Leaves carbonic anhydrase

Carbonic anhydrase, in turn, is located predominantly in the chloroplasts, and provides therefore a reasonable estimate of the specific 6 0 of water fraction with which CO2 equilibrate in the leaves, and 5i used in Equation (43) will simply be 5i = 5l, - -Seq, where Seq is the water-CO2 equilibrium fractionation. In difihising out of the leaf, a dififusional fractionation, sj must also be considered, such that the final 5 0 value of the CO2 reaching the atmosphere is 5i - - si. Note that considerable uncertainty is still associated with estimating the 5 0 value of leaf water on large scale and current estimates ranged between 4.4%o and 8.8%o (Bender etfll., 1985,1994 Dongmann, 1974 Forstel, 1978). 

Estimates of A using Equation (48) rely on the assumption of complete isotopic equilibrium between CO2 and water in the leaf chloroplasts, due to the action of carbonic anhydrase. This assumption has recently been tested, and revised (GUlon and Yakir, 2001, 2000a,b). The extent of CO2-H2O equilibrium may be derived from Mills and Urey (1940) as... 

Thus, there may be a requirement for this low amount of chloroplast carbonic anhydrase in photoautotrophic cell suspension cultures. Whether the enzyme is directly involved in supplying CO2 to ribulosebisphosphate carboxylase is unclear. The higher levels of carbonic anhydrase found in leaves may be required to deal with transitory changes not found under the growth conditions of the cells, or may aid in CO2 diffusion in the more complex morphology of the leaf. 

Table 1 Carbonic Anhydrase activity in leaf extracts of barley after treatment with Abscisic acid and Jasmonic acid.

The C4 pathway in the plants of group (i) (NADP-malic enzyme eq. 1) is shown in the Fig. Air passes through the open stomata of the leaf into the extensive intercellular spaces and bathes the mesophyll cells. COj passes into the cytoplasm of these cells where it dissolves and ionizes the latter process is probably facilitated by carbonic anhydrase (EC... 

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