Aspects of xylem anatomy and vulnerability to water stress-induced embolism were examined in stems of two droughtdeciduous species, Brachychiton australis (Schott and Endl.) A. Terracc. and Cochlospermum gillivraei Benth., and two evergreen species, Alphitonia excelsa (Fenzal) Benth. and Austromyrtus bidwillii (Benth.) Burret., growing in a seasonally dry rainforest. The deciduous species were more vulnerable to water stress-induced xylem embolism. B. australis and C. gillivraei reached a 50% loss of hydraulic conductivity at Ϫ3.17 MPa and Ϫ1.44 MPa, respectively; a 50% loss of hydraulic conductivity occurred at Ϫ5.56 MPa in A. excelsa and Ϫ5.12 MPa in A. bidwillii. To determine whether pit membrane porosity was responsible for greater vulnerability to embolism (air seeding hypothesis), pit membrane structure was examined. Expected pore sizes were calculated from vulnerability curves; however, the predicted inter-specific variation in pore sizes was not detected using scanning electron microscopy (pores were not visible to a resolution of 20 nm). Suspensions of colloidal gold particles were then perfused through branch sections. These experiments indicated that pit membrane pores were between 5 and 20 nm in diameter in all four species. The results may be explained by three possibilities: (a) the pores of the expected size range were not present, (b) larger pores, within the size range to cause air seeding, were present but were rare enough to avoid detection, or (c) pore sizes in the expected range only develop while the membrane is under mechanical stress (during air seeding) due to stretching/flexing.Xylem cavitation and embolism are recognized as major constraints affecting plants regularly exposed to water stress (Tyree and Sperry, 1989;Milburn, 1993). Water in the xylem is under negative pressure, or tension, i.e. it is held in a metastable state, below its vapor pressure, a condition that increases the likelihood of cavitation occurring (Oertli, 1971;Pickard, 1981). Cavitation is the process whereby a vapor phase is introduced to the xylem water column, creating an embolism. Embolisms are gas bubbles consisting initially of water vapor and later air, which become trapped within xylem conduits. Because of its inability to transmit tension, the vapor phase limits the volume flow of water through the conduit, reducing the plant's capacity to deliver water to the canopy (Meinzer et al., 2001). Plants must minimize this disruption to water transport to avoid effects on leaf water status that may result in limitations on stomatal conductance and photosynthesis.The structure of xylem vessels is seen as an important factor in determining the occurrence of water stress-induced cavitation (Zimmermann, 1983). Xylem vessels are bounded by pit membranes, through which water must pass to move from one vessel to the next. Pit membranes are the degraded primary cell walls and middle lamella of the vessels and are composed of tightly inter-woven cellulose microfibrils in a matrix of hemicellulose and pectin polysaccharide...

Hydraulic conductivity and xylem anatomy were examined in stems of two evergreen species, Alphitonia excelsa (Fenzal) Benth. and Austromyrtus bidwillii (Benth.) Burret., and two drought-deciduous species, Brachychiton australis (Schott and Endl.) A. Terracc. and Cochlospermum gillivraei Benth., from a seasonally dry rainforest in north Queensland, Australia. The deciduous species possessed hydraulic architecture typical of drought-sensitive plants, i.e. low wood density, wider xylem vessels, higher maximal rates of sapwood specific hydraulic conductivity (K s ) and high vulnerability to drought-induced embolism. In contrast, the evergreen species had lower rates of K h and leaf specific conductivity (K L ) but were less susceptible to embolism. The evergreen species experienced leaf water potentials <−4.0 MPa during the dry season, while the deciduous species shed their leaves before leaf water potentials declined below −2.0 MPa. Thus, the hydraulic architecture of the evergreens allows them to withstand the greater xylem pressure gradients required to maintain water transport to the canopy during the dry season. Our results are consistent with observations made in neotropical dry forests and demonstrate that drought-deciduous species with low wood density and high water storage capacity are

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Diurnal and seasonal patterns of leaf gas exchange and water relations were examined in tree species of contrasting leaf phenology growing in a seasonally dry tropical rain forest in north-eastern Australia. Two drought-deciduous species, Brachychiton australis (Schott and Endl.) A. Terracc. and Cochlospermum gillivraei Benth., and two evergreen species, Alphitonia excelsa (Fenzal) Benth. and Austromyrtus bidwillii (Benth.) Burret. were studied. The deciduous species had higher specific leaf areas and maximum photosynthetic rates per leaf dry mass in the wet season than the evergreens. During the transition from wet season to dry season, total canopy area was reduced by 70-90% in the deciduous species and stomatal conductance (g(s)) and assimilation rate (A) were markedly lower in the remaining leaves. Deciduous species maintained daytime leaf water potentials (Psi(L)) at close to or above wet season values by a combination of stomatal regulation and reduction in leaf area. Thus, the timing of leaf drop in deciduous species was not associated with large negative values of daytime Psi(L) (greater than -1.6 MPa) or predawn Psi(L) (greater than -1.0 MPa). The deciduous species appeared sensitive to small perturbations in soil and leaf water status that signalled the onset of drought. The evergreen species were less sensitive to the onset of drought and g(s) values were not significantly lower during the transitional period. In the dry season, the evergreen species maintained their canopies despite increasing water-stress; however, unlike Eucalyptus species from northern Australian savannas, A and g(s) were significantly lower than wet season values.