SIEVE-TUBE STRUCTURE AND TRANSLOCATION IN THE POTATO

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Abstract
Because the potato is particularly favorable for translocation studies, the anatomy and physiology of phloem tissues in this plant have been investigated. Young sieve tubes are nucleate and display the characteristics of normal living cells. As the sieve-tube elements mature, the nuclei and slime bodies disintegrate; and the protoplasm apparently changes its organization, becoming more and more permeable. Slime plugs are shown to be artifacts formed by the action of killing agents upon the vacuolar contents; pores within the protoplasmic strands of the sieve plates could not be demonstrated. Phloem exudate from the potato has a low surface tension, does not coagulate rapidly, and seems to emerge at a rate of flow that would account for normal translocation. Measurements indicate that a 10% sucrose solution would have to flow 19 cm. per hour through a conduit equal in transverse area to the total phloem to provide for tuber formation. The theories of protoplasmic streaming and pressure flow through phloem walls seem inadequate to explain this rate of movement. Sieve-tube lumina apparently afford the most available channels for this movement, the parietal protoplasm offering little resistance. Capillary spaces of from 0.01 to 0.06 [mu] would allow movement across end walls under the available pressure. The phloem increases greatly in cross-sectional area within the potato tuber. The rate of flow is correspondingly reduced. Protoplasmic streaming may accelerate lateral movement across non-vascular tissues. Calculations indicate a rate of 1.1 [mu] per hour for diffusion along plasmodesma of cross walls in the tuber. The corresponding diffusion rate in the leaf would be 1.5 u per hour. Diffusion along plasmodesma of cross walls and acceleration by protoplasmic streaming within non-vascular tissues, combined with pressure flow through permeable sieve tubes and phloem walls within specialized conducting organs, seem most satisfactorily to explain translocation in the potato.