Loading of Assimilates in Wheat Leaves. I. The Specialization of Vein Types for Separate Activities

Abstract

An autoradiographic study is reported of the movement of labelled assimilate into the vein network in the area of the leaf fed ¹⁴CO₂, and in the zone of the leaf proximal to the fed area. In the fed area the labelled assimilate is concentrated first in the intermediate veins and transported via the transverse veins to the lateral veins in which it rapidly moves basipetally. The intermediate veins distribute the label up and down the leaf 1-2 mm to or from the nearest transverse vein. They appear to function primarily as loading, collecting and distributing reservoirs. Proximal to the fed zone, the labelled assimilate translocated in the lateral veins spreads into the network of transverse and intermediate veins. The vein types have therefore different primary functions and may differ in their transport mechanisms. Measurements of the cross-sectional area of phloem in the three types of veins along the leaf blade are consistent with the idea that the laterals are translocating veins and the intermediates are collecting reservoirs. Quantitative estimates are made of the fluxes of assimilate in the three vein types.