Seasonal trends of mesophyll conductance and its importance in understanding photosynthesis in a California Mediterranean oak savanna

Background/Question/Methods Recently, much literature has addressed the process of mesophyll conductance (gm), the conductance of CO2 from the intercellular airspaces to the site of carboxylation, and shown that it is not infinite as previously assumed. Drought and temperature are two of the factors that have been shown to decrease gm (Flexas et al. 2008). These two factors are also defining characteristics of Mediterranean savannas during the summer, the period of foliation. Thus, we proposed that mesophyll conductance would be a large contributor in limiting photosynthesis. We set out to measure gm by a variety of methods as well as maximum photosynthetic rates (Amax), photosynthetic capacity (Vcmax), stomatal conductance (gs), and other photosynthetic parameters. Our goal was to understand how gm changes throughout the period of foliation and how these changes contribute to the changes of photosynthesis. Measurements were made on Quercus douglasii throughout 3 years using a combined gas-exchange and fluorescence system (LiCor 6400-40, Lincoln Nebraska). All work was done in situ at the Tonzi eddy-flux site just outside of Ione, CA. Measurements of gm were made using the ‘variable J' method (DiMarco et al. 1990) and the ‘curvature' method (Ethier and Livingston 2004).

Results/Conclusions Measured values of mesophyll conductance ranged from nearly 0 to 0.26 mol CO2 m-2 s-1. Stomatal conductance ranged from nearly 0 to 0.16 mol CO2 m-2 s-1. The maximum photosynthetic capacity achieved was 116 μmol CO2 m-2 s-2 (as calculated from A-Ci curves based on analysis method of Ethier and Livingston and utilizing field measurements of gm). Photosynthetic capacity, stomatal conductance, and mesophyll conductance had similar trends throughout the spring and summer although the trend was much more defined in Vcmax. . Values peaked in the late spring/early summer months while PAR was increasing, water was still available, and temperatures were ideal. In Vcmax, the peak period was very brief, lasting as few as 10 days. During the summer drought, values were stable and very low, barely one-third of the peak value for Vcmax. Mesophyll and stomatal conductance were positively linearly related, however the slope varied with year. Vcmax and Amax were also linearly related to gm but with a consistent slope. Mesophyll conductance limited photosynthesis nearly as much as stomatal conductance during most of the period of foliation and thus is a factor that cannot be ignored when considering the seasonal trends of photosynthesis in a Mediterranean Oak savanna.