Late Season Water Stress in Cotton: I. Plant Growth, Water Use, and Yield

The behavior of water stressed cotton (Gossypium hirsutum L.) is well documented but our knowledge of traits which can be genetically manipulated to improve drought tolerance is incomplete. This study was conducted to determine which morphological and physiological factors lessen the effects of water stress on the yield of two short-season cultivars [i.e., TAMCOT HQ95 (HQ95)and (G&P 74+ (GP74)] with a common parent in ancestry. Plants were grown in a rain shelter- lysimeter facility containing a Pedernales fine sandy loam soil (fine, mixed, thermic Udic Paleustalf)in 1990 and 1991 at Temple, TX. Water stress was imposed by replenishing a fraction of the water lost to evapotranspiration, beginning about 78 d after emergence. HQ95 and GP74 did not differ in leaf area index(LAI) or in the rate of leaf area development before water stress was imposed. The rate of LAI decline and average LAI were similar between cultivas when water stress was imposed. HQ95 used more water and used it at a faster rate than GP74 when water stressed. HQ95 produced more bolls and had higher yield under both well-watered and water stressed conditions than GP74 in each of the 2 yr. The largest difference in boll load between cultivars occurred on sympodia branches in the lower canopy, where HQ95 had 37, 60, and 182% more bolls than GP74 when plants received 0, 50 or 75, and 100% of the depleted soil water. Whether well watered or water stressed, individual boll weight did not differ between the two cultivars. However, the harvest index and the production efficiency of bolls (i.e., bossl pr unit leaf area)of HQ95 was consistently higher than GP74 for all water regimes. On average, HQ95 located 21% more dry matter to yield and produced 32% more bolls per square meter than GP74. While differences in yield between the cultivars mirror harvest index, large differences in boll production efficiency suggested that the intrinsic photosynthetic capacity of HQ95 may be greater than GP74. Contribution of the Texas Agric. Exp. Stn, Texas A&M University System, College Station, TX.