of photosynthesis independently of drought sensitivity of the different species and genotypes. On the
other hand, changes in light exposure, inorganic nutrition, leaf age, and general acclimation or long-term
stress effects have been suggested to produce differences in the slope of the correlation between Aandgs
and, consequently, may modify the stomatal limitations of photosynthesis [17]. Finally, several examples
of a lack of correlation between Aandgscan be found in the literature, probably reflecting a lack of con-
trol of stomatal conductance on photosynthesis and the strong involvement of nonstomatal limitations
[32,33]. As an example, it has been shown [33] that the inhibition of adenosinetriphosphatase (ATPase)
is responsible for nonstomatal effects on photosynthesis in plants subjected to water stress.
B. Effect of gmon Photosynthesis
Because sclerophyllous plants also have low stomatal conductance (Figure 2), it has been suggested that
photosynthesis in these plants may be limited by the inherently high resistances to gas diffusion and by
the consequently low molar fraction of CO 2 in the chloroplasts. However, similarly to what has been ob-
served for gs(see Sec. III.A), this implies that photosynthesis should drop proportionally less than gm. Al-
thoughgmcorrelates fairly well with photosynthesis irrespective of leaf sclerophycity [8,34], the slope of
this relationship is not as steep in mesophyllous plants as in sclerophyllous plants. This is particularly ev-
ident when gmis higher than 0.4 mol m^2 sec^1 , when increments of photosynthesis are poorly coupled
with increments of gm(e.g., Figure 2) [16]. We also noticed that the correlation between gmand photo-
synthesis is loose when results obtained with herbaceous species are pooled together (data from Refs. 8
and 9 and S. Delfine et al. unpublished results). In mesophytic plants there seems to be a poor association
between photosynthesis and stomatal conductance as well. For these plants the cumulative resistance to
CO 2 diffusion offered by stomata and the mesophyll is low and probably unable to effectively limit pho-
tosynthesis. It should be noticed, however, that the relationship between gmand photosynthesis mirrors
that between gsand photosynthesis in xerophytic plants and even in aquatic species such as Carexand
Phragmites.When both conductances are reduced concurrently, restrictions on CO 2 diffusion inside the
leaves become particularly high. Thus, we conclude that gmmay contribute to limit photosynthesis only
when it decreases ccsignificantly. Under these conditions, photosynthesis may be progressively limited
DIFFUSIVE RESISTANCES TO CO 2 ENTRY 331
Figure 3 Relationship between photosynthesis (A) and stomatal conductance (gs) in leaves of sorghum geno-
types (, SV1; , Red Swazi; , 8504H) grown in the field near Rome under irrigated (filled) and nonirrigated
(open) conditions and measured before anthesis at different times of the day.