Hydraulic architecture and water flow in growing grass tillers (Festuca arundinacea Scheb.)

The water relations and hydraulic architecture of growing grass tillers (Festuca arundinacea Schreb.) are reported. Evaporative flux density, E (mmol s(-1) m(-2)), of individual leaf blades was measured gravimetrically by covering or excision of entire leaf blades. Values of E were similar for mature and elongating leaf blades, averaging 2.4 mmol s(-1) m(-2). Measured axial hydraulic conductivity, Kh (mmol s(-1) mm MPa(-1)), of excised leaf segments was three times lower than theoretical hydraulic conductivity (Kt) calculated using the Poiseuille equation and measurements of vessel number and diameter. Kt was corrected (Kt*) to account for the discrepancy between Kh and Kt and for immature xylem in the basal expanding region of elongating leaves. From base to tip of mature leaves the pattern of Kt* was bell-shaped with a maximum near the sheath-blade joint (approximately equal to 19 mmol s(-1) mm MPa(-1)). In elongating leaves, immature xylem in the basal growing region led to a much lower Kt*. As the first metaxylem matured, Kt* increased by 10-fold. The hydraulic conductances of the whole root system, Lproot (mmol s(-1) MPa(-1)) and leaf blades, Lpblade (mmol s(-1) Mpa(-1)) were measured by a vacuum induced water flow technique. Lproot and Lpblade were linearly related to the leaf area downstream. Approximately 65% of the resistance to water flow within the plant resided in the leaf blade. An electric-analogue computer model was used to calculate the leaf blade area-specific radial hydraulic conductivity, Lpr (mmol s(-1) m(-2) Mpa(-1)), using Lpblade, Kt* and water flux values. LPr values decreased with leaf age, from 21.2 mmol s(-1) m(-2) MPa(-1) in rapidly elongating leaf to 7.2 mmol s(-1) m(-2) MPa(-1) in mature leaf. Comparison of Lpblade and Lpr values showed that approximately equal to 90% of the resistance to water flow within the blades resided in the liquid extra-vascular path. The same algorithm was then used to compute the xylem and extravascular water potential drop along the liquid water path in the plant under steady state conditions. Predicted and measured water potentials matched well. The hydraulic design of the mature leaf resulted in low and quite constant xylem water potential gradient (approximately equal to 0.3 MPa m(-1)) throughout the plant. Much of the water potential drop within mature leaves occurred within a tenth of millimetre in the blade, between the xylem vessels and the site of water evaporation within the mesophyll. In elongating leaves, the low Kt* in the basal growth zone dramatically increased the local xylem water potential gradient (approximately equal to 2.0 MPa m(-1)) there. In the leaf elongation zone the growth-induced water potential difference was approximately equal to 0.2 MPa.