Responses of Cotton Leaf Water Potential and Transpiration to Vapor Pressure Deficit and Salinity under Arid and Humid Climate Conditions

Y. Guo, J. A. Landivar, J. C. Henggeler, J. Moore


Low humidity combined with salinity may cause severe crop water stress in arid regions. High humidity causes low transpiration thus high leaf temperature in humid regions. Understanding of crop water response to environmental conditions is important for efficient management of water use and crop growth. The objectives of this study are to determine (1) the effect of vapor pressure deficit on leaf water potential, transpiration and leaf temperature; (2) the effect of salinity on osmotic adjustment, stomatal control and root resistance. Both field experiments and model simulations were conducted. Field experiments with different water treatments were conducted at a humid (Corpus Christi) and an arid (Pecos) area in Texas during 1993 growing season. Leaf water potentials in the arid area were much lower than in the humid area. The minimum leaf water potential was -28 bars at Pecos and -20 bars at Corpus Christi. No significant difference in leaf water potential were found in the middle of the day between the high and low water treatments at Pecos, while there were remarkable differences between irrigated and dryland at Corpus Christi. Predawn leaf water potential at Pecos, however, showed soil moisture difference between the treatments, while it did not show the difference at Corpus Christi. At Pecos, leaves were still turgid and transpiring at -28 bars leaf water potential, while at Corpus Christi, leaves started wilting and reducing transpiration significantly at -20 bars. Transpiration rates at Corpus Christi responded to soil moisture treatments significantly, while less response was shown at Pecos. Plant size and yield were lower in Pecos than in Corpus Christi. These results may be explained by vapor pressure deficit and salinity. A crop simulation model (ICEMM) was used to perform a sensitivity analysis of parameters controlling plant water relation. The simulated results clearly show that under arid conditions, high vapor pressure deficit results in low leaf water potential. Osmotic adjustment induced by salinity maintains plant turgor under low leaf water potential conditions. Increases in stomatal resistance and root resistance from salinity reduce transpiration rate and inhibit CO2 exchange, thus decreasing plant growth rate. In comparison, under humid conditions, lower vapor pressure deficit results in lower transpiration rate, higher leaf water potential and higher leaf temperature. Higher leaf water potential maintains stomata open and increases CO(2 )exchange rate. Leaf turgidity in humid regions results in a higher growth rate than in arid regions. In arid regions, high vapor pressure deficit and high hydraulic resistances induced by salinity result in low leaf water potential during the daytime regardless of soil moisture. Management practices aimed at maintaining soil moisture at a higher levels at night may be effective in maintaining leaf turgor for cell elongation. In contrast, in humid regions, vapor pressure deficit does not have as much influence on plant water status. Soil moisture, however, determines leaf water potential, transpiration and photosynthetic rate. Maintaining soil moisture availability to plants is essential for reducing water stress and heat stress.

Reprinted from Proceedings of the 1994 Beltwide Cotton Conferences pp. 1301 - 1308
©National Cotton Council, Memphis TN

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Document last modified Sunday, Dec 6 1998