Changes in axial hydraulic conductivity along elongating leaf blades in relation to xylem maturation in tall fescue

Zylem maturation in elongating leaf blades of tall fescue (Festuca arundina cea) was studied using staining and microcasting. Three distinctive regions were identified in the blade: (1) a basal region, in which elongation was occurring and protoxylem (PX) vessels were functioning throughout; (2) a ma turation region, in which elongation had stopped and narrow (NMX) and large (LMX) metaxylem vessels were beginning to function; (3) a distal, mature r egion in which most of the longitudinal water movements occurred in the LMX . The axial hydraulic conductivity (K-h) was measured in leaf sections from all these regions and compared with the theoretical axial hydraulic conduc tivity (K-t) computed from the diameter of individual inner vessels. K-t wa s proportional to K-h throughout the leaf, but K-t was about three times K- h. The changes in K-h and K-t along the leaf reflected the different stages of xylem maturation. In the basal 60 mm region, K-h was about 0.30 +/- 0.0 7 mmol s(-1) mm MPa-1. Beyond that region, K-h rapidly increased with metax ylem element maturation to a maximum value of 5.0 +/- 0.3 mmol s(-1) mm MPa -1, 105 mm from the leaf base. It then decreased to 3.5 +/- 0.2 mmol s(-1) mm MPa-1 near the leaf tip. The basal expanding region was observed to rest rict longitudinal water movement. There was a close relationship between th e water deposition rate in the elongation zone and the sum of the perimeter s of PX vessels. The implications of this longitudinal vasculature on the p artitioning of water between growth and transpiration is discussed.