THE EFFECT OF DEFRUITING AT DIFFERENT STAGES OF FRUIT-DEVELOPMENT ON LEAF PHOTOSYNTHESIS OF GOLDEN-DELICIOUS APPLE

Net photosynthetic rates (A) of leaves on 11-year-old, field-grown app le trees (Malus domestica Borkh. cv. Golden Delicious) were measured a fter removal of fruits at four different stages of development. Defrui ting decreased A by 21, 42, 27 and 7% when fruits were growing at 311, 293, 229 and 113 mg(DW) day(-1), respectively. Photosynthesis was inh ibited more in the afternoon than in the morning, but it was not affec ted during the first 8 h after fruit removal. Inhibition of A was posi tively correlated with crop sink strength, but it was not correlated w ith fruit relative growth rate or crop load. Defruiting decreased A at saturating irradiances (PPFD > 1000 mu mol m(-2) s(-1)), but did not modify the apparent quantum yield of single leaves. These results sugg est that the overall effect of defruiting on carbon fixation is neglig ible in dense canopies, but it may be significant in sparse canopies a nd in single shoots.