Some properties of phloem exudate collected from root of sugar beet.

Much has been written concerning the mechanism and rate of movement of organic solutes in plants. The information concerning the physical properties and the composition of the liquid content of the phloem of plants is limited. The exudate from the sieve tubes of cucurbits has been studied most extensively. Crafts (5) reviewed the earlier literature and made extensive studies ( 1, 2, 3) on the movement of solutes, using the exudate from the sieve tubes of the cucurbits. Crafts and Lorenz (4), after careful microscopic observations, concluded that the exudate from the cucurbits comes from the sieve tubes. This has been accepted by other investigators. After a reexamination of the data and further extensive studies, however, they concluded that the phloem exudate from the cucurbits cannot be considered a true sample of the assimilate stream. These investigators pointed out that the phloem exudate from other species of plants is relatively high in sugars and that possibly the phloem exudate from some of these species might, therefore, be a true sample of the assimilate stream. Under certain conditions which are not well understood. some sugar beet roots will exude sufficient quantities of liquid from the phloem in sufficient quantities for studies on its physical and chemical properties. The only properties of phloem exudate from the sugar beet root that have been reported are the pH and total solids content. Fife and Frampton (6) reported the pH to be near 8.0 and a total solids content of near 11 %. This pH value agrees closely with that reported by Moose (7). In this paper other properties of phloem exudate are reported and these properties are compared with those of juice expressed from the same sugar beet roots and with xylem exudate collected from roots taken at a time when there existed no appreciable positive pressure in the phloem.