15
Diffusive Resistances to CO 2 Entry in the Leaves
and Their Limitations to Photosynthesis
Angelo Massacci and Francesco Loreto
National Research Council of Italy, Rome, Italy
327I. LEAF RESISTANCES TO CO 2 DIFFUSION
A. Definitions
Driven by a gradient between CO 2 molar fraction in the air and at the active sites of the ribulose-1,5-
bisphosphate carboxylase (Rubisco) in the chloroplasts, CO 2 diffuses through leaf stomata in the inter-
cellular air spaces present in the mesophyll, then crosses the cell wall and plasmalemma and the chloro-
plast membrane, and finally reaches the chloroplasts. Each of these steps (Figure 1) constitutes a
physical resistance to CO 2 diffusion that progressively reduces the CO 2 molar fraction [1]. The drop of
CO 2 caused by the stomatal resistance has been calculated from gas exchange parameters exploiting the
fact that CO 2 and water vapor share the same diffusion pathway to and from the substomatal cavities
(for a review see Ref. 2). The molar fraction of CO 2 in the intercellular spaces (ci) can, therefore, be
estimated as follows:
cicaArs (1)wherecais the ambient CO 2 molar fraction, Ais the photosynthetic rate, and rsis the stomatal
resistance.
The drop in CO 2 between the intercellular air spaces and the Rubisco active sites is not paralleled by
the flux of water and is more difficult to calculate. However, the balance between photosynthesis and pho-
torespiration and the correct estimation of the catalytic properties of Rubisco through gas exchange are
based upon as reliable as possible calculations of the chloroplastic CO 2 molar fraction (cc).
Photosynthesis has been modeled assuming that ciis the same as cc[3]. Alternatively, it has been as-
sumed that mesophyll resistance reduces ccto the compensation point between photosynthesis and pho-
torespiration [4].
Three methods have been used to estimate in vivo the further resistances between the intercellular
spaces and the chloroplasts. These studies have indicated that a further drop of CO 2 molar fraction into
the leaf may be significant and is attributable to what has been called wall resistance [5,6], liquid phase
resistance [7], mesophyll resistance [8,9], internal resistance [10,11], CO 2 transfer resistance [12], or mes-
ophyll diffusional resistance [13].