Variation in the tissue water relations of two sympatric Hawaiian Dubautia species and their natural hybrid

At one site of sympatry on the Island of Hawaii, Dubautia ciliolata and D. scabra are restricted to different lava flows, even though individuals of the two species may be found growing within a few meters of one another. Associated with this habitat difference is a difference in the tissue water deficits experienced by these two species. Midday water potentials in D. ciliolata are typically 0.4-0.5 MPa lower than in D. scabra. These two species also exhibit significant differences in their tissue osmotic and elastic properties. Dubautia ciliolata exhibits a lower tissue osmotic potential at full hydration and a lower tissue elastic modulus near full hydration than D. scabra. As a result, high and positive tissue turgor pressures are maintained to significantly lower tissue water contents and water potentials in D. ciliolata than in D. scabra. These differences in tissue osmotic and elastic properties appear to have a marked influence on diurnal turgor maintenance. Thus, while diurnal water potentials in D. ciliolata are significantly lower than in D. scabra, the diurnal turgor pressures exhibited by these two species are very similar. The natural hybrid between D. ciliolata and D. scabra exhibits intermediate tissue osmotic and elastic properties. This is evident, in particular, for the turgor dependence of the elastic modulus. The degree of phenotypic variation in the tissue osmotic and elastic properties of D. ciliolata appears to be relatively limited. As a result, plants of D. ciliolata growing under both well-watered conditions in the glasshouse and under natural conditions in the field exhibit a large capacity for maintaining high turgor pressures as tissue water content decreases.