The relationship between SPAD values and leaf blade chlorophyll content throughout the rice development cycle

The Soil and Plant Analyzer Development (SPAD) meter is a chlorophyll meter used worldwide due to its convenience. The present study was conducted to investigate the relationship between SPAD values and rice leaf blade chlorophyll content throughout the entire rice development cycle, and determine whether it is possible to apply a nonlinear model to the relationship. A field experiment was conducted in which the rice communities of two cultivars ('Nipponbare' in 2000 and 'Kinuhikari' in 2002) with wide-ranging chlorophyll content were generated by varying nitrogen fertilizer rates. Data sets comprising SPAD values for leaves sampled from the various rice communities and actual chlorophyll content of the same leaf measured spectroscopically were created. A linear model for each cultivar was applied to the relationship between the SPAD value and chlorophyll content of the same leaf at each developmental stage. Their relationship revealed a good fit. Subsequently, linear and nonlinear models were applied to the data through all rice development stages of each cultivar. A nonlinear model for each cultivar revealed a better fit to the relationship between SPAD values and rice blade chlorophyll content than the linear model. The following nonlinear models were then generated to fit the data: Chlsub(Ni) = 0.965 exp (0.036 Sp) - 1 (r**2 = 0.93) Chlsub(Ki) = 0.925 exp (0.038 Sp) - 1 (r**2 = 0.94) where, Chlsub(Ni) and Chlsub(Ki) denote the leaf blade chlorophyll content (mg/g, flesh weight) for Nipponbare and Kinuhikari, respectively, and Sp is the SPAD value. The coefficients of the nonlinear models obtained for each cultivar were very similar. The possibility of applying a nonlinear model to the relationship between the SPAD value and rice leaf blade chlorophyll content was confirmed with high coefficients of determination. To establish a broadly applicable calibration equation based on a nonlinear model for evaluating the chlorophyll content in the rice leaf blade using a SPAD meter, more studies are needed using various cultivation data sets.