PARENT SESSION
Poster Session #28: Gas Exchange.
Tuesday, August 6. Presentation from 5:00 PM to 6:30 PM. Exhibit Hall B & C, TCC

Annual variation in the carboxylation efficiency of Larrea tridentata .

Naumburg, Elke^*,1, ¹ University of Nevada, Las Vegas & Desert Research Institute, Las Vegas, NV

ABSTRACT- Early studies of Larrea tridentata, an evergreen shrub of North American warm deserts, have established that this species can be photosynthetically active under a wide range of environmental conditions - resulting in positive carbon gain essentially year-round. High tolerance to water stress is one of the reasons behind this observation and has been studied extensively. Here I present data on seasonal adjustments in the photosynthetic apparatus, which has received much less attention. Five shrubs were measured with gas exchange techniques on 8 dates throughout one year to determine the carboxylation efficiency of Rubisco (V_cmax). For each date, photosynthetic CO₂ response curves were conducted at 3-6 measurement temperatures that were representative of actual daytime temperatures. Tissue samples were further analyzed for their nitrogen content. V_cmax was related to nitrogen content only during summer, when some shrubs showed a decrease in %N relative to the rest of the year. V_cmax measured at a common temperature was positively correlated with growth temperature. Integrated into a multiple regression model, however, growth temperature explained only 2% of the total variation in V_cmax suggesting only small seasonal adjustments in the photosynthetic apparatus of Larrea. During summer, a broad temperature optimum of net photosynthesis itself facilitates maximal photosynthesis under sufficient water availability. However, summer-time decreases in nitrogen content that were coupled with lower stomatal conductances suggest the potential for a coordinated down-regulation of the photosynthetic apparatus under increased water stress.

KEY WORDS: seasonal photosynthesis, desert, Larrea tridentata, nitrogen