Article Title

Water Relations of an Annual Grass, Bromus diandrus, in the Central Valley of California





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Annual grasses dominate uncultivated land in the Central Valley of California. The Mediterranean climate constrains growth of grasses to winter and early spring when most precipitation occurs and temperatures are moderate. Water is the key limiting resource, yet its dynamics in the annual grasses is not well understood. We determined diurnal and seasonal patterns in water potential and relative water content (RWC) in Bromus diandrus, evaluated the degree of coupling between water potential and RWC and also tracked growth of the plants. The work was conducted in a 8.0 ha exclosure which has had minimal disturbance since its establishment in 1974. Predawn water potential remained mostly un- changed and high through about 80% of the growing season. Midday water po- tential was consistently lower than at predawn and showed greater fluctuation. Relative water content declined to low values of 73% and 56% at predawn and midday, respectively. This condition was reversible since plants were able to recover to higher RWC levels following rainfall. Predawn coupling of water po- tential and RWC was variable. The two were tightly coupled early and late in the growing season but diverged during the middle of the season. Alternatively, water potential and RWC were tightly coupled throughout the growing season at midday. Growth followed a sigmoid function Y = 0.00923 + 0.00232x - 0.0000985x 2 + 0.00000 193x 3 (r 2 = 0.95). We conclude that the growth of Bromus diandrus is linked to water availability but that there is a predetermined time span for the life cycle. The end of the growing season then, is likely more a function of the onset of germination in the fall/winter than a consequence of the water environ- ment in the spring.