Osmotic adjustment in two temperate oak species [Quercus pyrenaica Willd and Quercus petraea (Matt.) Liebl] of the Iberian Peninsula in response to drought

Abstract

One-year-old seedlings of Quercus pyrenaica and Quercus petraea were subjected in a greenhouse to three consecutive water stress cycles. Watering was withdrawn in water stressed seedlings until 5% volumetric soil water content in the two first cycles and up to 3.5% in the third one. At the end of each cycle, predawn water potential (Ψpd) was measured by a pressure bomb. Furthermore, other water parameters: osmotic potential at full (Ψπ100) and at turgor loss point (Ψπ0) as well as bulk modulus of tissue elasticity at maximum turgor (Emax), were derived from the establishment of pressure-volume curves at the end of the second and third water stress cycles. In the three drought periods there was a significant decrease of Ψpd in both species as compared to the controls which were maintained at field capacity. This decrease progressed from the first to the following cycles in Quercus pyrenaica up to –1.7 ± 0.31 MPa minimum value. In Quercus petraea the Ψpd value reached at the end of the first cycle (±1.8 ± 0.52 MPa), was no significantly different from those reached afterwards. A large decrease in Ψπ100 and Ψπ0 together with an increase of Emax were recorded in both species after the second water stress cycle, without any new significant variation in the third one. When well-watered and water-stressed seedlings were compared, Quercus pyrenaica showed a decrease in Ψπ100 of 0.8 MPa with regard to 0.3 MPa in Quercus petraea. These results show the occurrence of osmotic adjustment in both species. However, the osmoregulation capacity was higher in Quercus pyrenaica, which pointed out to a higher ability to develop mechanisms of water stress tolerance. This could be recognized as one of the physiological traits that explain why Quercus pyrenaica is better adapted to xeric conditions than Quercus petraea.