Liquid Phase Conductance

Liquid phase component of mesophyll conductance is quite complex depending on many leaf anatomical traits such as mesophyll cell surface area, chloroplast surface area exposed to intercellular air spaces (Evans and Loreto 2000) , chloro-plast rearrangements, and cell wall thickness (Terashima et al. 2006). However, the CO2 diffusion coefficient in the liquid phase is suggested to be much slower compared to that in intercellular air spaces, indicating significant lower gliq values (Morison and Lawson 2007) . Actually, a reevaluation of gias using the comparison of dif-fusivities in air and helox, revealed that gliq was much smaller than gias, and therefore it consists the most limiting factor for photosynthesis (Piel et al. 2002) . Furthermore, Gorton et al. )2003) using a photoacoustic technique, also concluded that low gliq as the most limiting factor for CO2 diffusion in the mesophyll. Gillon and Yakir (2000) combining 13C and 18O discrimination by leaves were able to partition gm into the two components, gias and a cellular component that was termed chloroplast conductance (gchl) instead of gliq (Fig. 2.6). As chloroplasts are usually tightly coupled to cell membranes facing intercellular air spaces, it was assumed that CO2 would not have to cross the cytosol, entering the chloroplasts directly after crossing the cell wall and plasma and chloroplast membranes. These authors showed that gias was lower than gchl in thick leaves of oaks, but gchl was lower than gias in the mesophytic leaves of soybean and tobacco. In contrast, Piel et al. (2002) showed that even in the sclerophyll oak Quercus ilex rliq (the inverse of gliq) contributed to 70% of the total internal resistance, while rias (the inverse of gias) contributed the remaining 30%. Therefore, gliq (or gchl) seems to be the most important component of gm not only in mesophytic species but probably also in sclerophylls.

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