Four indices based on the readings
of the first and third fully expanded leaf, 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), were developed. They were all significantly
and exponentially related with leaf total N content.