fact, the leaves have sunken stomata in channels of the lower epidermis filled with wax and attached to
subsidiary cells resembling the crypt invagination (M. Centritto et al., unpublished).
The data set of Figure 2 shows that a clear specificity exists for the capacity of gsthat reflects sub-
stantially the adaptation of stomata and leaves to the water status of the different habitats. This is in agree-
ment with the interpretation that the dominant stimulus for adaptation is the prevention of water loss im-
pairment of growth and the relevant biophysical and biochemical processes [26].
B. Species-Specific Capacity for gm
Mesophyll conductance has the same order of magnitude of stomatal conductance, and reported values
range between 0.02 and 0.7 mol m^2 sec^1. Loreto et al. [8] showed an empirical relationship between
gsandgmwithgm1.4gs. However, mesophyllous plants (included in the hydrophyte and mesophyte
groups of Figure 2) apparently have a higher gmthan sclerophyllous trees when gsis similar. The rea-
son for this difference is unknown. Plants with hypostomatous leaves, thick mesophyll, or several lay-
ers of palisade cells, such as those showing xerophytic adaptations (see Figure 2), may have high in-
ternal resistances to CO 2 diffusion. Tree leaves, in fact, possess all of the described features, and the
lowgmobserved in these plants may be simply caused by the longer distance between substomatal sites
and Rubisco active sites compared with that of herbaceous leaves. Trees may also have a high density
of cells and, consequently, a low porosity in the mesophyll, which would make the CO 2 path toward
the chloroplasts more tortuous and difficult. Nobel [1] observed that the morphological parameter that
correlates better with mesophyll resistances is the ratio between mesophyll cell wall area and leaf area.
Following an early suggestion of Laisk et al. (27), Evans et al. [12] pointed out that diffusion resis-
tances increase when the chloroplast surface exposed to the intercellular spaces decreases while the to-
tal mesophyll surface exposed to intercellular spaces does not change. Sharkey et al. [28] noticed that
gmwas low in mutants of Nicotianacharacterized by cupped chloroplasts, a feature that made inho-
mogeneous chloroplasts adhere to cell walls and may have created further resistance to gas diffusion
while crossing the cytoplasm. Although results do not conclusively identify the factors involved in the
relationship between leaf anatomy and diffusion resistances, they suggest that a reduction of both pho-
tosynthesis and gmmay in fact be caused by anatomical changes related to chloroplast exposure to air
spaces and shape.
III. EFFECT OF LEAF RESISTANCES ON PHOTOSYNTHESIS
A. Effect of gson Photosynthesis
Mostly hydrophytes and xerophytes show a good proportionality between the capacity for gsand photo-
synthesis. In mesophytes, however, the relationship is not so clear as shown by the large variation of gs
in plants having similar photosynthesis (Figure 2). In addition, C 4 plants may have much higher photo-
synthesis than C 3 plants, but a comparable gs, because of the biochemical mechanism that concentrates
CO 2 to very high levels inside the mesophyll.
Thus, there are indications that gsmay limit photosynthesis. The limitation is clear at low gsand less
evident at high gsdespite the outlined differences in the leaf morphological characteristics. Wong et al.
[29] showed that the relationship between conductance and photosynthesis is due to the tendency of plants
to maintain a proportionality between the calculated internal and the measured external CO 2 molar frac-
tions. This observation, made on plants of several species grown under different light environments and
subjected to different nitrogen nutrition, was further investigated. Farquhar and Wong [30] explained that
this proportionality depends on the response of stomata to the pool size of a photosynthetic substrate. Sub-
sequently, Jarvis and Davies [31] held this concept in their model of the stomata response to photosyn-
thesis and developed the idea that stomata respond to “a signal in proportion to the degree to which the
photosynthetic capacity is realized.”
In Figure 3 the relationship between Aandgspassing through the origin is shown for three different
genotypes of sorghum grown in the field under irrigated and nonirrigated conditions. Similar results have
been reported for three genotypes of wheat under the same water conditions of sorghum [21]. In both
cases, it appears that the proportionality between Aandgsholds. Therefore, even when the leaf water
deficit reduces stomatal conductance, the stomatal limitations probably contribute to the overall reduction
330 MASSACCI AND LORETO