PLANT NITROGEN STATUS AND BELL LOAD OF COTTON

Nitrogen management is important in determining cotton (Gossypium hirsutum L.) yield and fiber quality. Plant tissue tests are often used to estimate cotton N status and determine if further fertilization is needed during the growing season. This study was conducted to examine the relationship between plant tissue tests of petiole NO3 concentration and leaf N content and cotton performance as measured by vegetative growth (i.e., leaf area) and bell load. Nitrogen rates of 0, 18, 36, 72, and 144 mM N pot(-1) were applied at 2-wk intervals beginning 13 and 26 d after emergence in 1987 and 1988 through bell maturity to a greenhouse-grown cultivar, Stoneville 213. The strong relationship between leaf N and leaf area and boll number (r(2) = 0.80 and r(2) = 0.89) 1 wk after first flower appearance suggests that the transition period between vegetative and boll development is a critical time to assess cotton's N status in relation to yield. Nitrogen deficits had little effect on bell weight, boll period duration, and number of main stem nodes. Although boll weight decreased with increasing N deficit, final boll weight of the high-N plants were within 20% of the plants receiving low N supply. The relationship between leaf N and petiole NO3 content, was best described by Y = a + bX(C); however, with this function only one-half of the variation in leaf N was accounted for by variation in petiole NO3 content (r(2) = 0.48). Leaf area and boll number were poorly correlated with petiole NO3 content. Leaf N content was a better predictor of cotton vegetative growth (as indicated by LAI) and bell number than petiole NO3 content, particularly when measurements are made 1 wk after first flower