stress is rapid it cannot change the gmof leaves. On the contrary, slowly developing stresses, which are
more common in nature, cause a wide range of changes in leaf anatomy that, in turn, seem to affect gas
diffusion resistances. However, no conclusive indication of the stress effect on gmcan be inferred until
heterogeneities of stomatal opening are quantitatively assessed because methods used to estimate gmare
sensitive to this error.
Aging is another factor likely to affect leaf anatomy; consequently, gmwas expected to change in
ageing leaves (Figure 4). Mesophyll conductance of wheat leaves also decreased dramatically during leaf
senescence and reached a value (0.15 mol m^2 sec^1 ) typical of trees and sclerophyllous plants [9], but
the morphological parameter associated with this reduction was unclear.
Reduction of photosynthetic capacity in aging leaves may be related to low gm. However, age also
causes a reduction of leaf nitrogen [12] and of the amount of Rubisco [9]. Therefore, it is difficult to par-
tition between age-related metabolic and diffusive limitation of photosynthesis. Intuitively, if the reduc-
tion of gmwere as strong and immediate as that shown by aging wheat leaves, it might significantly con-
tribute to the age-related reduction of photosynthesis. But we have also found that the reduction of
photosynthesis in aging leaves of Quercus ilexL., a tree species characterized by a constitutively low gm
[8], is caused by reduced activity of Rubisco rather than by further reduction of gm, as previously specu-
lated by Di Marco et al. [53, and unpublished results]. Therefore, the available information indicates that
leaf age is likely to affect gmonly in mesophyllous leaves characterized by constitutively low diffusive
resistances.
V. CONCLUDING REMARKS
Knowledge of the variation of the cumulative diffusive resistances to CO 2 entry in leaves of different
species enables us to understand how the plasticity of leaf characteristics to the environment contributes
to the photosynthetic performances in contrasting habitats. This knowledge is essential to accomplish the
goals of modern ecophysiology applied to agriculture: to widen the geographical distribution of valuable
plants, to optimize plant phenotypes in response to predictable climate changes, and to achieve stable crop
yields in areas exposed to environmental constraints. We have presented results indicating the likely ef-
fects of intrinsic (plant species, age) and environmental (water availability, salt stress, suboptimal tem-
peratures, CO 2 changes) factors on the two main components of diffusive resistances to CO 2 : stomatal
and mesophyllous. We have shown a generally clear association between these two resistances and have
highlighted the possible combined effect of both resistances in limiting carbon uptake and photosynthe-
sis, particularly under stress conditions. These conclusions may be of use for modeling photosynthesis
and predicting plant growth in a changing environment.
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334 MASSACCI AND LORETO