Plant Water Deficits, Osmotic Properties, and Hydraulic Resistances of Hawaiian Dubautia Species from Adjacent Bog and Wet-Forest Habitats!

Functional responses of two closely related Dubautia species from a mosaic of Hawaiian bogs and wet forest were compared to help explain their differential distributions. Dubautia paleata is largely restricted to saturated bogs, while D. raillardioides is restricted to the surrounding, better-drained wet forest. Minimum diurnal tissue water potentials of D. paleata are significantly lower than those of D. raillardioides, despite the moister condition of bog soil. The tissue osmotic potential at full hydration (nJ of D. paleata is significantly lower than that of D. raillardioides. As a result , the tissue water potential at which turgor reaches zero for D. paleata is significantly lower than that of D . raillardioides. Dubautia paleata is thus able to maintain positive turgor to lower water potentials than D. raillardioides. Lack ofa lowered n, in D. raillardioides may therefore contribute to exclusion of that species from the bog habitat. Preliminary data suggest a significantly greater hydraulic resistance for D . paleata than for D . raillardioides, probably due to higher root resistance caused by the reduced condition of the waterlogged bog substrate. The difference in hydraulic resistance could help account for the contrasting water deficits of the two species. creating sucstantial hydraulic resistance in bog plants (Marchand 1975, Bradbury and Grace 1983). One physiological mechanism capable of buffering the effect of tissue water deficits is a decrease in tissue osmotic potential. A lowered osmotic potential enables turgor to be maintained to lower tissue water contents (Tyree and Jarvis 1982). Stomatal opening and photosynthetic carbon assimilation are among the physiological processes adversely affected by water deficits. Whether moderate water deficits are transduced into metabolic changes through a reduction in turgor pressure is still a matter of debate (Hsiao et al. 1976, Turner and Jones 1980, Schulze 1986). It has been argued that the ability of plants to maintain turgor at lower tissue water contents may promote their growth and survival under moisture-limited conditions (Turner and Jones 1980, Bradford and Hsiao 1982).Low osmotic potentials (e.g., -3 MPa) have been reported for bog plants (Harris 1934, Ingram 1983). This paper addresses the water status ofbog plants by examining two closely related species of Dubautia (Asteraceae) from adjacent