Abstract
Abstract Leaf susceptibility to blackening differs between Protea neriifolia R. Br. and Leucospermum ‘Rachel’ [( L. lineare × L. vestitum ) × L. glabrum ]; members from two different genera of the family Proteaceae. Protea leaf discs had a greater dry weight percentage and a higher protein and total phenolic content, than Leucospermum leaf discs. No significant difference was observed in ascorbic acid content in fresh leaf tissues of the two genera. Polyphenol oxidase (PPO) activity was high in Protea leaf homogenates with none being detected in Leucospermum leaf tissue at any pH. A significantly higher pH was observed in Protea leaf homogenate (pH 5.2) than in Leucospermum leaf homogenate (pH 4.4), with pH 5.2 being the optimum pH for PPO activity in Protea leaf. Mixing Protea leaf extract with Leucospermum leaf extract at pH 4.4–6.8 inhibited PPO activity, suggesting that Leucospermum leaf extract contained an unknown PPO inhibitor. The absence of leaf blackening in Leucospermum could be due to either or both the lower leaf homogenate pH and the presence of an inhibitor.
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