CRITICAL PETIOLE NITRATE-NITROGEN FOR LINT YIELD AND ENHANCED MATURITY IN PIMA COTTON

Petiole NO3-N concentration (PNN) has been recommended as an indicator of midseason N deficiency in upland cotton (Gossypium hirsutum L.). Its use in Pima cotton (G. barbadense L.) management, especially when early maturity is the goal, is not well documented. A 3-yr study evaluated PNN as an indicator of N status in Pima cotton in order to identify a critical minimum PNN level for yield and early maturity. Treatments were five preplant applied N rates of 0 to 180 kg ha(-1) in 1990 and 0 to 269 kg ha(-1) in 1991 and 1992. The PNN was measured throughout boil development, starting approximate to 2 wk before or at first flower. Applied N significantly (P less than or equal to 0.05) affected PNN in 1991 and 1992, when residual soil N was low but not in 1990, when residual N was high; differences among treatments in 1991 and 1992 were largest during early flowering. PNN was a good indicator of plant N status within but not across seasons. Based on early-flowering samples, the critical PNN level for lint yield was 3.84 g kg(-1) in 1991 and only 1.75 g kg(-1) in 1992; in 1990, residual soil N alone resulted in PNN much higher than the critical levels in the other years. PNN < 2 g kg(-1) in 1991 and < 1 g kg(-1) in 1992 resulted in earlier maturity, but also resulted in less lint yield. Pima cotton PNN did not exceed an average of 11 g kg(-1), in contrast to 24 to 30 g kg(-1) reported for upland cotton. Plant growth or petiole anatomical variables that cause year-to-year variation in PNN should be identified and the necessary normalization made before PNN can be used as a midseason diagnostic tool in Pima cotton.