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

Transport of carbon-11 labelled photo-assimilate was monitoredin Phaseolus vulgaris. Beta vulgaris, Zea mays, and Cucwbitopepo. The region of leaf to be labelled was first abraded anda solution passed over it to gain access to its apoplast andmonitor changes in label therein. With PCMBS in the bathingsolution the rate of washout of label into the bathing solutionincreased, but the effect of phloem loading was very variablefor 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 acceptedand suggests that PCMBS affects symplastic transport or thatthere is an apoplastic step in Cucurbito pepo. Apoplastic pHhad little effect on transport or label washout unless a veryacid (pH 40) buffer was introduced, contrary to notions of hydrogenion co-transport for sugar uptake. Anoxia caused phloem loadingto decrease immediately and label washout to increase in allspecies. It is suggested that both symplastic and apoplasticpathways can operate in all species but that their proportionvaries according to species and ambient and/or growth conditions. Key words: Phloem loading, photo-assimilate transport