B. Location of Mesophyll Resistance
Stomatal resistance is attributable only to stomatal movements; it is commonly believed that the resis-
tances encountered by diffusion in the mesophyll are shared between gas phase and liquid phase, but there
is no consensus about the relevance of each component. On the one hand, it has been found that decreas-
ing the gas phase resistances by partially substituting helium for the air sometimes stimulates photosyn-
thesis. This would indicate that the gas phase resistances to diffusion are substantial [14]. On the other
hand, Loreto et al. [8] found only a limited association between mesophyll resistances and porosity. More-
over, it has been pointed out that CO 2 diffusion in liquid is 10,000 times less than in air [7]. Although it
is likely that the path length in the liquid phase is short [15], liquid phase resistances may also be consid-
erable. Parkhurst [7] concluded on the basis of porosity and path length that the two resistances may be
similar, but gas phase resistances may be prevalent when the mesophyll cells are tightly packed and CO 2
entry in the leaf is structurally limited by stomatal distribution (i.e., more in hypostomatous than in am-
phistomatous leaves). However, this viewpoint has been challenged by Syvertsen et al. [11]. Their diffu-
sional model predicts that diffusion resistances are higher in the liquid phase than in the gas phase in hy-
postomatous tree species.
In summary, all methods used to estimate the resistances to diffusion within the leaf do not distin-
guish between resistances encountered at the cell wall, plasmalemma, cytoplasm, or chloroplast mem-
brane. They are also ineffective in partitioning between gas and liquid phase resistances. Because of this,
we will denote the series of resistances between intercellular air spaces and Rubisco sites by the generic
term mesophyll resistance (rm), which includes both gas and liquid phase resistances. These resistances
are directly proportional to the gradient of carbon molar fraction and inversely proportional to the flux
density of carbon (the photosynthetic rate A):
rm(cicc)/A (2)The total diffusion resistance (rtot) is the sum of the stomatal and mesophyll resistances. Because the dif-
fusion resistances are expressed in term of flux, it is frequently convenient to use their inverses, i.e., con-
ductances. By knowing the stomatal (gs) and mesophyll (gm) conductances, the total conductance to CO 2
can be calculated:
gtot1/gs1/gm (3)We will not address the biophysics of the diffusion in the leaf, which has already been extensively
described by Parkhurst [7]. Comprehensive reviews of the methods used and of their accuracy have also
328 MASSACCI AND LORETOFigure 1 Representation of the resistances to CO 2 diffusion within a leaf. Resistances caused by diffusion
through stomata lower the CO 2 molar fraction from ambient (ca) to intercellular (ci). Resistances encountered
in the mesophyll lower the CO 2 molar fraction from cito chloroplastic CO 2 (cc). Stomatal and mesophyll con-
ductances can be calculated when CO 2 fluxes and molar fraction gradients are known. The CO 2 fraction that
reaches Rubisco in the chloroplasts drives the photosynthetic process.