PHYSIOLOGICAL-RESPONSES TO WATER-STRESS AND WATERLOGGING IN NOTHOFAGUS SPECIES

Gas exchange and water relations were investigated in Nothofagus solan dri var, cliffortioides (Hook. f.) Poole (mountain beech) and Nothofag us menziesii (Hook. f.) Oerst (silver beech) seedlings in response to water stress and waterlogging. At soil matric potentials (Psi(soil)) a bove -0.005 MPa, N. solandri had significantly higher photosynthetic r ates (A), and stomatal and residual conductances (g(sw) and g(rc)), an d lower predawn xylem water potentials (Psi(predawn)) than N, menziesi i. The relative tolerance of plants to water stress was defined in ter ms of critical soil matric potential (Psi(cri)) and lethal xylem water potential (Psi(lethal)). The estimated values of Psi(cri) and Psi(let hal) were -1.2 and -7 MPa, respectively, for N. solandri, and -0.7 and -4 MPa, respectively, for N. menziesii. Photosynthesis was sustained to a xylem water potential (Psi(xylem)) of -7 MPa in N. solandri compa red with -4 MPa in N. menziesii. Following rewatering, both A and Psi( xylem) recovered quickly in N. solandri, whereas the two variables rec overed more slowly in N. menziesii. During the development of water st ress, nonstomatal inhibition significantly affected A in both N. solan dri and N. menziesii. Nothofagus menziesii was more susceptible to inh ibition of A by waterlogging than N. solandri. However, the tolerance of N, solandri to severe waterlogging was also limited as a result of a failure to form adventitious roots, suggesting a lack of adaptation to these conditions. The differences in tolerance to water stress and waterlogging between the two species are consistent with the distribut ion patterns of N, solandri and N, menziesii in New Zealand.