An increased ambient CO 2 molar fraction might be considered a particular stress factor for stomata.
Besides, the interaction of a CO 2 increase in the atmosphere with increased air temperature is well docu-
mented. The effect per se of an increase of CO 2 is a reduction of stomata opening. The mechanism un-
derlying this response is not fully understood, and we invite readers to consult specialized papers dealing
with this basic topic [46]. However, this reduction of gsis very unlikely to be solely responsible for the
limitation of photosynthesis that certain species show under high CO 2 , particularly because the feedback
limitation mechanism for sugar accumulation also seems to make a significant contribution under these
conditions [47].
B. Effect of Environmental Stresses on gm
Almost all of the currently published studies assessed gmin leaves grown and exposed to nonstressful con-
ditions. Growth under environmental constraints that cause leaf anatomy changes probably cause changes
in mesophyll resistances as well, as a few studies seem to indicate (Figure 4).
The effect of a saline environment on the growth of olive leaves is an increase of leaf thickness and
doubling of the palisade layers, associated with a decrease of gm[5]. A reduction of gmwas also noticed
after growing spinach with saline water [48,49]. In this case, gmdecreased from a value typical of meso-
phyllous leaves (about 0.50 mol m^2 sec^1 ). However, no increase in leaf thickness was noticed in
spinach leaves, but the spongy cells became more appressed and the intercellular spaces were signifi-
cantly reduced with respect to control leaves. The authors, therefore, suggested that gmis dependent on
leaf porosity and that a low porosity increases the tortuosity of the path leading to chloroplasts. Delfine
et al. [49] also demonstrated that the reduction of gmunder salinity could be partially reversed if the in-
ternal salt concentration was lowered by irrigation with salt-free water. This is the only report indicating
that reduction of mesophyll conductance under stress conditions is not a permanent feature.
An association between photosynthesis and gmwas also found in leaves of spinach and chestnut ex-
posed to increasing water stress [50,51]. On the contrary, we did not notice any change in the gmofRosa
leaves exposed to rapid dehydration (B. Genty, S. Meyer, E. Brugnoli, F. Loreto, unpublished). The last
experiment was done by using video images of chlorophyll fluorescence. This technique can map and
quantify heterogeneities in stomatal closure and photosynthetic activity of leaves [52] that frequently oc-
cur in stressed leaves and cause errors in ci,cc, and gmcalculations [53]. These results suggest that if the
DIFFUSIVE RESISTANCES TO CO 2 ENTRY 333
Figure 4 Reduction of photosynthesis (white bars) and total conductance as partitioned in mesophyll (dark
bars) and stomatal (gray bars) components following environmental stresses or in aging leaves. Data from Refs.
9, 48, and 49 are reported.