PHOTOSYNTHESIS AND CARBOHYDRATE PARTITIONING FOR THE C-3 DESERT SHRUBENCELIA-FARINOSA UNDER CURRENT AND DOUBLED CO2 CONCENTRATIONS

Changes in photosynthesis (A) and carbohydrate partitioning were studi ed for Encelia farinosa, a common C-3 desert shrub in the southwestern United States, after a 3-month exposure to the current or a doubled C O2 concentration (750 mu L L(-1)). A remained unchanged under the curr ent CO2 concentration but decreased during the day under the doubled C O2 concentration, resulting in a 46% enhancement in the early morning, 26% at midday, and 15% in the late afternoon by the elevated CO2. The decrease during the day under the doubled CO2 concentration may repre sent end-product inhibition, because the sucrose and the starch conten ts increased during the day proportionally more than under the current CO2 concentration. The (CO2)-C-14 activity in sink leaves was maximal 3 h after labeling under the doubled and at 5 h under the current CO2 concentration, indicating faster movement of photosynthate out of sou rce leaves and into sink tissues under the doubled CO2 concentration, which may have been responsible for the sustained enhancement in A und er the doubled CO2 concentration.