appeared [16]. This chapter will focus on the ecological significance of diffusion resistances, including
the species-specific capacity for diffusion of gases in trees and herbaceous plants, and the effect of envi-
ronmental stresses, leaf ontogeny, and increasing CO 2 concentration on diffusion resistances. In conclu-
sion, the photosynthesis limitations caused by diffusion resistances are investigated.
II. SPECIES-SPECIFIC LEAF RESISTANCES
A. Species-Specific Capacity for Stomatal Conductance
The capacity for leaf gsoften varies significantly between different species, within leaves of genotypes of
the same species, and between leaves of the same plant depending on leaf age and insertion [17]. This
large variation may be attributed to the leaf morphological characteristics, which, in turn, are mainly con-
trolled by the leaf water status. Stomatal conductances shown in Figure 2 have been selected from mea-
surements on plants under nonlimiting conditions (see figure legend) for each species and have been pri-
marily grouped according to a general classification as hydrophytes, mesophytes, and xerophytes.
Among hydrophytes, we take as an example Phragmites australisandCarexspp., Graminaceae
species from wetland habitats. Leaves of these plants are broad and flat and have many invaginations of
the upper epidermis, facilitating gas exchanges. Stomata are mostly in the lower epidermis and bulliform
cells are present in the upper epidermis. The mesophyll tissue is highly lacunose [18,19]. The gsof hy-
drophytes on average is 0.20–0.30 mol m^2 sec^1.
The mesophyte group includes many species typical of land habitats with no severe moisture and
temperature stresses. It includes woody trees and grass species. The latter are further distinct as C 3 and
C 4 species. Despite the differences in biochemistry and anatomy, there is no significant difference in gs
between these subgroups. Stomatal conductance is, for instance, around 0.2 mol m^2 sec^1 in the woody
mesophytePrunus avium[20], in C 3 herbaceous species such as wheat [21], and in C 4 leaves such as those
of sorghum [22]. Leaves of both these C 3 and C 4 herbaceous species are amphistomatous, but C 4 plants
present a special bundle sheath surrounding the veins containing large, conspicuous chloroplasts and from
which the mesophyll radiates [23].
The last group, the xerophytes, includes many plant species (particularly trees) typical of arid or
semiarid environments. These species have a very thick cuticle, epidermis, and palisade. The lower epi-
dermis layer often creates an invagination that contains many trichomes as well as all of the stomata [24].
Leaves of these plants have low gsas in the case of the Mediterranean sclerophyllous tree Quercus ilex
(0.1–0.15 mol m^2 sec^1 ) [8] and of the boreal tree Larix x eurolepsis(0.1 mol m^2 sec^1 ) [25]. Larix
leaves are characteristic because they show xerophytic features despite vegetating in a cold habitat. In
DIFFUSIVE RESISTANCES TO CO 2 ENTRY 329
Figure 2 Relationship between photosynthesis and stomatal (gs) and mesophyll (gm) conductances in differ-
ent groups of plants. Data for hydrophytes (Carexspp. and Phragmitesspp.) are unpublished. Data for C3 and
C4 mesophytes are from Refs. 9, 12, 13, 21, 48 and 4, 22, 39, respectively. Data for xerophytes are from Refs.
5, 6, 10, 19, 25, 50.