Influence of Soil Water Potential and Environment on the Internal Water Status of Grasses

The water relations of two grass species, smooth brome (Bromus inermis Leyss.) and intermediate wheatgrass (Agropyron intermedium (Host) Beauv.), were studied under a range of environmental conditions in a growth chamber. The environmental conditions included three temperature regimes (day/night temperatures of 15/10, 20/15, and 25/20 degrees centigrade), and three vapor pressure deficits within each temperature regime (6.2, 8.8, and 11.4 millimeters of mercury). Short wave radiation was maintained at 0.5 calories per square centimeter per minute with a 14 hour photoperiod, and wind speed was constant at 45 centimeters per second. Six plants of each species were studied simultaneously under each of the nine environments, and each set of conditions was replicated once. The plants were subjected to a single severe wilting cycle, usually requiring about 4 days. Just prior to when the lights came on and the temperature increased each morning, the soil water potential, leaf water potential, and leaf osmotic potential (of frozen-and-thawed tissue samples) were measured for each plant using Peltier thermocouple psychrometers. At this time of the day the water relations of the plant are the most favorable, and measurements taken then will reflect a base-line condition under which the plant can potentially recover from daytime extremes. Leaf pressure potential was calculated as the difference between leaf water and osmotic potentials. One hour after the lights came on and the temperature increased, leaf resistance was measured with a diffusion porometer, and leaf temperature was measured with a Barnes infrared radiometer. Leaf water potential, osmotic potential, and soil water potential all decreased progressively each day under continued water potential all decreased progressively each day under continued water stress in both species, while leaf resistance and leaf temperatures increased. Generally, leaf and soil water potentials decreased more rapidly and to a lower extreme under the warmer environments than under the cooler ones. Soil water potentials remained about 10 bars higher than leaf water potentials throughout the entire wilting cycle, as measured one hour before the lights came on, with no significant differences between species. Leaf resistance increased as leaf water potential decreased, becoming asymptotically higher as the leaf tissues became drier, with no significant differences between species. A critical leaf water potential was not identified at which leaf resistance increased sharply, perhaps because these factors were determined only once daily. Leaf temperatures also increased as leaf water potential decreased in both species. The differences between leaf and air temperatures were greater at low leaf water potentials under the cooler temperatures were greater at low leaf water potentials under the cooler temperatures. For both species under all conditions, the difference between leaf and air temperatures increased steeply at the higher leaf water potentials, but tended to increase less steeply at the higher leaf water potentials, but tended to increase less steeply at low leaf water potentials. The relationship between leaf pressure potential and leaf water potential is believed to be of considerable importance. As the plant progressively dries out, this relationship shows that the pressure potential declined from an initial high of about 10 bars, eventually reaching zero, and then becoming negative. With continued drought, the pressure potential became progressively more negative, ultimately reaching a minimum, and then increased toward zero bar again. This J shaped relationship appears to describe the wilting process in plants. The leaf water potential at which pressure potential first reaches zero was termed the physiological wilting point , and appears to provide a quantitative measure of incipient wilting. It is proposed that when the minimum pressure potential is reached, individual cells begin to collapse as air penetrates between the cell wall and the plasma lemma. The leaf water potential at which this occurs was termed the point of protoplast collapse , and is believed to represent permanent cell damage. It is believed that when the pressure potential reaches zero the second time, a condition of tissue death exists wherein virtually all of the cells have collapsed. The results of this study provide important information concerning the wilting phenomena in plants, and illustrate the possible mechanisms by which it occurs. The use of these techniques and relationships provide quantitative criteria by which the water relations of different plant species can be evaluated. Also, they suggest some potentially important ecological implications regarding the adaptability of plants to arid environments