Water Potential and Component Potentials in Expanded and Unexpanded Leaves of Two Xeric Grasses

Water (ψ), osmotic (ψ_s+ψ_m) and pressure (ψ_p) 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 (ψ_m+ψ_m). Absolute water content (AWC) was measured gravimetrically. The ψ and ψ_p were slightly lower in the emerging leaf blade (EBI) than in the last fully emerged leaf blade (FEBI); (ψ_s+ψ_m) 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 (ψ_s+ψ_m) and ψ_p were generally higher in the EBs, the gradients diminished as the level of stress increased in the shoot. Under moderate water stress the ψ_p of the EBs remained constant relative to the ψ_p 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.