Kinetics of Sugar Transport in Isolated Vascular Bundles and Phloem Tissue of Celery

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Abstract
Celery (Apium graveolens L.) is a favorable plant material for phloem loading studies, because vascular bundles and phloem tissue can be isolated from fleshy petioles with minimal manipulations. Uptake kinetics for sucrose loaded into phloem are different than uptake by phloem-free tissue. In isolated vascular bundles, uptake of phloem sugars (sucrose and mannitol) were biphasic kinetics, consisting of an active, saturating component operating at low sugar concentrations, and linear kinetics at higher sugar concentrations. Glucose uptake exhibited linear kinetics. However, when the glucose analog, 3-0-methyl glucose (unmetabolized) was used, biphasic kinetic profiles were obtained. Sucrose and mannitol uptake in isolated phloem tissue consisted of one saturating component showing Michaelis-Menten kinetics. In the homogenous storage parenchyma tissue, sucrose uptake kinetics were by diffusion and insensitive to carbonyl cyanide m-chlorophenyl hydrazone (identical to that of L-glucose). The data suggested the presence of different mechanisms for sucrose uptake across the sieve-tube companion cell complex and nonphloem cells. It is proposed that celery may be a useful system to obtain direct evidence on mechanism of solute loading. Chemical names used: carbonyl cyanide m-chlorophenyl hydrazone (CCCP).

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