WATER POTENTIAL AND COMPONENT POTENTIALS IN EXPANDED AND UNEXPANDED LEAVES OF 2 XERIC GRASSES

Water (ψ), osmotic (ψs+ψm) and pressure (ψp) potentials were measured in three leaf regions of Agropyron dasystachyum and A. smithii grown in the field. Spanner‐type thermocouple psychrometers were used to measure ψ and (ψm+ψm). Absolute water content (AWC) was measured gravimetrically. The ψ and ψp were slightly lower in the emerging leaf blade (EBI) than in the last fully emerged leaf blade (FEBI); (ψs+ψm) and AWC were similar in the two regions. A gradient as large as 0.7 MPa was observed between the EBI and the base of the same emerging leaf (EBs); the latter included the meristematic regions. Although (ψs+ψm) and ψp were generally higher in the EBs, the gradients diminished as the level of stress increased in the shoot. Under moderate water stress the ψp of the EBs remained constant relative to the ψp in the exposed blades. The large ψ gradient within the growing leaf could have resulted from high resistance imposed by poor vascular development in the intercalary meristem. Ability to maintain a relatively large ψ gradient may be of general significance in buffering the growing region of xeric grass leaves from extreme, short‐term fluctuations in water stress that occur in exposed leaf blades. Copyright © 1978, Wiley Blackwell. All rights reserved