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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.

Carbohydrate-hybridization probes (Vreeland and Laetsch, 1989, Planta (177, 423–434) were used to localize the homogalacturonan (pectate) component of pectins in the cell walls of leaves and soybean root nodules. Leaves of two species of the dicotyledon Dubautia were compared; these species contain much pectin but differ in their tissue water relations with respect to their cell-wall properties. Maturation of the primary cell walls in nodules was studied in the Bradyrhizobium japonicum-Glycine max symbiosis. Probe labelling was based on the divalent-cation-mediated association between pectate in tissue sections and fluorescein-conjugated pectate fragments. Pectate was also labelled by mixed-dimer formation with fluorescent polyguluronate derived from alginate. The specificity of the probe for unesterified polygalacturonate was indicated by increased cell-wall labelling after chemical or enzymatic deesterification of tissue sections, in contrast to elimination of labelling by chemical esterification. Postfixation of tissue sections improved retention of soluble pectate. Pectate differences were found in the leaves among cell types, in degree of esterification, and between plant species. The cell walls of soybean nodules were strongly labelled by the pectate probe in nodules one week and three weeks after infection. Pectate was more highly esterified in the central infected zone than in the surrouding cortex. Within the infected zone, walls of uninfected cells and infected cells were similarly labelled by the pectate probe. The results indicate that the pectate molecular probe provides detailed information on pectate distribution at the cellular level for investigations of cell-wall structure, development and physiology.

Mature leaves of Argyroxiphium grayanumexhibit an unusual structure‐function relationship. The leaves are traversed by several large channels filled with extracellular polysaccharide. Infrared spectral data suggest that the polysaccharide is mainly pectin, which is consistent with previous data based on its histochemical staining properties. The polysaccharide exhibits a very high water‐holding capacity, which appears to result in a very high leaf capacitance at high water contents and water potentials. The volume‐normalized leaf capacitance of A. grayanumis 0.67 MPa–1over the water potential range of –0.1 to –0.5 MPa. Coupled with the low time constant for water flux within the leaves (57 sec), the high leaf capacitance of A. grayanummay enable it to avoid rapid declines in leaf water potential. Thus, in its native bog habitat in Hawaii, the high leaf capacitance of A. grayanummay be of benefit in terms of minimizing the development of short‐term leaf water deficits.

Four endemic Hawaiian Euphorbia species range in habitat from open arid coastal strand to shaded mesic forest and in growth-form from small prostrate shrubs to trees. As shown in the present study, these large differences in habitat and growth-form are paralleled by equally large differences in maximal photosynthetic rate (13.7 to 37.1 µmol CO2 m-2s-1), dark respiration rate (0.7 to 4.1 µmol CO2 m-2s-1), light level for saturation of photosynthesis (0.9 to over 2.0 mmol m-2s-1), light compensation point (0.01 to 0.11 mmol m-2s-1), leaf conductance to CO2 (1.7 to 4.9 mm s-1), and mesophyll conductance to CO2 (3.7 to 8.5 mm s-1). A principal consequence of this differentiation is that the capacity for photosynthesis at high light levels is higher in open site species, such as E. celastroides and E. degeneri, and at low light levels is higher in shade species, such as E. forbesii. E. hillebrandii, a species from intermediate semiopen habitats, exhibits an intermediate photosynthetic capacity at both high and low light levels. Despite this remarkable diversity, all four species exhibit the distinguishing physiological features of C4 photosynthesis.

The C4 species, Euphorbia forbesii, and the C3 species, Claoxylon sandwicense, occupy cool, shaded habitats in Hawaii. Both of these species exhibit the photosynthetic characteristics of typical shade plants: low light-saturated photosynthetic rates, low dark respiration rates, low light levels for saturation of photosynthesis, and low light compensation points. In addition, the quantum yields of the two species are similar at leaf temperatures near 22°C, reflecting a significant increase in the quantum yield of E. forbesii over that of C4 species from open habitats. C. sandwicense has a lower dark respiration rate than E. forbesii. Hence, since the quantum yields of the two species are similar at cool temperatures, C. sandwicense has a higher photosynthetic rate than E. forbesii at low incident photon flux densities. As a consequence, C. sandwicense should have a greater carbon gain than E. forbesii under the diffuse radiation conditions of their native habitat. However, since E. forbesii has a higher light-saturated photosynthetic rate than C. sandwicense, E. forbesii may have a greater carbon gain than C. sandwicense during sunflecks.

The endemic Hawaiian species of Scaevolaand Euphorbiagrow in a wide variety of native habitats and exhibit a wide range of variation in photosynthetic responses. Light‐saturated photosynthetic capacities range from 12.0 to 24.7 μmol CO2m−‐2s−‐1in the Scaevolaspecies and from 18.2 to 51.4 μmol CO2m−‐2s−‐1in the Euphorbiaspecies. Within each genus, differences in light‐saturated photosynthetic capacity are paralleled by differences in mesophyll and leaf conductances to CO2. Within each habitat, the C4Euphorbiaspecies exhibits a significantly higher photosynthetic capacity and a significantly higher mesophyll conductance than the corresponding C3Scaevolaspecies. These differences are greatest in the dry scrub habitat and least in the wet forest habitat. One photosynthetic characteristic that exhibits little variation among the species within each genus, yet that exhibits a consistently large difference between the species within each habitat, is photosynthetic water‐use efficiency. The C4Euphorbiaspecies possess water‐use efficiencies that are 2–3½ times as high as those of the C3Scaevolaspecies, regardless of whether these species are native to very dry or very wet habitats. At present, the ecological significance of this large inherent difference in photosynthetic water‐use efficiency is unknown. Indeed, it appears that neither photosynthetic pathway has imposed any major inherent constraints on the ability of the Scaevolaand Euphorbiaspecies to diversify into a wide variety of habitats.

Eight Hawaiian Dubautia species grow in habitats as varied as exposed lava, dry scrub, mesic forest, wet forest, and bog. These species also differ in diploid chromosome number, with four species having 13 pairs of chromosomes and four species having 14 pairs. This ecological and chromosomal variation is paralleled by significant interspecific variation in tissue elastic properties. The four 13-paired species from dry habitats exhibit significantly lower tissue elastic moduli near full hydration (Ei) than the four 14-paired species from mesic to wet habitats. Values of Ei range from 2 to 4 MPa among the former species and from 9 to 18 MPa among the latter species. The turgor dependence of the elastic modulus also differs markedly between the two groups of species. As a result of these differences in tissue elastic properties, the capacity for maintaining high turgor pressures as tissue water content decreases is much greater in the 13-paired species from dry habitats than in the 14-paired species from mesic to wet habitats. These results indicate that the evolutionary diversification of the Dubautia species has been accompanied by a significant degree of change at the physiological level.