Effect of ethylene on enzymatic activities involved in the browning of Hevea brasiliensis callus

Browning, which is of varying intensity depending on species, develops in Hevea brasiliensis callus in vitro and can affect somatic embryogenesis. Endogenous ethylene appears to be involved since application of aminocyclopropane carboxylic acid (ACC) strongly enhances this browning. At the same time, peroxidases (EC 1.11.1.7), bonded polyphenol oxidase (EC 1.10.3.1) and NADH-quinone reductase (EC 1.6.99.5) are enhanced by ACC whereas superoxide dismutase (SOD: EC 1.15.1.1) and ascorbate peroxidase (EC 1.11.1.11) are decreased. In contrast, application of aminoacetic acid (AOA) causes spectacular decreases in bound and soluble polyphenol oxidase, peroxidase and NADH-quinone reductase activities whereas SOD and ascorbate peroxidase activities increase. Enzymatic activities in the control callus present a medium value, probably due to a considerable ethylene production in the culture medium. After 47 days culture. the initial catalase activity (EC 1.11.1.6) in the explants had disappeared completely. In contrast, after only 22 days, the silver nitrate and especially AOA treatments promoted considerable activities of catalase. The beneficial effect of the inhibitors of ethylene synthesis was reflected in the enhanced formation of embryogenic tissue, which varied from 1% (control or ACC) to 34% (AOA treatment).