SIGNIFICANCE OF PHLOEM EXUDATE OF CUCURBITA PEPO WITH REFERENCE TO TRANSLOCATION OF ORGANIC MATERIALS

Abstract

The quantity and composition of phloem exudate of the cucurbit were determined several times during the day from plants at 3 metabolic levels: (1) Those which were young and growing very rapidly, (2) those which were fruiting but which had an adequate supply of N, and (3) those of the same age as (2) but whose metabolic level was very low because of the withholding of nitrates from the culture solution. The quantity of exudate bore no relationship to the quantity of translocation but was shown to be correlated with the osmotic relations of the tissues at the time of cutting. All the evidence indicates that exudations do not represent flow within the intact plant. More than 1/3 of the dry matter of the exudate is protein, pointing to a dense, active cytoplasm of the sieve tubes from which the exudate is shown largely to arise. Sugars represent less than 1% of the fresh wt. The composition of the exudate shows no relation to the composition of the adjacent tissues. In plants with a high metabolic rate, the sugars of the tissues are very low while that of the exudate is high. In plants with a low metabolic rate the sugar content of the tissues is very high, that of the exudate is low. The intermediate plants are intermediate in their composition. The quantities of exudates and their composition are used as evidence that the mass flow hypothesis is untenable, and that it appears that translocation intensity is correlated with the metabolic level of the plant.