The Carbon Budget of a Young Apple Tree1

Photosynthesis and dark respiration of the above ground components of young orchard-grown apple trees (Malus domestica Borkh.) were measured by infrared gas analysis. Soil and root respiration were determined by CO2 absorption. Maximum net photosynthesis (Pn) of shoots was 2.0 g CO 2 m –2 hr –1 and light saturation level was 500 Wm –2 (global solar radiation). Photosynthetic efficiencies of shoots were at a maximum (6%) at 100 Wm –2 was 2 to 3% at light saturation. Pn rates of stems and fruits were very low and even at high irradiance did not exceed the compensation point. Dark respiration (Rd) increased exponentially with temperature. Rd per unit weight was comparable for leaves and stems, less for roots and very little for fruits; per unit area the rates for leaves and fruit were small, while those for stems were large. Estimates of dry matter accumulation by growth analysis and by net CO 2 uptake were remarkably similar, despite a gross simplifying assumption with regard to light interception by the leaves.