Great Basin Naturalist Memoirs


The distribution of photosynthate among plant parts subsequent to its production is needed to fully understand behavior of vegetation in any ecosystem. The present study, undertaken primarily to obtain information on transport of assimilates into roots of desert vegetation, was conducted in the northern Mojave Desert, where the mean annual rainfall is about 10 cm. Shoots of Ambrosia dumosa (A. Gray) Payne plants were exposed to 14CO2 in1971, and the distribution of 14C in roots, stems, and leaves was subsequently measured at 1 week, 2 months, and 5months. Only about 12 percent of the 14C photosynthate was stored in the root. Much of that stored in stems was available for new leaf growth. Photosynthate was labeled with 14C for 24 plants representing eight species in 1972. Results showed that after 127 days the mean percentage of 14C in roots as compared with the estimate of that originally fixed was 11.8; the percentage in stems was 43.8. The mean ratio of root to root plus stem for 14C was 0.212, but this value was only half that of the ratio for actual weights of these parts of field plants. The correlation coefficient for (14 < C in roots)/(14C in root + stem) × (dry wt of root)/ (dry wt of root + stem) was +0.89. Small stems were the major storage organ for the 14C. To check the validity of the 14C data, root growth of eight perennial desert plants grown in the glasshouse was followed as plants increased in size. The mean percent of the whole plant that was root for eight species was 17.7 percent. The mean proportion of the increase in plant weights that went below ground for the eight species was 19.5 percent. This value is higher than the fraction of 14C found below ground, and therefore the 14C technique underestimates the movement of C to roots. Results of an experiment designed to test the value of the 14C-pulse technique for determining current root growth for some perennial species from the desert indicated that the transition part of roots where root growth continued after exposure to 14C was highly labeled. Old growth contained less 14C than new growth.