Abstract
Histological observations were made on the taro (Colocasia esculenta Schott cv. Dotare) tuber during development of internal browning due to chilling injury at 4°C. An attempt was also performed histochemically to detect phenolic substances and polyphenol oxidase associated with internal browning.
It was found that tannin cells were scattered in the epidermal and vascular tissues, and also in the inner parenchyma tissues. The shape of tannin cells was generally elliptical, but those in the vascular tissues was rectangular.
Before appearing of internal browning, slight browning occurred already in some tannin cells and surrounding parenchyma cells in each tissues. With progress of internal browning, numbers of browned tannin cells with granulation increased, and also the degree of browning of surrounding parenchyma cells augmented.
Further, in parenchyma cells, nuclei swelled, and leucoplasts and amyloplasts assembled around nuclei before appearance of internal browning. In the stage internal browning progressed, nuclei shrank with plasmolyses and collapsed accompanying with destruction of leucoplasts and amyloplasts.
Although phenolic substances of taro tuber were detected mainly in tannin cells, some of them were also found in secretory canals. Polyphenol oxidase was detected in the parenchyma cells, in addition to tannin cells and secretory canals.
As internal browning progressed, normal color reactions became indiscernible to the reagents for phenolic substances, while some reaction remained to the reagent for polyphenol oxidase.
