Source: Brown & Hattersley (1989).

increasing atmospheric CO2 concentrations the irra-diance at which light saturation is reached shifts to higher levels also in C3 plants.

The high concentration of CO2 in the vascular bundle sheath of C4-plants, the site of Rubisco, allows different kinetic properties of Rubisco. Table 9 shows that indeed the Km(CO2) of Rubisco from terrestrial C3 plants is lower than that from C4 plants. A high Km, that is a low affinity, for CO2 of Rubisco is not a disadvantage for the photosynthesis of C4 plants, considering the high Cc in de the bundle sheath. For C3 plants a low Km for CO2 is vital, since the Ci is far from saturating for Rubisco in their mesophyll cells. The advantage of the high Km of the C4 Rubisco is thought to be indirect in that it allows a high maximum rate per unit protein of the enzyme (Vmax or kcat). That is, the tighter CO2 is bound to Rubisco, the longer it takes for the carboxylation to be completed. In C3 plants, a high affinity is essential, so that kcat cannot be high. C4 plants, which do not require a high affinity, do indeed have an enzyme with a high kcat, allowing more moles of CO2 to be fixed per unit Rubisco and time at the high Cc (Table 10). Interestingly, the alga Chlamydo-monas reinhardtii, which has a CO2-concentrating mechanism (Sect. 11.3), also has a Rubisco enzyme with a high Km (low affinity) for CO2 and a high Vmax and kcat (Table 10). Apparently, there is a trade

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