Four indices were selected to
construct aggregated indicators to diagnose rice N status. The four
SPAD indices were all significantly and exponentially related with
leaf total N content and the R-squared values were 0.8305, 0.8205,
0.8205 and 0.8442 for NDSI (the normalized difference SPAD index
between 1LFT and 3LFT), RDSI (the SPAD relative positional difference
index between 1LFT and 3LFT), RSI (the SPAD ratio of 1LFT and
3LFT) and DSI (the SPAD difference between 1LFT and 3LFT), respectively