The growth and performance of rain-grown cotton in a tropical upland environment: I. Yields, water relations and crop growth

Abstract

SUMMARYThe effects of environment and genotype on growth and yield of cotton were studied in three experiments done at Namulonge, Uganda, from 1966 to 1969. Treatments were date of sowing, variety, fertilizer, plant population and water. Variety BPA66 sown in June at 4–10 plants m-2 outyielded other varieties, sowing dates and population densities. Compound fertilizer at l.25 t ha-1 increased yield by 15% and irrigation increased yield by 38%. The soil water deficit (CSWD) was calculated from meteorological data, and the relative water content (RWC) of the plants was measured. CSWD did not affect growth until a critical value (CD) was reached, which increased from 20 to 50 mm as the crop aged. When CD was reached RWC was s0·094 at dawn and 0·83 at 1400 h. Growth stopped when CSWD > CD, except while any rain, insufficient to make CSWD < CD, was being consumed. Days while such rain was being consumed and days when CSWD < CD were added to give the effective numbers of growing days which accounted for differences in numbers of mainstem nodes caused by sowing date and spacing, and for differences in plant dry weight and leaf area caused by sowing date. Variation in light transmitted by the crop canopy depended on leaf area index (L) alone; spacing, fertilizer and CSWD had no independent effects. The measured extinction coefficient was 1.1 compared with 0·9 predicted by de Wit's (1965) model. Measured value of crop growth rate (C) agreed with values predicted by the de Wit model for the vegetative phase. Spacing and fertilizer only affected C through L. During the reproductive phase C became much less dependent on L, and the form of the relationship changed. Some varieties including BPA66 had a greater net assimilation rate. Maximum C was expected when L ~ 3, but L was seldom > 2. Crops sown at current and previously recommended spacings had sparse canopies and did not fully use light available for dry matter production.