Supply of C02 Through Stomata
The relationships described above may only be applied if the supply of C02 to the chloroplast (Cc) is known. The C02 supply in the leaf is a diffusion process, limited by stomatal conductance (g). Equation (2.4.8) may also be expressed as diffusion-limited C02 assimilation, A
where pCa is the C02 partial pressure in the air which surrounds the leaf and P is the air pressure. It should be noted that the C02 concentration at the leaf surface also depends on the boundary layer conditions (Chap. 2.2.4).
The calculation of photosynthesis from the model requires simultaneous solution of Eqs. (2.4.8) and (2.4.17). Unfortunately, there is no biochemically based model for stomatal responses corresponding to carboxylation. To solve Eqs. (2.4.8) and (2.4,17) a set of equations is produced which determines the reaction of stomata empirically and this is coupled to the photosynthesis model. In this model g is a function of air humidity, temperature, light, soil water, atmospheric C02 and other growth factors, insofar as they influence stomata (Jarvis 1976; Leuning 1995; Lloyd et al. 1995b; Arneth et al. 2002).
Although the original equations by Farquhar et al. (1980) were based solely on cellular processes, this model may also be applied to whole leaves, whole plants and whole ecosystems
(Lloyd et al. 1995 a; Leuning et al. 1998; Arneth et al. 1999; de Pury and Farquhar 1999). Depending on the question, the successful use of the model requires not only information on Vmax and Jmax> but also knowledge about the state of nutrition and water supply from the soil.
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