Re-sorption of organic compounds by roots of Zea mays L. and its consequences in the rhizosphere IlL Characteristics of sugar influx and efflux

The influx and efflux of sugar-C and the cycling of C within intact maize roots (Zea mays L. ) was studied in sterile solution culture. Using metabolic inhibitors it was shown that roots could take up sugars against the concentration gradient probably via H +-ATPase dependent plasmalemma proton cotransporters. In contrast to this, no evidence was found for an ATPase mediated efflux of sugars from the root. All parts of the root were capable of taking up exogenous sugars. Examination of sugar exudation sites along the root slowed effiux at all locations, with the amount of efflux linearly correlated with internal cellular concentration. The results clearly indicated that the influxefflux mechanisms are linked both spatially, temporally and with respect to the sugars capable of transportation. The turnover of C within the root was found to be extremely rapid with turnover of the soluble sugar pool being 0.8 to 15 times daily depending on root spatial location. The results strongly suggest that the recapture of sugars from outside the root plays an important role in regulating the amount of C lost to the soil which in turn will reduce both pathogen attraction and the size of the rhizosphere microbial population and will also increase the plant's C efficiency.