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

The water relations and hydraulic architecture of growing grass tillers (Fe stuca arundinacea Schreb,) are reported. Evaporative Aux density, E (mmol s (-1) m(-2)), of individual leaf blades was measured gravimetrically by cove ring 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, K-h (mmol s(-1) mm MPa-1), of excised leaf segment s was three times lower than theoretical hydraulic conductivity (K-t) calcu lated using the Poiseuille equation and measurements of vessel number and d iameter. K-t was corrected (K-t*) to account for the discrepancy between K- h and K-t and for immature xylem in the basal expanding region of elongatin g leaves, From base to tip of mature leaves the pattern of K-t* was bell-sh aped with a maximum near the sheath-blade joint (approximate to 19 mmol s(- 1) mm MPa-1). In elongating leaves, immature xylem in the basal growing reg ion led to a much lower K-t*. As the first metaxylem matured, K-t* increase d by 10-fold, The hydraulic conductances of the whole roof system, L-proot (mmol s(-1) MPa-1) and leaf blades, L-pblade (mmol s(-1) MPa-1) were measur ed by a vacuum induced water flow technique. L-proot and L-pblade were line arly related to the leaf area downstream. Approximately 65% of the resistan ce to wafer flow within the plant resided in the leaf blade. An electric-an alogue computer model was used to calculate the leaf blade area-specific ra dial hydraulic conductivity, L-pr (mmol s(-1) m(-2) MPa-1), using L-pblade, K-t* and wafer flux values. L-pr 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 L-pblade and L-pr values showed that ap proximate to 90% of the resistance to wafer flow within the blades resided in the liquid extra-vascular path. The same algorithm was then used to comp ute the xylem and extravascular water potential drop along the liquid water path in the plant under steady state conditions. Predicted and measured wa ter potentials matched well. The hydraulic design of the mature leaf result ed in low and quite constant xylem water potential gradient (approximate to 0.3 MPa m(-1)) throughout the plant. Much of the water potential drop with in mature leaves occurred within a tenth of millimetre in the blade, betwee n the xylem vessels and the site of water evaporation within the mesophyll. In elongating leaves, the low K-t* in the basal growth zone dramatically i ncreased the local xylem water potential gradient (approximate to 2.0 MPa m (-1)) there. In the leaf elongation zone the growth-induced water potential difference was approximate to 0.2 MPa.