Transport of sucrose, not hexose, in the phloem

Abstract

Several lines of evidence indicate that glucose and fructose are essentially absent in mobile phloem sap. However, this paradigm has been called into question, especially but not entirely, with respect to species in the Ranunculaceae and Papaveraceae. In the experiments in question, phloem sap was obtained by detaching leaves and placing the cut ends of the petioles in an EDTA solution. More hexose than sucrose was detected. In the present study, these results were confirmed for four species. However, almost identical results were obtained when the leaf blades were removed and only petiole stubs were immersed. This suggests that the sugars in the EDTA solution represent compounds extracted from the petioles, rather than sugars in transit in the phloem. In further experiments, the leaf blades were exposed to ¹⁴CO₂ and, following a chase period, radiolabelled sugars in the petioles and EDTA exudate were identified. Almost all the radiolabel was in the form of [¹⁴C]sucrose, with little radiolabelled hexose. The data support the long-held contention that sucrose is a ubiquitous transport sugar, but hexoses are essentially absent in the phloem stream.