Abstract
To investigate the accumulation of sugars and other soluble solids into Citrus juice cells, we analyzed vesicle sink strength and sucrose uptake in Murcott mandarin (Citrus reticulata Blanco) fruit tissue. Plasmalemma and tonoplast vesicles isolated from fruits (juice sacs) collected at different developmental stages were analyzed for their transport capabilities. There was an active transport mechanism for sucrose in energized plasmalemma vesicles. Unexpectedly, tonoplast vesicles were shown to lack active transport mechanism of sucrose into the vacuole. Citrus juice cells were shown to incorporate membrane impermeable dyes into their vacuoles in the presence of sucrose. High definition confocal microscopy revealed the co-localization of membrane impermeable markers in cytoplasmic vesicles and the formation of vesicles at the plasmalemma. In addition, cells were able to take up artificial nano-particles. The data provide evidence for an endocytic system of transport that allows direct transport of sucrose from the apoplast to the vacuole in Citrus juice cells. Having identified the factors regulating sucrose uptake and solute accumulation, we can proceed to design experiments aimed at increasing the rate of sugar uptake resulting in higher Brix.
