Peels from the inner epidermis of onion bulbs are a model system in plant cell biology. While the inner epidermis of red onions is characteristically white, small patches of cells sometimes redden, containing vacuolar anthocyanin. This study investigated the spectroscopic properties of these anthocyanic cells. When fluorescent dyes were loaded into the vacuole of onion epidermal cells, the anthocyanic cells showed decreased dye fluorescence. This decrease was observed for fluorescein and carboxyfluorescein that are pumped into the vacuole by anion transporters, for acridine orange which acid loads into the vacuole, and for the fluorescent sugar analogue esculin loaded into the vacuole by sucrose transporters. Similar decreases in carboxyfluorescein fluorescence were observed when dye was loaded into the vacuoles of several other plant species, but decreases were not observed for dyes resident in the tonoplast membrane. As cellular physiology was unaffected in the anthocyanic cells, with cytoplasmic streaming, vacuolar and cytoplasmic pH not being altered, the decreased dye fluorescence from the anthocyanic cells can be attributed to fluorescence quenching. Furthermore, because quenching decreased with increasing temperature. It was concluded, therefore, that vacuolar anthocyanin can statically quench other fluorescent molecules in vivo, an effect previously demonstrated for anthocyanin in vitro.
Keywords: Allium cepa; anthocyanin; fluorescence quenching; onion; onion epidermis.