by the competition between the high concentration of O 2 and the low concentration of CO 2 , which, in turn,
favor photorespiration [see Eq. (2) for further comments].
To make the chloroplast environment favorable to CO 2 fixation, the gas composition can be changed
by either increasing the CO 2 or decreasing the O 2 molar fraction. It is conceivable that species character-
ized by low gsand low gm, such as trees and sclerophyllous plants, will greatly benefit from exposure to
CO 2 molar fractions higher than ambient. It has been demonstrated that although the gmofQuercus ilex
andCitrus aurantiumdoes not change with increasing CO 2 molar fraction, photosynthesis of these scle-
rophyllous plants is by far more sensitive to CO 2 than photosynthesis of herbaceous plants [8]. Thus, the
photosynthetic capacity of trees may exceed that of herbaceous plants, but this does not result in higher
photosynthesis at ambient CO 2 because of the low amount of substratum reaching the chloroplasts. If this
is true, trees may have an evolutionary advantage over other plants during the current trend of atmospheric
CO 2 increase. However, it should be pointed out that such an advantage may be lost whether leaf photo-
synthesis is limited by end-product removal in plants grown at high CO 2 [35] or whether other factors be-
gin to be limiting for plant growth. For instance, fast growth could lead to depletion of the soil content of
N and to early competition for light caused by canopy closure.
IV. EFFECT OF ENVIRONMENTAL STRESSES ON LEAF
RESISTANCES AND PHOTOSYNTHESISA. Effect of Environmental Stresses on gs
Under high evaporative atmospheric demand, stomata closure minimizes leaf water loss to avoid imme-
diate restrictive effects on the rates of biochemical and biophysical processes. At the same time, an ade-
quate supply of CO 2 through the stomata to the carboxylation sites is required for optimal performance of
photosynthesis when energetic resources and nutrients are not limiting. These two contrasting stimuli af-
fect the resulting degree of stomatal opening when the leaf is not water stressed. Indeed, Jones [36] sug-
gested that the actual dominant stimulus that drives stomatal movements is that to prevent irreversible
damage to leaves. It has been shown that this stimulus is amplified when leaves are just mildly water
stressed and thus photosynthesis becomes limited by the supply of CO 2 to the carboxylation sites if other
factors are not concurrently limiting [26,37]. Cornic and Massacci [38] have reviewed results showing
that when leaf water deficit is induced slowly, the photosynthetic apparatus becomes very resistant to
drought and limitation to photosynthesis might be completely attributed to resistances to CO 2 diffusion
inside the leaves, primarily stomata closure but also mesophyll resistances (see Sec. IV.B and Figure 4).
Results of Figure 3 for sorghum and of Di Marco et al. [21] for wheat represent slow field development
of water stress that does not alter the proportionality between changes in Aandgs. Recently, Meyer and
Genty [39] showed that also under severe water stress rapidly developed the reduction of stomatal con-
ductance is the main limitation of photosynthesis and the main cause of heterogeneous stomatal closure
inrosa rubiginosaL. Slowly developing salt stress apparently mimics water stress because their effect is
a coordinate reduction of photosynthesis and both stomatal and mesophyll conductances to CO 2 diffusion
(Figure 4).
The interaction of temperature stress with gsandAis much more complex than that attributable to
water deficit and salt stress. In fact, changes of a few degrees Celsius have a great effect on evapotran-
spiration and thus on gsthrough a hydraulic feedback [40]. Besides, if such changes are fast, they may un-
evenly alter the water status of some stomata areoles and induce heterogeneous variations in their degree
of closure. This may lead to underestimation of the actual rate of photosynthesis and evapotranspiration
and to artifactual changes in the relationship between Aandgs[41,42]. The occurrence of heterogeneous
stomatal closure may also impair the calculation of mesophyll conductance (see Sec. IV.B).
In C 3 plants the effects of temperature changes on Aandgsare even more complicated by the more
competitive response of photorespiration with respect to photosynthesis for their substrata [43] and by the
frequent occurrence of feedback limitation of photosynthesis by accumulation of phosphorylated sub-
strata in the cytosol [44]. These temperature effects on photosynthesis as well as those typical of elevated
temperatures (alteration of photosynthetic protein and membrane conformation) have, however, only an
indirect effect on gs.
In C 4 plants, on the other hand, a decrease of temperature may also increase gsand lead, in the case
of these plants, to leaf wilting because of the partial loss of stomatal control [45].
332 MASSACCI AND LORETO