Loading and translocation of assimilate in the fine veins of sunflower leaves

The time course of loading and transport of assimilate in sunflower leaves was examined by pulse labelling with 14 CO 2 , followed by freeze drying or freeze substitution, and dry autoradiography at both low and high resolution. The five classes of veins, V1‐V5 (V5 being smallest), show a division of function: V5 and V4 are engaged in loading and short distance transport; V3 to V1, in long distance translocation. The first high concentration of 14 C is found in two or three phloem parenchyma cells (intermediary cells) of V5 and V4 veins. The sieve elements of V5 and V4 veins do not show comparable concentrations of 14 C at any time. Recently assimilated 14 C is transported by the intermediary cells for distances of about 0.5 mm to the V3 veins. In V3 to V1 veins translocation is in the sieve tubes. Transport in V5 and V4 veins is in two directions, that in V3 to V1, in one direction towards the petiole. The high concentration of 14 C formed in the intermediary cells does not increase further as the assimilate moves to the sieve tubes of the V3 veins, and so is probably the origin of the gradient that drives translocation.