Water balance was studied in developing leaves of Curatella americana, Byrsonima crassifolia, Bowdichia virgilioides and Casearia sylvestris, four widespread tropical savanna evergreen woody species that renew their leaves during the dry season. Water potential components of leaves of different ages were estimated in the laboratory by determination of pressure-volume (PV) curves. Data from PV curves were used to help interpret the significance of variations in leaf water potential (Ψ) and stomatal conductance observed in leaves of different ages in the field.Initial osmotic potential at full turgidity as well as osmotic potential at the turgor loss point changed considerably during leaf development. Values of these water potential components for mature leaves were similar to those reported for temperate mesophytic woody species. Passive osmotic adjustment defined as the change in osmotic potential between full turgidity and turgor loss averaged only 0.3 MPa, much smaller than values for temperate mesophytic and drought hardy woody species. Since environmental conditions, especially soil moisture in the rooting zone, were essentially constant during leaf development, changes in leaf water potential components were not seasonal, but rather ontogenetic responses to diurnal water stress.In all species except C. sylvestris there were differences in Ψ between adjacent leaves exposed to the same environmental conditions, with immature leaves generally maintaining higher Ψ than mature leaves. The probable causes for this were 1) lower liquid flow resistance and 2) lower transpiration rates in younger leaves. At low transpiration rates liquid flow resistance was flux-dependent in all species, but became nearly independent of transpiration flux density (E) as E increased. This and their lower flow resistance permit high E to be sustained in developing leaves without excessive ψ drop across the leaf.During two typical dry season days E was high and even though complete or nearly complete turgor loss was sustained, these water deficits were not severe enough to cause complete stomatal closure. The possible relationship between a high stomatal conductance and the maintenance of a favorable carbon balance in these species is discussed.All of the water relations features observed suggested that in spite of their scleromorphic appearance, leaves of the four species under the conditions studied were adapted only to short term diurnal water deficits.

The foliage of coniferous plants in the area to the northeast of Mt. St. Helens, Washington State, was exposed to heavy ashfall during the May 18, 1980 eruption of the volcano. Significant damage to the pre-1980 foliage occurred after the eruption and continued through the summer. The amount of damage seen on the needles was significantly related to the amount of ash on the foliage.Elevated temperatures caused the foliage damage. The presence of ash on the foliage increased the dimensions of the shoot, thus increasing the boundary layer resistance. In turn this change in geometry elevated needle temperatures. Typical maximum needle temperatures for ash-laden foliage of Abies amabilis were in the range of 35° to 45° C and were 10° C above those of plants without ash. Damage occurred to needles at 40° C after a short growth-chamber exposure. Temperatures within the ash on the foliage also exceeded 40 degrees C.Neither chemical nor mechanical (abrasion) damage occurred. There was no melting of the cuticle. The plants with ash-covered foliage did not experience lower water potentials than those of control plants. The total radiation reflected from the needles was similar for foliage with and without ash.

Evidence obtained on the relation between the pH of the medium and the growth of intact stem sections is compatible with the acid-growth theory only if the proton conductance of the cuticle is so low that the cuticle is an effective barrier to the entry or exit of protons from the tissue. By measuring the rate at which protons cross frozen-thawed epidermal strips of sunflower (Helianthus annuus L.) and soybean hypocotyls (Glycine max Morr.) and enzymically isolated cuticles of Berberis aquifoliun Persh. and tomato (Lycopersicum eseadentum Mill.) frlit, we have now demonstrated the low proton conductance of the cuticular layer. Unless the conductance is enhanced by abrasion of the cuticle or by removal of the cuticular waxes, proton movement into and out of a tissue across the cuticle will be signlifcant only over long time periods.The aerial portions of all higher plants are covered by a cuticle, which is believed to act as a barrier to the entry and exit of materials from tissues (8). The ability of the cuticle to limit movement of cations such as K+ (7, 21), organic substances (4), and water (17) has been established, but the proton conductance2 of a cuticle has been examined only after long periods (3). Knowledge about the proton conductance of the cuticle is of importance in regard to the acid-growth theory of auxin-induced growth (15).The acid-growth theory states (3,15) that auxin causes the cells to excrete protons into the wall solution where the resulting increased acidity activates wall-loosening enzymes. The excreted protons might be expected to acidify the external solution, too, but it has been difficult to demonstrate any auxin-induced acidification (11,12,19) unless the cuticle is abraded (3, 9, 16) or removed (1, 13). This led us to suggest (1, 13, 15) that proton conductance of the cuticle is so low that protons excreted into the wall solution are trapped within the tissue unless they escape through cut surfaces. A low proton conductance would also explain why, in order to achieve a maximum rate of acid-induced growth, it is necessary to add a 50-fold greater proton concentration to intact A vena coleoptile sections as compared with sections whose cuticle had been removed (13 the cuticle may not be a barrier to protons. In this study, we have made use of a simple assay that allows us to assess the ability of protons to cross cuticular layers, and we show for the first time that the cuticle is an effective barrier to the entry and exit of protons. The cuticle of some seedlings was abraded by rubbing the hypocotyl five to ten times with a slurry of emory powder (American Optical, Seattle, WA, grade M180) before isolation of the epidermal strips. Other seedlings were wiped twice with a 3:1 (v/ v) mixture of chloroform:ethanol to remove some of the cuticular waxes. MATERIALS AND METHODSLeaves of Berberis aquifolium Dursh. were selected from a plant growing outside the laboratory. To isolate the cuticle, leaf pieces were infiltrated and incubated for at least 2 days with a solution containing...

Seven sites ranging from 15 to 135 km from Mount St. Helens were selected to study the impact of air-fall tephra on the growth of Abiesamabilis (Dougl.) Forbes, A. procera Rehd., Pseudotsugamenziesii (Mirb.) Franco, Tsugaheterophylla (Raf.) Sarg., and T. mertensiana (Bong.) Carr. As tephra depth increased, there was a corresponding increase in visible foliar damage and associated decreases in diameter and height growth. Reduction in diameter growth was greater than reduction in height growth. The reduction in diameter growth approached 50% in both trees and saplings of A. amabilis. Growth reduction in true firs was greater than in associated species. This difference was related to their greater capacity for interception and retention of air-fall tephra. Damage to trees, and resulting growth reductions, were due to tephra coverage of both the foliage and the soil. Coverage of the foliage resulted in foliar damage, foliage abscission and reduction of total tree foliar area, and increased fine root mortality. Tephra coverage of the soil had the potential to restrict oxygen diffusion into the soil. However, soil oxygen concentrations less than 10% were measured only once over a 2-year period.

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