Osmotic and Specific Salt Effects on Growth of Cotton

Studies of plant response to a saline environment are generally performed using nutrient culture techniques or on soils artificially salinized with a single salt or a salt mixture, usually NaCl and CaCl₂. The soil is frequently irrigated to maintain a low matric suction, in contrast to the multiple salt system and variable soil water content encountered in normal field conditions. The objectives of this study were to determine the effects of seasonal changes in water availability on growth and ionic composition of cotton (Gossypium hirsutum L.) and the relative importance of water stress and ion accumulation on growth under field conditions. Plots were established on the saline and non-saline phases of Raymondville clay loam (Vertic Calciustolls). Total soil water suction (TSWS), osmotic suction of root zone (OS), plant height, and leaf mineral composition were determined periodically on each plot through the growing season. Cotton stem elongation decreased with increasing TSWS, and ceased as TSWS approached 12 bars. Regression coefficients suggested that an increase of 1 bar in the TSWS decreased growth by 3.4 and 0.9 cm, respectively, during the 70 to 85 and the 85 to 115-day growth periods. However, plant size differences at the same TSWS suggested that growth depression on the saline soil was related to ionic composition effects in addition to an osmotically. induced water deficit. The Ca, Mg, and Na concentration of the cotton leaves increased significantly as TSWS increased, whereas K was not affected. Consequently, the K/(Ca+Mg) ratio decreased as OS and TSWS increased. The Cl concentration of the cotton leaves increased with increasing TSWS. On day 85, the leaf Mg concentration and OS were almost equally correlated with plant size. Later in the season (115 and 128 days after planting), plant size was significantly related to the OS and Na concentration of the leaves. The results indicated that plant analyses could be used to separate the specific nutritional effects from the osmotic effects on growth in a saline environment.