Effects of doubled atmospheric CO2 concentration on apple trees I.: Growth analysis

Two year old apple (Malus domestica Borkh. cv. 'Golden Delicious') trees on 'M9' were exposed to ambient (350 +/-10ppm) and doubled (700+/-10ppm) atmospheric CO2 for 120d in growth chambers situated in an apple orhard. Doubled CO2 promoted tree growth, indicated by more leaves and shoots, a larger average leaf area, and total leaf dry weight per tree, as well as a wider canopy and trunk diameter. Average shoot length and diameter, shoot length per tree, tree height, and total leaf area increased, whereas specific leaf area and leaf area ratio of the trees decreased at 700 ppm CO2 When compared to trees grown under 350 ppm CO2, 700 ppm CO2-treated trees stimulated the mean daily growth rate of total leaf area by 73-130%, total shoot length by 161-162%, and plant dry mass by 134-158%, respectively. Impacts of long-term atmospheric CO2 enrichment on dry weight ratio in different components of the trees were more prominent in above-ground organs than in roots. High CO2 raised leaf and shoot dry weight ratio, but reduced stem dry weight ratio, whereas root dry weight ratio and root/shoot ratio of the trees were not affected throughout the experiment. On average, net assimilation rate and relative growth rate were 0.6-1.7gm(-2)d(-1) and 1.2-3.9gkg(-1)d(-1) higher in trees grown at 700 than in those at 350ppm CO2 but tended to decline with the duration of CO2 treatment. The physiological and morphological mechanisms for the increased apple tree growth and plant development by the doubled atmospheric CO2 concentration were associated with net assimilation rate and relative growth rate of the plants as well as cell differentiation and elongation of the plant organs. The adjustment of plant growth under long-term atmospheric CO2 enrichment resulted from the plant adaptation to high CO2 conditions, which was affected by carbon sink/ source availability and strength.