Synchronous changes in tissue 2

Tissue water relations using pressure-volume analysis were examined in mature foliage of continuously well-watered and twice droughted seedlings of two contrasting tree species over a 75 d period. One species, Liriodendron tulipifera L., exhibited no osmotic adjustment in droughted plants, but had significant decreases in ψ_x in well-watered plants. Droughted plants of Quercus prinus L. had significant decreases and increases in ψ_x during and after drought, but synchronous changes in ψ_x in the well-watered control plants accounted for 40-89% of this variation. During a one month period, ψ_x in droughted and well-watered Q. prinus decreased by 0.36 and 0.32 MPa, respectively, indicating that almost all of the osmotic adjustment in droughted plants could be attributed to phenological shifts. Liriodendron had higher mean (±s.e.) relative solute content (RN_s) (4.1 ±0.3 mmol g^-1) and relative symplast volume (RN_sv) (46.8 ±3.3%) than did Q. prinus (2.5 ± 0.2 mmol g_-1 and 35.7 ±2.9%, respectively). However, there were no differences in mean RN_s and R_sv or the relationship of RN_s) and R_sv versus ψ_x between well-watered and droughted plants within each species. The results of this study indicate that synchronous changes in tissue water parameters may occur in well-watered and droughted plants, that drought effects need to be defined as the difference in change between well-watered and droughted plants, rather than the change in droughted plants alone, and that seasonal changes in RN_s, and R_sv may be responsible for the previously unexplained phenological shifts in mature leaves of well-watered plants.