Water Pathways in Wheat Leaves. II. Water-Conducting Capacities and Vessel Diameters of Different Vein Types, and the Behaviour of the Integrated Vein Network

Abstract

The water-conducting capacity of the lateral veins of the wheat leaf, measured by the velocity of movement of a dye marker, decreases with distance along the leaf towards the tip. The laterals contain a number of small (up to 10 µm) diameter vessels as well as two or three relatively large (up to 45 µm) diameter vessels. The largest vessel in each decreases in diameter with distance along the leaf towards the tip, resulting in the decreased velocity of conduction. The large vessels represent physical spaces through which laminar flow can occur; however, the flow rate is slower than that predicted by the Hagen-Poiseuille law for pipes of equivalent diameter. The intermediate veins contain only several 7-10 µm diameter vessels, and the diameter of the largest vessel in these veins does not change along the length of the leaf. The number of 7-10 µm diameter vessels per vein also remains unchanged. The water- conducting capacity of an intermediate vein is therefore constant along the length of the leaf. The transverse veins that cross-connect neighbouring longitudinal veins all have similar water-conducting capacity regardless of location in the leaf. These measurements support the view that the lateral veins serve to supply water from the base of the leaf to the tip, while the intermediate and transverse veins form a distribution network carrying the water across the leaf to the mesophyll cells. A theoretical analysis is made of the behaviour of the network as an integrated system. Assumed transpiration rates are imposed on the model to find what the pressure gradient along the leaf blade is, what pressures would be produced at the nodes of the network, and what fluxes would result through the vein elements of it. The model predicts movements of water in the intermediate veins very similar to those observed in a previous paper and suggests that the network responds to changes in supply or demand by producing minima of similar pressure in the distributing veins at different distances from the supplying longitudinal veins.