Studies on Sugar Diffusion From Sugarcane Storage Tissue

Abstract

Sugar diffusion studies were performed with sugarcane stem tissues bathed in distilled water, Tris buffer, and aqueous solutions of various chemical additives. Rind-free tissue slices were prepared with mature and immature internodes from field-grown sugarcane, and from plants propagated in the greenhouse with constant water and nitrogen supply. Sucrose diffusion rates were of sufficient magnitude to have physiological, and possibly economic, significance. Sugar movement to the external media conformed with the operation of an active accumulation mechanism as described by workers in Australia. Initial efflux was rapid up to about 4 hours, achieving a maximum external sugar concentration between 8 and 12 hours, followed by depletion of sucrose in the bathing medium. The depletion phase was attributed to reentry of sugar into the tissue slices rather than inversion. Inclusion of cycloheximide and 2,4-dinitrophenol in the bathing medium permitted a continuing efflux of sucrose amounting to 5- to 10-fold increases over that obtained with distilled water. Actinomycin D produced no appreciable effect on initial efflux rate. 6-methyl purine appeared to block reentry of exited sugar without causing leakage of vacuolar sucrose. Diffusion patterns for mature storage tissue suggested that a single mechanism is operative for field- and greenhouse-reared cane. Tris buffer did not affect sucrose diffusion from mature tissue, but increased diffusion from immature storage tissue owing to an apparent inhibition of acid invertase. In general, sucrose diffusion was of greater magnitude from mature than immature tissue preparations, and reducing sugars diffused at a much lower rate which seldom exceeded 10 percent of the sucrose level. Addition of borate to the bathing medium appreciably increased the quantity of exiting sugar without altering the shape of the diffusion curve. The existence of a sugar-borate carrier system operating in the passage of cellular membranes is suggested. Physiological roles of a diffusible sugar pool in the intercellular spaces or dissolved in cell-wall water are evaluated. The potential significance of diffusion in promoting sucrose losses from stalks mechanically injured during harvest and post-harvest operations is also mentioned.