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An experiment was conducted to investigate the morphologic characteristics and photosynthetic response of sun and shade leaves of mature pecan (Carya illinoinensis (Wangenh.) K. Koch) trees. Treatments were established according to leaf type (sun or shade leaves) and cultivar (Pawnee and Stuart). Sun leaves were chosen from those growing on exterior portions of the tree canopy and exposed to full sunlight for most of the day ($1500 mmolm -2 s -1 photosynthetic photon flux (PPF)). Shade leaves were those growing in interior parts of the tree canopy (#100 mmolm -2 s -1 PPF). Epidermis characteristics, leaf area, and chlorophyll (Chl) content were also measured. Results indicated that stomatal density (stomata/mm 2 ), leaf area, and leaflet area were greater in sun leaves than in shade leaves in both cultivars investigated. Specific leaf area was greater in shade leaves than sun leaves. Chlorophyll fluorescence, total Chl content, Chl a, Chl b, and Chl a/b were unaffected by leaf type or cultivar. In both cultivars, photosynthetic light response curves showed that area-based maximum assimilation rate (Amax) in shade leaves was about half of that measured in sun leaves in June through August. However, in October, Amaxof sun leaves dropped to values similar to those measured in shade leaves. Light compensation point of photosynthesis and dark respiration rate were always lower in shade leaves than in sun leaves. Overall, there were only minor differences between the cultivars. Pecan trees require careful canopy management to avoid self shading and to maintain productivity. These results could help determine optimal levels of canopy light interception and could be used to develop canopy and crop management practices.

Photosynthetic Light Response and Epidermal Characteristics of Sun and Shade Pecan Leaves