This study compared the relationship between chlorophyll (Chl) content, gas exchange, Chl fluorescence characteristics, and leaf color, using paired near-isogenic lines (NILs) of a medium-green leaf inbred line SN12 and a yellow-green leaf mutant SN62 to explore the photosynthesis of the yellow-green mutant. The SN62 was found in a female parent, Xianyu 335, which grew normally, although there were small yellow spots on the leaves at the seedling stage and yellow-green leaves appeared from the seedling to the maturation stage. The results indicated that Chl a (b), quantum efficiency of PSII, and maximal quantum yield of PSII photochemistry of SN62 were significantly lower than those of SN12, but there were almost no differences in the net photosynthetic rate (PN). There was no significant correlation between Chl a (b) and P N of inbred lines with different leaf colors. In the reproductive stage, photochemical quenching, effective quantum yield of PSII photochemistry, and the electron transport rate of SN62 increased obviously, and all parameter values exceeded the values of SN12. It explained that increasing the openness of the PSII reaction center was able to compensate for the lower Chl content, which was beneficial for harvesting more light energy for photochemical reactions. It also ensured that P N was not reduced.

Core Ideas This study aims to understand the relationships of Na, Nw, NNI, and SPAD values.We find it is more specific to analyze the effect of factors on SPAD values.Some efficient measures may enhance the accuracy of SPAD values.Relative personnel may use more efficiently and accurately the SPAD meter in rice. Determination of the nitrogen nutrition index (NNI) of a crop requires measurement of the amount and status of N in shoot biomass, which restricts its wide application in paddy fields under field conditions. Thus, a convenient and efficient method is required. We estimated the N concentration of leaves in the rice canopy on the basis of dry matter weight, Nw (%) and per unit area, Na (mg cm−2), and measured Soil–Plant Analysis Development (SPAD) values as a potential replacement for NNI. Two rice cultivars with six N fertilization rates (0, 75, 150, 225, 300, and 375 kg N ha−1) were used in a field study from 2010 to 2012 in Hangzhou, China. The SPAD values, Na, and Nw of the upper four leaves in the rice canopy were measured at three developmental stages (tillering, booting, and heading) alongside the NNI. The results showed that with increasing N rates the SPAD values, Na, Nw, and NNI increased in all developmental stages and leaf positions each year. The SPAD values, Na, and Nw showed linear regression with NNI, but the regression parameters varied between developmental stages. Once developmental stages were determined, the equation parameters did not differ among years, leaf positions, and cultivars. The relationships between Na and NNI, and SPAD values and NNI were closer than those between Nw and NNI, suggesting that SPAD values and Na may replace NNI for a quick and approximate prediction of rice plant N nutrition in the field.

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