Photosynthetic acclimation of apple spur leaves to summer-pruning
A/Ci and photosynthetic light response curves for gas exchange characteristics were measured for spur leaves of 25-year-old 'Golden Delicious' and 'Granny Smith' apple trees (Malus domestica Borkh.) to investigate their acclimation capacity to a shade-to-sun transition in a commercial apple orchard. Spur leaves of both cultivars adapted to summer-pruning within 2 weeks, regardless of the time of the season at which pruning was conducted. There were no significant differences between the spur leaves of later pruned trees and corresponding leaves on trees that were continuously pruned during the summer period in terms of net photosynthetic rate and carboxylation efficiency, for both cultivars. The shape of the photosynthetic light response curves also proved the acclimation potential of both cultivars to shade-to-sun transitions. The rapid increase of the chlorophyll a/b ratio after pruning indicated that the acclimation of the photosynthetic system to avoid over-excitation of the photosynthetic reaction centers occurred within 1 week. Nevertheless, leaf dry mass per leaf area was intermediate between 'continuous summer-pruning' and 'no-pruning' treatments for leaves on later pruned trees, indicating that the acclimation to shade-to-sun transitions was not perfect in either cultivar. The present data also support the hypothesis that nitrogen, phosphorus, calcium, and magnesium are distributed to leaves growing under the highest photosynthetic photon flux density in order to maximize photosynthesis. © 2002 Elsevier Science B.V. All rights reserved.