Movement of Sugars through Plants by Cytoplasmic Pumping

Abstract

MÜNCH'S hypothesis¹ for the mechanism of translocation in plants depends on a pressure gradient in sieve tubes driven by osmosis. It is supported by mass flow movements of mobile compounds^2,3 in the phloem. Other hypotheses^4–6 suggest that the transport mechanism requires a direct supply of energy from metabolism; they are supported by the demonstration of high levels of respiration in the phloem⁷, and the sensitivity of phloem transport to light⁸, temperature⁹ and metabolic inhibitors¹⁰. This apparently conflicting evidence is reconciled in hypotheses by Fensom¹¹ and Spanner¹², who independently proposed electrokinetic mechanisms which involve metabolic pumping at sieve plates and a mass flow of solution in the lumina of sieve elements. Each hypothesis implies a particular structure of the sieve element protoplast, but cytological studies are at present inconclusive because some indicate a degradation of the protoplast of sieve elements while others suggest a specialized organization of cytoplasm. In this article I wish to propose that sugars and other solutes are pumped over long distances through plants by transcellular strands of cytoplasm in sieve tubes.