Great Basin Naturalist Memoirs


Six Larrea tridentata (Sesse & Moc. ex DC) Cov. plants were exposed to 14CO2 in a field experiment for 2 h. Three of the plants had been irrigated regularly in the preceding year. Ten small twigs from each plant were moved and counted for 14C activity at the end of 2 h. The stem portion of the twigs was of equal dry weight for the two sets of plants, but those irrigated had a greater weight of leaves per twig. The activity of 14C in leaves was equal for the two groups, but was higher in stems for watered plants than for unwatered plants. The results were best expressed as ratios. Dry weight of leaves dry weight of stems was high for watered plants; cpm/g dry weight leaves ÷ cpm/g dry weight of stems was higher for unwatered plants. In another experiment in which leaves were removed before exposing stem portions of twigs to 14CO2, small green stems accounted for about the total photosynthesis for a plant; the coefficient of variation was around 100 percent.