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

Abstract

Xylem maturation in elongating leaf blades of tall fescue (Festuca arundinacea) 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 maturation region, in which elongation had stopped and narrow (NMX) and large (LMX) metaxylem vessels were beginning to function; (3) a distal, mature region 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 conductivity (K_t) computed from the diameter of individual inner vessels. K_t was 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±0.07 mmol s⁻¹ mm MPa⁻¹. Beyond that region, K_h rapidly increased with metaxylem 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 restrict longitudinal water movement. There was a close relationship between the water deposition rate in the elongation zone and the sum of the perimeters of PX vessels. The implications of this longitudinal vasculature on the partitioning of water between growth and transpiration is discussed.