Water relations of seedlings of three Quercus species: variations across and within species grown in contrasting light and water regimes

We compared seedling water relations of three Mediterranean Quercus species (the evergreen shrub Q. coccifera L., the evergreen tree Q. ilex L. subsp. ballota (Desf.) Samp. and the deciduous or marcescent tree Q. faginea L.). We also explored seedling potential for acclimation to contrasting growing conditions. In March, 1-year-old seedlings of the three species were planted in pots and grown outdoors in a factorial combination of two irrigation regimes (daily (HW) and alternate day watering (LW)) and two irradiances (43 and 100% of full sunlight). At the end of July, predawn and midday water potentials (Psi(pd), Psi(md)) were measured, and pressure-volume (P-V) curves were obtained for mature current-year shoots. Species exhibited similar Psi(pd) and Psi(md) values, but differed in leaf morphology and water relations. The evergreens possessed larger leaf mass per area (LMA) and were able to maintain positive turgor pressure at lower water potentials than the deciduous species because of their lower osmotic potential at full turgor. However, the three species had similar relative water contents at the turgor loss point because Q. faginea compensated for its higher osmotic potential with greater cell wall elasticity. Values of Psi(pd) had a mean of -1.12 MPa in LW and -0.63 MPa in HW, and Psi(md) had a mean of -1.13 MPa in full sunlight and -1.64 MPa in shade, where seedlings exhibited lower LMA. However, the P-V curve traits were unaffected by the treatments. Our results suggest that Q. faginea seedlings combine the water-use characteristics of mesic deciduous oak and the drought-tolerance of xeric evergreen oak. The ability of Q. coccifera to colonize drier sites than Q. ilex was not a result of higher drought tolerance, but rather may be associated with other dehydration postponement mechanisms including drought-induced leaf shedding. The lack of treatment effects may reflect a relatively low contrast between treatment regimes, or a low inherent responsiveness of these traits in the study species, or both.