Photosynthetic responses of black walnut (Juglansnigra) to shading

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Abstract
Photosynthetic characteristics and morphology of leaves of black walnut (Juglansnigra L.) seedlings grown under different types and degrees of shade were investigated by measuring insitu gross photosynthesis (Ps) and by microscopic study of leaf material. During the growing season of 1979, seedlings were subjected to artificial shading of two types: the first type simulated two overstory densities (leaf-area indices of 1 and 2) with corresponding proportions of sunflecks and total transmission of 50 and 21%, respectively, of incident photosynthetically active radiation (PAR, 400–700 nm); the second consisted of two densities of greenhouse shading screen which transmitted approximately 16 and 3% of incident PAR. From quantum efficiency values derived from Ps – quantum flux density response curves it was determined that the walnut seedlings adjusted photosynthetically to shade. Quantum efficiency increased as much as 44% with the densest shading. Light-saturated Ps did not appear to decline with increased shading if sunflecks were present, and it declined only 18% under complete shade that transmitted about 16% of incident PAR. Estimated daily photosynthesis per unit leaf area for black walnut growing under heavy and complete shade (3% of incident PAR) was nearly one-half that of the unshaded control. Shading resulted in plants that possessed leaves that were thinner, had less palisade mesophyll, had lower stomatal density, and had more chlorophyll per unit of leaf area. These data indicate that black walnut seedlings have the capacity for substantial photosynthesis under shade and may be more shade tolerant than silvicultural classifications suggest.