Great Basin Naturalist Memoirs


Ambrosia dumosa (A. Gray) Payne cuttings grown in solution culture were exposed to 14CO2 to measure the distribution of labeled photosynthate among leaves, stems, and roots after 4, 24, and 48 h. For all sampling periods, the highest levels of 14C were found in leaves and the lowest in roots; however, considerable 14C had moved to roots in 48 h. In a 12-week study of A. dumosa in solution culture, plants increased in size more than 17 times and flowered and produced seeds. The plants had received 14CO2 in photosynthesis at the start. The gradual loss of 14C from the plants in the 12 weeks averaged 3.5 percent per week (coefficient of variation = 58 percent). This represents an average respiration rate of 0.21 mg C g dry weight-1 h-1. This compares favorably with other means for determining respiration rate. The percentage of 14C in the root portion of the plant varied little over 6 sampling periods, indicating that essentially none of the initially fixed 14C left the roots during the 12 weeks of test. The 14C entering fruits and seeds came from leaves only. The biomass of fruit parts resulted more from new photosynthate than from retranslocation from leaves. In a study in which A. dumosa plants were defoliated, little 14C moved from roots to new shoot growth.