Investigation of SPAD meter-based indices for estimating rice nitrogen status

One solution-culture and two field experiments with four rice genotypes were conducted to relate SPAD-502 chlorophyll meter readings to variables reflecting nitrogen (N) status at different developmental stages. We systemically compared differences of SPAD readings from different measurement points of the same leaf and their relationships with leaf nitrogen and chlorophyll concentrations. Suitable SPAD indices for estimating rice nitrogen status were developed on the basis of the relationships between different SPAD indices calculated from the SPAD readings of various leaf positions and leaf nitrogen and chlorophyll concentrations. The results showed that there was a close relationship between the average SPAD readings at the midpoint of each leaf blade, 30 mm apart and one side of the midrib, and approximately one-third of the way down from the leaf tip, and leaf N and chlorophyll concentration, leading to the selection of four indices of foliar N status of the first and third fully expanded leaf from the top, including NDSI (the normalized difference SPAD index between the first fully expanded leaf (1LFT) and the third fully expanded leaf (3LFT)), RDSI (the SPAD relative positional difference index between the first fully expanded leaf (1LFT) and the third fully expanded leaf (3LFT)), RSI (the SPAD ratio of the first fully expanded leaf (1LFT) and the third fully expanded leaf (3LFT)) and DSI (the SPAD difference between the first fully expanded leaf (1LFT) and the third fully expanded leaf (3LFT)). These indices can be used to predict foliar N concentration in rice without significant interferences of rice genotypes and growth stages. The indices were validated in an independent data set using the integrated effects of the coefficients of determination, the RMSE and the RE. There was evidence that the indices RSI and RDSI were more reliable SPAD indices for estimating foliar N status in rice plant.