SYNCHRONOUS CHANGES IN TISSUE WATER PARAMETERS OF MATURE FOLIAGE FROM WELL-WATERED AND PERIODICALLY DROUGHTED TREE SEEDLINGS

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 Psi(pi) in well-watered plants. Droughted plants of Quercus prinus L. had significant decreases and increases in Psi(pi) during and after drought, but synchronous changes in Psi(pi) in the well-watered control plants accounted for 40-89% of this variation. During a one month period, Psi(pi) 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,) (4.1+/-0.3 mmol g(-1) and relative symplast volume (R(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, and R, or the relationship of RN(S) and R(SV) versus Psi(pi) 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.