A series of experiments with Protea neriifolia R. Br. found evidence that leaf blackening is associated with the inhibition of photosynthesis and its resultant effect on oxidative metabolism. Leaf blackening was significantly reduced by constant illumination greater than 25-mu-mol m-2 s-1. The light compensation point for P. neriifolia leaves was estimated to be 24-mu-mol m-2 s-1. Treatment of cut stems with 100-mu-M 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of electron transport in Photosystem II, increased leaf blackening, indicating the necessity of active photosynthesis in the control of blackening. The nature of the oxidative processes resulting in blackening was examined. Dipping P. neriifolia stems in an anti-oxidant (diphenylamine, 1.5 mg l-1) resulted in a significant reduction in leaf blackening after 7 days. Storage in a low oxygen, controlled atmosphere (1% O2, 5% CO2) provided effective control against leaf blackening for up to 14 days at 25-degrees-C. It is proposed that leaf blackening in P. neriifolia leaves is inhibited by active photosynthesis. possibly through the production of carbohydrates as well as anti-oxidant or reducing compounds.
