VALIDATION OF GOSSYM: EFFECTS OF IRRIGATION, LEAF SHAPE AND PLANT POPULATION ON CANOPY LIGHT INTERCEPTION, GROWTH AND YIELD OF COTTON (GOSSYPIUM HIRSUTUM L.)

Field experiments were conducted for two summers to: (1) validate canopy light interception model in GOSSYM and to characterize the decline in crop canopy light interception during natural defoliation; (2) develop a validation data set for the cotton stimulation model GOSSYM and to validate the model under irrigated and unirrigated conditions; (3) study the effect of water stress on stomatal resistance and leaf water potential under field conditions and (4) study the effects of irrigation and plant population on certain growth characteristics, floral bud, boll formation and yield of normal-leaf and okra-leaf cotton.;Slight increases in stomatal resistance were observed as leaf water potentials decreased although complete stomatal closure due to water stress was not observed. The decline of leaf water potentials during drought was due to decline in soil water potential and high evaporative demand.;Irrigated plants were significantly taller than unirrigated plants. Plant height and mainstem node numbers were reduced in the high plant population treatments. The okra-leaf crops had lower leaf area indices than the normal-leaf crops. Okra-leaf plants produced significantly more squares than normal-leaf plants but the differences in number of bolls were not significant, indicating that okra-leaf plants shed fruits either as squares or young bolls. Leaf shape and plant population had no significant effect on the seed cotton yield. The okra-leaf type was earlier in maturity than the normal-leaf type.;Irrigated plants produced more squares and green bolls than plants without irrigation. The irrigated treatments yielded more than the unirrigated treatments. Unirrigated cotton matured earlier than irrigated cotton.;Normal-leaf plants intercepted more light than okra-leaf plants throughout the season. However, okra-leaf plants, at a given leaf area index, intercepted more light than did the normal-leaf plants. Equations for calculation of light interception from seedling stage to full canopy closure were developed for normal-leaf and okra-leaf cotton. Decline of light interception was observed below leaf area index of 3.1. The data collected under irrigated and unirrigated conditions were used to validate the model GOSSYM. Model predictions were quite close to real data on all plant parameters collected.