Photosynthetic down-regulation in Larrea tridentata exposed to elevated atmospheric CO2: interaction with drought under glasshouse and field (FACE) exposure

The photosynthetic response of Larrea tridentata Cav,, an evergreen Mojave Desert shrub, to elevated atmospheric CO2 and drought was examined to assis t in the understanding of how plants from water-limited ecosystems will res pond to rising CO2, We hypothesized that photosynthetic down-regulation wou ld disappear during periods of water Limitation, and would, therefore, like ly be a seasonally transient event. To test this we measured photosynthetic , water relations and fluorescence responses during periods of increased an d decreased mater availability in two different treatment implementations: (1) from seedlings exposed to 360, 550, and 700 mu mol mol(-1) CO2 in a gla sshouse; and (2) from intact adults exposed to 360 and 550 mu mol mol(-1) C O2 at the Nevada Desert FACE (Free Air CO2 Enrichment) Facility. FACE and g lasshouse wed-watered Larrea significantly down-regulated photosynthesis at elevated CO2, reducing maximum photosynthetic rate (A(max)), carboxylation efficiency (CE), and Rubisco catalytic sites, whereas droughted Larrea sho wed a differing response depending on treatment technique. A(max) and CE we re lower in droughted Larrea compared with well-watered plants, and CO2 had no effect on these reduced photosynthetic parameters. However, Rubisco cat alytic sites decreased in droughted Larrea at elevated CO2. Operating C-i i ncreased at elevated CO2 in droughted plants, resulting in greater photosyn thetic rates at elevated CO2 as compared with ambient CO2. In well-watered plants, the changes in operating C-i, CE andA(max) resulted in similar phot osynthetic rates across CO2 treatments. Our results suggest that drought ca n diminish photosynthetic down-regulation to elevated CO2 in Larrea, result ing in seasonally transient patterns of enhanced carbon gain. These results suggest that water status may ultimately control the photosynthetic respon se of desert systems to rising CO2.