FLUXES OF CARBON IN ROOTS OF BARLEY PLANTS

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Abstract
Summary: Sucrose entering barley roots is unloaded symplastically from the phloem, as indicated by anatomical considerations, the unexchangeability of [14C]sucrose of shoot origin, and the characteristics of uptake of exogenous sucrose. It then enters a cytoplasmic sucrose pool which, together with an apoplastic pool exchanging rapidly with it, accounts for 40 to 50% of sucrose in the root. The remainder is in vacuoles, from which it is released with a half‐time of 8 h. Storage polysaccharides have a half‐time of about 17 h. About one‐quarter of all sucrose leaving the cytoplasmic plus apoplastic pool is temporarily stored as sucrose or polysaccharide. The net flux of carbon to growth of structure is about 55% of that entering the root; over 40% is lost in respiration. On root excision, the flux of sucrose out of the vacuole rose but the breakdown of storage carbohydrate did not.