VEGETATIVE AND REPRODUCTIVE DRY-WEIGHT INHIBITION IN NITROGEN-DEFICIENT AND PHOSPHORUS-DEFICIENT PIMA COTTON

Whether the extent of dry weight inhibition by nitrogen (N) or phospho rus (P) deficiencies on different plant parts is the same and whether imposing moderate N and P deficiencies selectively suppress undesirabl e vegetative growth has not been studied in Pima cotton (Gossypium bar badense L.). The purpose of this study was to determine the extent to which dry matter accumulation in leaves, stems, and reproductive struc tures is inhibited by N and P deficiencies in Pima cotton. The study w as conducted in 1991 and 1992 in a Uvalde silty clay loam soil (fine-s ilty, mixed, hyperthermic Aridic Calciustolls). The treatments include d applied rates of 0, 67, 135, 202, and 269 kg N ha(-1) in a factorial combination with 0, 15, 29, and 44 kg P ha(-1). Nitrogen deficiency ( 0 kg N ha(-1)) significantly (P less than or equal to 0.05) reduced le af (LDW) and stem (SDW) dry weights in both years and reproductive dry weight (RDW) in 1992. Nitrogen deficiency suppressed dry weight accum ulation in leaves to a greater extent than in stems. Relative to 269 k g N ha(-1), the 0 kg N ha(-1) treatment resulted in a maximum LDW redu ction of 62% at 144 DAP (days after planting) in 1991 and 36% at 121 D AP in 1992, compared with a corresponding SDW reduction of only 39% in 1991 and 25% in 1992. Dry weight accumulation in reproductive parts w as the least affected by N deficiency. The decline in LDW associated w ith senescence and defoliation began earlier in treatments that receiv ed 0 or 67 kg N ha(-1) than treatments that received greater than or e qual to 135 kg N ha(-1). Phosphorus affected LDW and SDW in 1991, but its differential effect on LDW, SDW, and RDW was much smaller than tha t of N. Imposing a moderate level of N deficiency, not P deficiency, m ay be an effective Pima cotton management strategy to selectively supp ress undesirable vegetative growth and enhance maturity.