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Interior diffuse light readings were taken in trees of semi-dwarf ‘Wayne’ and ‘Golden Delicious’ apple ( Malus domestica Borkh.) on days of varying cloudiness. Interior light to shaded spurs was found to be greatest when the total light outside the tree was 60 to 90% of maximum, due to higher diffuse light with haze or light cloudiness. Diffuse light available to a shaded site within the canopy is a function of exterior diffuse light levels and canopy density as affected by cultivar. Calculations of whole canopy photosynthetic potential from light data and photosynthetic light response, indicate that haze or cloudiness is needed to obtain maximum whole canopy rates.
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Lakso et al. (1976) studied this question.
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