142
Robert Save. Carles Castell and Jaume Terradas
while stomatal resistance does not show significant changes. Above ca. 35 °C,
the transpiration rate decreases while the stomatal resistance increases
sharply. Shade leaves show a similar behaviour, although the change of tendency in the transpiration rate and in the stomatal resistance occurs at air
temperatures around 25 °C (Save 1986).
In winter, the xylem water potential can reach very low values in holm
oak. Predawn values of less than -4.0 MPa were measured at Prades in a
ridge-top plot and near -2.0 MPa in a valley-bottom plot (Sala et al. 1990).
During the day, the xylem potential increa$es to between -1.0 and -2.2 MPa.
At Montseny, the minimal values of winter xylem potential of holm oak decrease along an altitude gradient from 350 to 1130 m a.s.l. (Save et al. 1982,
1988). Electrolyte leakage and cuticular transpiration rate of the trees growing at the highest altitude are 150 and 90% higher, respectively, than those of
trees growing at the lowest elevation. These changes agree with the deleterious effects of frost drought stress described by Tranquillini (1982) on plant
growth and development.
10.8 Xylem Embolism
Xylem embolism has been recognized as a serious cause of lowered plant
productivity when plants are submitted to abiotic (drought, cold, freeze) or
biotic (tracheomycoses, fungus, nematodes) stress (Gartner 1995; Sperry
1995). Observations made at Prades and elsewhere in the Catalan pre-coastal
range during the extreme drought of 1994 suggest that, in comparison with
other Mediterranean tree species, holm oak could be quite sensitive to embolism, since it suffered heavy crown withering, while Phillyrea latifolia and
Pinus halepensis fared much better. Plants with smaller xylem conduits develop cavitation at lower water potentials (Tyree and Dixon 1986). Thus, despite their lower efficiency in water conduction, short and narrow xylem
conduits would be advantageous to plants exposed to xylem embolism
(Zimmerman 1983; Lo Gullo et al. 1995). Compared to Euro-Siberian deciduous oaks, holm oak has small xylem conduits: in 1-year-old internodes, 35%
of the conduits have diameters < 20 f..Lm, and only 20% have diameters
> 40 f..Lm (Lo Gullo and Salleo 1993). However, there is evidence that vulnerability to embolism is determined by the pit-membrane pore diameter rather
than by the conduit diameter (Zimmerman 1983). This agrees with the "airseeding" hypothesis (Sperry and Tyree 1.990), which proposes that gaseous
emboli would enter xylem conduits through the pit-membrane pores, whenever the pressure differences across them are high enough to make the radius
of the air-water meniscus smaller than that of the pore.
Embolism can occur in summer under drought stress. When holm oak
plants dried under a mild water stress [a value of about 0.9 in the ratio (leaf
water potential)/(turgor loss point water potentia!)], only the conduits wider
Robert Save. Carles Castell and Jaume Terradas
while stomatal resistance does not show significant changes. Above ca. 35 °C,
the transpiration rate decreases while the stomatal resistance increases
sharply. Shade leaves show a similar behaviour, although the change of tendency in the transpiration rate and in the stomatal resistance occurs at air
temperatures around 25 °C (Save 1986).
In winter, the xylem water potential can reach very low values in holm
oak. Predawn values of less than -4.0 MPa were measured at Prades in a
ridge-top plot and near -2.0 MPa in a valley-bottom plot (Sala et al. 1990).
During the day, the xylem potential increa$es to between -1.0 and -2.2 MPa.
At Montseny, the minimal values of winter xylem potential of holm oak decrease along an altitude gradient from 350 to 1130 m a.s.l. (Save et al. 1982,
1988). Electrolyte leakage and cuticular transpiration rate of the trees growing at the highest altitude are 150 and 90% higher, respectively, than those of
trees growing at the lowest elevation. These changes agree with the deleterious effects of frost drought stress described by Tranquillini (1982) on plant
growth and development.
10.8 Xylem Embolism
Xylem embolism has been recognized as a serious cause of lowered plant
productivity when plants are submitted to abiotic (drought, cold, freeze) or
biotic (tracheomycoses, fungus, nematodes) stress (Gartner 1995; Sperry
1995). Observations made at Prades and elsewhere in the Catalan pre-coastal
range during the extreme drought of 1994 suggest that, in comparison with
other Mediterranean tree species, holm oak could be quite sensitive to embolism, since it suffered heavy crown withering, while Phillyrea latifolia and
Pinus halepensis fared much better. Plants with smaller xylem conduits develop cavitation at lower water potentials (Tyree and Dixon 1986). Thus, despite their lower efficiency in water conduction, short and narrow xylem
conduits would be advantageous to plants exposed to xylem embolism
(Zimmerman 1983; Lo Gullo et al. 1995). Compared to Euro-Siberian deciduous oaks, holm oak has small xylem conduits: in 1-year-old internodes, 35%
of the conduits have diameters < 20 f..Lm, and only 20% have diameters
> 40 f..Lm (Lo Gullo and Salleo 1993). However, there is evidence that vulnerability to embolism is determined by the pit-membrane pore diameter rather
than by the conduit diameter (Zimmerman 1983). This agrees with the "airseeding" hypothesis (Sperry and Tyree 1.990), which proposes that gaseous
emboli would enter xylem conduits through the pit-membrane pores, whenever the pressure differences across them are high enough to make the radius
of the air-water meniscus smaller than that of the pore.
Embolism can occur in summer under drought stress. When holm oak
plants dried under a mild water stress [a value of about 0.9 in the ratio (leaf
water potential)/(turgor loss point water potentia!)], only the conduits wider