EFFECT OF FRUITING ON CARBON BUDGETS OF APPLE TREE CANOPIES

Carbon budgets were calculated from net photosynthesis and dark respir ation measurements for canopies of field-grown, 3-year-old apple trees (Malus domestica Borkh.) with maximum leaf areas of 5.4 m(2) in a tem perature-controlled Perspex tree chamber, measured in situ over 2 year s (July 1988 to October 1990) by computerized infrared gas analysis us ing a dedicated interface and software. Net photosynthesis (Pn) and ca rbon assimilation per leaf area peaked at respectively 8.3 and 7.7 mu mol CO2 m(-2) s(-1) in April. Net photosynthesis (Pn) and dark respira tion (Rd) per tree peaked at 3.6 g CO2 tree(-1) h(-1) (Pn) and 1.2 g C O2 tree(-1) h(-1) (Rd), equivalent to 4.2 mu mol CO2 (Pn) and 1.4 mu m ol CO2 (Rd) m(-2) s(-1) with maximum carbon gain per tree in August an d maximum dark respiration per tree in October 1988 and 1989. In May 1 990, a tree was deblossomed. Pn (per tree) of the fruiting apple tree canopy exceeded that of the non-fruiting tree by 2-2.5 fold from June to August 1990, attributed to reduced photorespiration (R1), and resul ting in a 2-fold carbon gain of the fruiting over the non-fruiting tre e. Dark respiration of the fruiting tree canopy progressively exceeded , with increasing sink strength of the fruit, by 51% (June-August), 1. 4-fold (September) and 2-fold (October) that of the non-fruiting tree due to leaf (i.e. not fruit) respiration to provide energy (a) to prod uce and maintain the fruit on the tree and (b) thereafter to facilitat e the later carbohydrate translocation into the woody perennial parts of the tree. The fruiting tree reached its optium carbon budget 2-4 we eks earlier (August) then the non-fruiting tree (September 1990). In t he winter, the trunk respired 2-100 g CO2 month(-1) tree(-1). These da ta represent the first long-term examination of the effect of fruiting without fruit removal which shows increased dark respiration and with the increase progressing as the fruit developed.