Gas Exchange and Water Relations
141
tial in holm oak sprouts allows them to maintain high leaf conductance and
high photosynthesis rate during the summer, with a less conservative use of
water than mature plants (Fig. 10.2). However, the observed increase in leaf
conductance and net photosynthesis may not be fully explained just by the
increased leaf water potential, and enhanced nutrient availability might be
involved in this response (Fleck et al. 1996). Growth of sprouts can also be
fostered by the mobilization of carbohydrates from burls and large roots. The
morphological and physiological traits exhibited by young holm oak sprouts
are involved in the fast recovery of the leaf area index and canopy carbon
and water fluxes after disturbance (Leonardi and Rapp 1990; Chap. 23).
Seasonal courses of gas exchange in potted holm oak seedlings (Espelta
1996; Chap. 7) are similar to those found in mature plants and very different
from those observed in sprouts, thus confirming the role of the root to shoot
ratio in determining the water status and gas exchange rates in holm oak.
10.6 Drought-Tolerance Mechanisms
Pressure-volume curves (Tyree and Richter 1981,1982) reveal that sun leaves
are more drought-tolerant than shade leaves. Osmotic potential at full turgor
('PrtIOO ), osmotic potential at zero turgor ('Prto ) and volumetric modulus of
elasticity (e) of l-year-old leaves are -1.49, -1.82 and 7.31 Mpa, respectively,
in the upper canopy and -0.98, -1.35 and 6.64 MPa, respectively, in the lower
canopy of the permanent plot at La Castanya (Terradas and Save 1992).
These data agree with the results reviewed by Morgan (1984), and indicate
the different capacity of sun and shade leaves for osmotic adjustment. However, the relatively high values of 'II rtO and the relatively low values of e seem
to indicate that holm oak shows higher capacity for elastic than osmotic adjustment as a tolerance mechanism to drought stress (Salleo et al. 1997).
A moderate osmotic adjustment under drought conditions (Kyriakopoulos
and Richter 1991) may also contribute to drought tolerance, but this is only
possible by maintaining gas exchange at relatively low levels.
10.7 Effects of Temperature on Plant Water Relations
Since water availability is a key factor in holm oak performance, many ecological factors can affect holm oak through their effects on water relations.
We will try here to analyze the indirect effects of temperature through
changes in ecophysiological parameters related to water relations. Sun and
shade leaves show significant differences in their responses to high temperatures, reaching the turgor loss point at 33 and 26 DC, respectively. In sun
leaves, transpiration rate increases with increasing temperatures up to 33 DC,
141
tial in holm oak sprouts allows them to maintain high leaf conductance and
high photosynthesis rate during the summer, with a less conservative use of
water than mature plants (Fig. 10.2). However, the observed increase in leaf
conductance and net photosynthesis may not be fully explained just by the
increased leaf water potential, and enhanced nutrient availability might be
involved in this response (Fleck et al. 1996). Growth of sprouts can also be
fostered by the mobilization of carbohydrates from burls and large roots. The
morphological and physiological traits exhibited by young holm oak sprouts
are involved in the fast recovery of the leaf area index and canopy carbon
and water fluxes after disturbance (Leonardi and Rapp 1990; Chap. 23).
Seasonal courses of gas exchange in potted holm oak seedlings (Espelta
1996; Chap. 7) are similar to those found in mature plants and very different
from those observed in sprouts, thus confirming the role of the root to shoot
ratio in determining the water status and gas exchange rates in holm oak.
10.6 Drought-Tolerance Mechanisms
Pressure-volume curves (Tyree and Richter 1981,1982) reveal that sun leaves
are more drought-tolerant than shade leaves. Osmotic potential at full turgor
('PrtIOO ), osmotic potential at zero turgor ('Prto ) and volumetric modulus of
elasticity (e) of l-year-old leaves are -1.49, -1.82 and 7.31 Mpa, respectively,
in the upper canopy and -0.98, -1.35 and 6.64 MPa, respectively, in the lower
canopy of the permanent plot at La Castanya (Terradas and Save 1992).
These data agree with the results reviewed by Morgan (1984), and indicate
the different capacity of sun and shade leaves for osmotic adjustment. However, the relatively high values of 'II rtO and the relatively low values of e seem
to indicate that holm oak shows higher capacity for elastic than osmotic adjustment as a tolerance mechanism to drought stress (Salleo et al. 1997).
A moderate osmotic adjustment under drought conditions (Kyriakopoulos
and Richter 1991) may also contribute to drought tolerance, but this is only
possible by maintaining gas exchange at relatively low levels.
10.7 Effects of Temperature on Plant Water Relations
Since water availability is a key factor in holm oak performance, many ecological factors can affect holm oak through their effects on water relations.
We will try here to analyze the indirect effects of temperature through
changes in ecophysiological parameters related to water relations. Sun and
shade leaves show significant differences in their responses to high temperatures, reaching the turgor loss point at 33 and 26 DC, respectively. In sun
leaves, transpiration rate increases with increasing temperatures up to 33 DC,