Phloem Loading and Transport of Endogenously or Exogenously Labelled Photo-Assimilate in Bean, Beet, Maize and Cucurbit

Abstract

Transport of carbon-11 labelled photo-assimilate was monitored in Phaseolus vulgaris. Beta vulgaris, Zea mays, and Cucwbito pepo. The region of leaf to be labelled was first abraded and a solution passed over it to gain access to its apoplast and monitor changes in label therein. With PCMBS in the bathing solution the rate of washout of label into the bathing solution increased, but the effect of phloem loading was very variable for each species: on some occasions transport was hardly affected, on others it was halted. This was true even for Cucurbito pepo, where a symplastic pathway of loading has been widely accepted and suggests that PCMBS affects symplastic transport or that there is an apoplastic step in Cucurbito pepo. Apoplastic pH had little effect on transport or label washout unless a very acid (pH 40) buffer was introduced, contrary to notions of hydrogen ion co-transport for sugar uptake. Anoxia caused phloem loading to decrease immediately and label washout to increase in all species. It is suggested that both symplastic and apoplastic pathways can operate in all species but that their proportion varies according to species and ambient and/or growth conditions.