Xylem tension, lower apoplastic water content and high tissue rigidity in woody plants improves supercooling capacity during winter

Abstract

Olive is an evergreen tree considered highly drought resistant and moderately resistant to low temperatures. Olive tissues do not tolerate ice formation and present supercooling as a mechanism to resist low temperatures. The aim of this work was to evaluate water relations and the mechanisms involve in the frost resistant in five olive cultivars growing in pots during winter and summer in the patagonian steppe in Southern Argentina. We determined leaf pressure volume curves and measured midday (Ψmin) and predawn (Ψmax) leaf water potentials, ice nucleation temperature (INT) and flux density (δ) at the base of the trunk. All cultivars exhibited Ψmin and Ψmax significantly lower in winter than in summer. In winter Ψmin varied between -2.68 MPa and -3.21MPa, while in summer none cultivars presented Ψmin lower than -2.1 MPa. Bulk elastic modulus (e) increased in winter in all cultivars. The apoplastic water fraction (AWF) was lower in winter compared to summer in three of the study cultivars. In winter the observed range of leaf osmotic potential at turgor loss point (π 0 )was -1.2 to -1.7 MPa, but in summer the range was of -4.1 to -4.7 MPa. During winter INT ranged between -10°C and -13°C, while in summer ranged between -2°C and -6°C. We found strong correlations between INT and Ψmin, e, AWF and π 0 . Flux density was significant lower in winter compared to summer. The results suggest that the maintenance of water in an unfrozen state in olives trees during winter is the consequence of the low apoplastic water content, substantially high xylem water tension and tissue rigidity which contribute to improve supercooling capacity.