|Abstract or Summary
- The relationships among total water potential (psi), osmotic potential ([pi]), pressure potential (P), and relative water content (R*) were determined during four consecutive soil drying cycles for shoots of Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) seedlings. In each cycle, seedlings were sampled for pressure-volume determinations as soil water was depleted gradually from saturation to pre-dawn plant [psi] of 0.8-1.0 MPa (after 7-9 d) and 2.O-2.4 MPa (after an additional 4-6 d); tissue responses were compared against that of frequently irrigated controls. In non-stressed seedlings, [pi], the relative water content of the apoplasm (R*a), and the weight of symplasm water each exhibit significant decline during the period from 2 June to 16 September; the relative water content at the turgor loss point and the volumetric elastic modulus (e) each exhibit significant increases during this period. The osmotic potential at full turgor of irrigated seedlings was not different from that of non-irrigated seedlings in any of the four drying cycles. However, increases in R*a, and decreases in e are induced in shoots of stressed plants relative to unstressed plants in each of soil drying cycles 2, 3, and 4. Both variables are sensitive to water deficit but return to seasonal baseline values during periods of stress relaxation. Diurnal changes in [psi], [pi], P, R*a, and [epsilon] were measured on two days in both irrigated and non-irrigated seedlings. Osmotic adjustment plays a small, though significant, role in diurnal turgor regulation of non-stressed seedlings; release of water from cell walls and increases in wall elasticity are more important features of turgor regulation. In stressed seedlings, osmotic adjustment and changes in R*a act to maintain positive P in shoot tissue. Phenological and morphological responses to water deficit were compared in progeny of 5-7 trees from each of four populations. Highly significant differences were observed among populations for each of 19 traits. Further, a significant fraction of total variability for each trait was attributable to differences among families within populations. The irrigation X population interaction was non-significant for all traits with the exception of seedling height, average rate of growth, and leaf area : sapwood area ratio. Populations and families within populations each exhibit significant interaction with irrigation in multivariate analyses of variance. A canonical discriminant analysis shows distinct separation of groups when the classification variable is based on the region of origin and level of irrigation and when the quantitative variables include six measures of seedling morphology. Coefficients of the allometric formulae of seedling progeny of both coastal and southwestern Oregon populations are significantly altered by the level of soil moisture.