AbstractsBiology & Animal Science

Under adverse soil moisture conditions a critical water stress or water potential develops in the plant and productivity is reduced, or in extreme cases, terminated. Research to determine the soil moisture status for maximum production of specific crops has been conducted for decades. The results have done much to increase the productivity of the irrigable acreage. Yet, even with irrigation water available, under adverse atmospheric conditions water stress will still develop in the agronomic crops. The degree of stress that develops and the duration, however, is generally unknown. Limited research in controlled environments has been conducted, but actual plant water stress levels under field conditions have not been studied. The purpose of this research was to determine the levels of leaf water potential (water stress) that develop in specific horticultural crops under conditions of optimum soil moisture. Further studies were made to relate the leaf water potential to atmospheric parameters and leaf temperature. Three horticultural crops, sweet corn (Zea mays), tomatoes (Lycopersicon esculentum), and bush beans (Phaseolus vulgaris) were grown on a Cloquato silt loam soil under high soil moisture conditions and exposed to the natural atmospheric conditions common to the Willamette Valley of Oregon. A pressure chamber was used to monitor, in the field, the leaf water potential that developed during each of five days throughout the summer months. An infrared radiometer was utilized to monitor the temperatures of the leaves during the same periods. Sweet corn consistently developed the highest leaf water potential, reaching as high as 13.0 atmospheres. The tomatoes and beans developed leaf water potentials above 10.0 atmospheres. On all five days the corn plants developed leaf water potentials above the critical 8.0 to 9.0 atmospheres suction levels reported in the literature by other investigators. On four of the five days, the Ψl of the tomato and bean plants equalled or exceeded these critical levels although not to the same degree as the corn plants. The length of time that the leaf water potential remained above 8.0 atmospheres suction varied, but lasted up to 9 1/4 hours in the corn and up to 7 hours in the beans and tomatoes. During these periods photosynthesis formation, soluble nitrogenous and phosphorous compounds formation, RNA accumulation, carbohydrate synthesis, and cell growth are reportedly impaired, limiting productivity. Leaf temperature under these stress conditions followed closely the same general trends as ambient air temperature. High levels of leaf water potential did not necessarily cause the temperature of the leaf to rise significantly. Leaf temperatures exceeded ambient air temperature by a maximum of 3.8 °C, but were generally in close agreement with or below ambient air temperature. The results of this study indicated that many horticulture crops may be limited in productivity by fairly frequent periods of critical leaf water potential even…