Abstract:

The objective of this research was to investigate the positional distribution of SPAD values of the main-stem leaves and their relationships with the nitrogen content of cotton (Gossypium) leaves. The plant and leaf samples were analyzed for concentrations of N and SPAD values from the field experiments in 2004 and 2005 on the farm of Jiangsu Academy of Agricultural Sciences with two cotton cultivars and five N fertilization rates applied. The five N rates were 0, 120, 240, 360, and 480 kg ha^-1 and the two cotton cultivars were Meimian 33B and Simian 3 which have obvious difference in leaf greenness. The results showed that the SPAD values of the two cultivars increased significantly with N rate increasing, and they all could be used as indices for diagnosis of the plant nitrogen nutrition. The leaf SPAD values at different positions on main stem of two cultivars responded differently to the N rate increasing, of which the 1^st fully-expanded leaf from the top on the main stem (SL1) were more sensitive while the 4^th fully-expanded leaf (SL4) responded slowest. The differences of SPAD values between SL4 and SL1 of the two cultivars decreased with N rate increasing. They could be well characterized by the relative positional difference index (PDI) reflecting the greenness difference between the SL1 and the SL4. The PDI was related to plant leaves nitrogen concentrations, which could be combined in a linear regression for Meimian 33B and Simian 3. In particular, the PDI can accurately indicate plant leaves nitrogen status of cotton varieties under varied nitrogen supply levels over different growth stages. It was concluded that PDI can be an ideal indicator of cotton nitrogen status.

Key words: Cotton, Main-stem leaves, SPAD values, Leaf positional distribution, Plant nitrogen diagnosis, Leaf positional difference index