Effect of shading on leaf SPAD values and the characteristics of photosynthesis and morphology of rice canopy

Soil-plant analysis and development （SPAD） meter is a quick, portable，non-destructive diagnostic tool for the measurement of relative chlorophyll or nitrogen（N）content in leaves. SPAD meter is time, labor and resources saving compared to traditional N nutrition diagnosis methods. The experiment with and without shading treatments was conducted in paddy field with rice cultivars YY9 and B9363. The physiomorphology indices including leaf SPAD values, chlorophyll contents, chlorophyll fluorescence and photosynthetic parameters, specific leaf weight and blade thickness in rice canopy were measured to establish the regression relationship between SPAD values and the maximum quantum output in photosystem II (Fv/Fm of PSII) at rice growth stages. The results show that under the shading treatment, there is decline in the leaf SPAD values, chlorophyll a/b ratios, specific leaf weights and electron transport rate. The thickness of leaves becomes thin in rice canopy; the initial slopes of rapid light curves do not change obviously. The respiration rate, the maximum net photosynthetic efficiency (Pmax), the quantum efficiency, light compensation point and saturation point of leaves are decreased to reduce photosynthetic capacity and consume through breath, and increase the utilization of limited light energy under weak light environment. Under both the shading and without shading treatments, exponential relationships exist between the leaf SPAD values and the Fv/Fm of PSII in rice canopy. For cultivar YY9, the coefficient of determination R2 is 0.896 and for B9363 is 0.833. Therefore, the leaf photosynthetic process in rice canopy can be evaluated quickly, non\|destructively and efficiently using SPAD meter, and the SPAD reading of lower than 35 may be used to indicate the impairment in photosynthetic process of rice.