Effects of Soil Salinity and Plant Density on Yield and Leaf Senescence of Field-Grown Cotton

The response of cotton to constant salinity has been well documented under controlled conditions, but its response to changing salinity under field conditions is poorly understood. Using a split-plot design, we conducted a 2-year field experiment to determine the effects of soil salinity and plant density on plant biomass, boll load, harvest index and leaf senescence in relation to cotton yield in three fields with similar fertility but varying salinity. The main plots were assigned to weak (electrical conductivity of soil saturated paste extract, ECe = 5.5 dS m-1), moderate (ECe = 10.1 dS m-1) and strong (ECe = 15.0 dS m-1) soil salinity levels, while plant density (3.0, 4.5 and 7.5 plants m-2) was assigned to the subplots. Soil salinity had a negative effect on seedcotton yield, but the negative effect was compensated for by increased plant density under strong-salinity conditions. Seedcotton yield under weak salinity changed little with varying plant density, but the medium plant density yielded better than the low or high plant density under moderate salinity. Plants accumulated 49 and 112 % more Na+ in leaves under moderate and strong salinity than under weak salinity. Strong salinity also led to higher boll load and early leaf senescence. Plant density had no effect on Na+ accumulation in leaves, but greatly reduced boll load and delayed leaf senescence. Plant biomass, maximum leaf area index and harvest index were greatly affected by salinity, plant density and their interaction. Accelerated leaf senescence under strong salinity was attributed to the high boll load and increased accumulation of toxic ions like Na+ in leaves, while delayed leaf senescence with increased plant density was attributed to the reduced boll load. Optimal yield can only be obtained with proper coordination of total biomass and harvest index by modification of plant density based on salinity levels.