Stem Anoxia Temporarily Interrupts Translocation of11C-Photosynthate in Sunflower

Treatment of 5.0 cm of stem or petiole of sunflower with nitrogen gas for 5 min or longer reduced phloem translocation of 11C-photosynthate or stopped it entirely for a 15—20 min period after which time it slowly resumed. When the N2 was replaced by air the transport restarted immediately and soon reached speeds higher than pre-anoxia rates. Thus anoxia, which we know inhibits export of photosynthate by preventing dark respiration in the leaf (Grodzinski, Jahnke, and Thompson, 1984), also inhibited transport in veins. It reduced the speed of photosynthate movement eventually stopping it, probably by inhibiting the loading—unloading mechanisms along the sieve-tubes. These mechanisms are, apparently, themselves under control of a metabolic respiratory mechanism (Fensom, 1981). Short-term anoxia of the stem did not appreciably affect the loading and transport in the leaf; it probably did not destroy the integrity of membranes at the point of treatment nor cause a physical blockage of sieve-plate pores nor change the viscosity of sieve-tube sap. A metabolic component which assists transport is indicated in the mechanism of short-term translocation in sunflower.