Net photosynthesis, specific leaf weight and growth of apple leaves as affected by canopy position and leaf age
Shoots were selected in both interior and exterior canopy positions on 20-year-old 'Delicious' apple trees located at the VPI & SU Horticulture Farm orchard, Blacksburg, Va. The most recently unfolded leaf was tagged on April 28, May 12, and May 26, 1976. Shoot length was unaffected by canopy position but leaf number was greater on exterior shoots. One shoot from each canopy positions was detached for determination of net photosynthesis (Pn) and specific leaf weight (SLW) of tagged leaves on June 6, July 7, and August 4. Net photosynthesis and SLW of exterior leaves were greater than that of interior leaves. As leaf age increased, Pn tended to decrease while SLW increased. No significant correlation was found between SLW and Pn. Young container trees were placed under the canopy of the orchard trees and in a full sun location nearby. The most recently unfolded leaf was tagged on June 6, June 17, June 30, and July 14. Shoot length of the single stem tree was not affected by exposure, but leaf number was greater on sun trees. Net photosynthesis was determined on the same attached leaves at each time and was significantly higher for sun leaves than shade leaves. There were no differences in Pn as leaves aged. Specific leaf weight was determined once and was significantly higher for sun leaves. Maximum Pn rates were reached at approximately 13 klx for shade leaves while sun leaves had not reached maximum Pn at 27 klx.