leaf age, and the rate was lowered by N application. The lower leaves were physiologically older than the upper leaves, and hence its rate of decrease was faster than the upper leaves. Therefore, the differences in SPAD readings of upper and lower leaves became larger with plant development, and then the SPAD readings distribution was skewed by the increase in plant age. Moreover, the differences in the rate of the upper and lower leave development was reduced by the application of N, which led to less steep gradient of SPAD reading distribution.The SPAD readings, leaf thickness, chlorophyll a/b of rice grown under shaded condition declined compared with rice leaf grown under unshaded condition. Moreover, the maximum net photosynthesis and chlorophyll fluorescence parameters were also changed by shading. Shading also significantly reduced theΦPSII, which may induce lower ETR.