PHOTOSYNTHETIC ACCLIMATION OF LEAVES OF 3 APPLE CULTIVARS AS AFFECTEDBY GROWTH UNDER DIFFERENT LIGHT REGIMES

Photosynthetic acclimation of leaves of young 'Delicious,' 'Golden Del icious,' and apple trees (Malus domestica Bork.) as affected by growth under full sun or 15% sun and subsequent transfer to the contrasting light regime was studied. Photosynthetic capacity was reduced in 'Gran ny Smith,' 'Golden Delicious' and 'Delicious' apple leaves when grown under 15% sun. Leaf nitrogen per area (N) was highly correlated with C O2 assimilation (A), CO2 assimilation at saturating CO2 (A(max)), and carboxylation efficiency (g(m)'). The highest N was in leaves grown un der the highest photosynthetic photon flux (PPF). Photosynthetic capac ity under 15% sun was reduced to a greater extent in 'Granny Smith' co mpared to 'Golden Delicious' or 'Delicious'. 'Granny Smith,' however, acclimated photosynthetically to shade-to-sun transitions, whereas 'De licious' did not acclimate. Since photosynthetic capacity was greatly reduced by shade in 'Granny Smith' and since 'Delicious' and 'Golden D elicious' did not acclimate to shade-to-sun transitions the greatest p hotosynthetic productivity in these apple cultivars would be obtained by exposing the greatest portion of the canopy to full sun throughout the season.