Different responses to drought and freeze stress of Quercus ilex L. growing along a latitudinal gradient

The vulnerability to drought and freeze stress was measured in young plants of Quercus ilex L. growing in the field in two natural sites within the It alian distribution area of this species, i.e. Sicily (Southern Mediterranea n Basin) and Venezia Giulia (Northeastern Italy), respectively. In particul ar, the resistance strategies adopted by Q. ilex to withstand the two stres ses were estimated in terms of seasonal and/or diurnal changes in leaf cond uctance to water vapour (g(L)), water potential (psi(L)) and relative water content (RWC) as well as of xylem embolism in the stem and root hydraulic conductance (K-RL). Sicilian (SI) plants showed to reduce water loss by sto matal closure (g(L) decreased) in summer, thus maintaining average RWCs at 88-90%. Moreover, SI plants showed considerable resistance to xylem cavitat ion in the stem (the loss of hydraulic conductance, PLC, was less than 12% throughout the year) and to maintain the hydraulic conductance of their roo ts (K-RL), constantly high even in summer. Plants growing in Venezia Giulia (VG plants), on the contrary, underwent leaf dehydration in the winter due to freeze stress so that RWC measured in April was still 78% on a diurnal basis. This was apparently due to consistent xylem embolism in the stem. In fact, PLC was as high as 40% between November and March. Only in the summe r was PLC similar to that recorded in SI plants. Moreover, K-RL of VG plant s decreased in November from about 1.5 to 0.8x10(-)4 kg s(-)1 m(-)2 MPa(-)1 , i.e. about 50%, and in February K-RL dropped further to 0.4x10(-)4 kg s(- )1 m(-)2 MPa(-)1. On the basis of the above, we conclude that: (a) Q. ilex was more sensitive to freeze than to drought stress so that freeze stress c an be considered as a factor limiting the distribution area of this species ; (b) drought and freeze stress were faced by Q. ilex adopting two differen t resistance strategies, i.e. drought avoidance based on water saving in Si cily and freeze tolerance in Venezia Giulia.