The rate of photosynthetic carbon fixation and of root and shoot xiv respiratory carbon use was measured in the laboratory and in the field (shoots only) for Agropyron desertorum (Fisch. ex Link) Schult. and Agropyron spicatum (Pursh) Scribn. and Smith. The rate of respiratory carbon use of the root system declined within hours of the shading or defoliation of the shoot system, resulting in as much as 60% reduction in specific rate of root respiration. The mean whole-plant growth efficiency (the ratio of whole-plant net carbon gain to gross photosynthetic carbon fixation) in full irradiance in the laboratory was 0.53 and was reduced both by shading and defoliation. The mean conversion efficiency was o. 70 and o. 73, and the mean maintenance coefficient 20°c was 10.8 and 9.9 mmol C mol C-1 d-1 for A. desertorum and A. spicatum, respectively. These maintenance coefficients are lower than previously reported for fast growing crop plants. The rate of respiratory carbon use and the dynamics of labile carbon compounds were simulated both for intact plants and for plants regrowing following defoliation. The partitioning of assimilates between root and shoot was explicitly modeled to make the separate simulation of root and shoot respiration possible . The simulated daily net mobilization of labile carbon compounds exceeded carbon input from photosynthesis for only the first one-to-two days of regrowth, depending on the severity of the defoliation. The instantaneous rate of respiratory carbon use of the shoot system in the field during short-term light exclusion during the day was higher than the rate at the same temperature during the subsequent night. The Qio of shoot respiration was estimated to be 2.1-2.2. The mean growth efficiency in the field for the shoots only was 0.65 for sunny days. This efficiency was higher than the whole-plant growth efficiency in the laboratory because root respiration was not measured in the field .

I quantified seasonal variations in corticular photosynthesis in 1st through 5th order branches of Myrica cerifera L. (Myricaceae) in order to determine whether corticular photosynthesis contributes to whole plant carbon gain by reducing respirational CO2 loss. Maximum % refixation was 110 ± 39 % of CO2 efflux in the dark (Rd) in 1st order branches during winter, minimum was 18 ± 3 % in 5th order branches during summer. Variations in % refixation paralleled changes in photosynthetically active radiation (PAR). As light attenuated with increasing branch order % refixation decreased. Increased PAR in the winter due to a more sparse canopy lead to increases in % refixation. Total chlorophyll content and chlorophyll a:b ratios were consistent with shade acclimation as branch order increased. Corticular photosynthesis may be a mechanism to enhance shrub expansion due to increased whole plant carbon use efficiency (CUE) and water use efficiency (WUE) attributed to refixation.