CHARACTERIZATION OF PROTON AND SUGAR-TRANSPORT AT THE TONOPLAST OF SWEET LIME (CITRUS LIMMETIOIDES) JUICE CELLS

Citrus fruits accumulate high levers of sucrose and hexoses, although most photoassimilates arrive in the form of sucrose. In sweet limes, f aster rates of sugar accumulation take place early in development when sucrose catabolic enzymes are most active. The present investigation was aimed at providing information on the mechanisms of sucrose (and h exose) uptake into the Vacuole of cells containing high levels of sucr ose hydrolytic activity. Tonoplast vesicles of high purity were isolat ed in a discontinuous sucrose gradient. The vesicles were capable of f orming a pH gradient in the presence of ATP. Both bafilomycin and NO3- (but not vanadate) inhibited ATP hydrolysis and prevented the formati on of the pH gradient, confirming the tonoplast origin. Energized vesi cles (either by addition of ATP or by artificial pH gradient) did not accumulate sucrose or hexoses against a concentration gradient. In the presence of either sucrose or hexoses, the established Delta pH was n ot disrupted as was the case with tonoplast Vesicles from red beet hyp ocotyl. Therefore, a sucrose/H+ (hexose) antiport may not be the mecha nism of sucrose and hexose transport into the vacuoles of sweet lime j uice cells. The data indicated that sucrose uptake into vacuoles of sw eet lime occurs by facilitated diffusion. Hexoses originate from the h ydrolytic action of acid invertase on sucrose within the vacuole, and by the action of cytosolic sucrose synthase.