Localization of 14C-labeled assimilates in sieve elements of Macrocystis integrifolia by histoautoradiography

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Abstract
Macrocystis integrifolia fronds are known to translocate photoassimilates at velocities of 35–72 cm/h. By means of 14C-labeled histoautoradiography it is shown that this translocation occurs in sieve elements, and, further, that it is the mature, fully differentiated sieve elements that have undergone selective hydrolysis of cell contents, including nucleus, and have wide open pores that are involved in this translocation. Cortical cells, young differentiating and old callosed sieve elements, and hyphal cells show no label above background. Sieve elements immediately below the 14C-fed blade are the ones that show the label which indicates that only specific sieve elements are involved in translocation of photoassimilate from a blade. Since the sieve elements form a three-dimensional longitudinal system and since label is seen in the cross-connecting sieve elements, it is probable that at increasing distances from the fed blade the label will be distributed in many more sieve elements. The cytology of sieve elements that are shown here to be conducting is supportive of an osmotically driven pressure flow movement of assimilates in Macrocystis.