EFFECTS OF ELEVATED CO2 ON GROWTH AND CHLOROPLAST PROTEINS IN PRUNUS-AVIUM

To predict the future carbon sequestering capacity of trees, we need information about the possible acclimatory mechanisms of plant growth and photosynthesis in rising atmospheric CO2 under a variety of environmental conditions. We have, therefore, studied the growth response of a tree species (Prunus avium L. Stella (wild cherry)) to elevated CO2 and characterized the associated changes in photosynthetic machinery of the leaf tissue. Self-pollinated seedlings and mature cuttings (clones) from the same parent plant of P. avium were grown for two consecutive growing seasons (about 60 days each) in ambient CO2 (350 mumol mol-1 CO2) or elevated CO2 (700 mumol Mol-1 CO2) with a high or low nutrient supply. The degree of acclimation of leaf biochemistry and growth response to elevated CO2 was dependent on the plant material (seedling or mature cutting) and nutrient supply. There was little or no growth response to elevated CO2 in seedlings or cuttings in the low nutrient supply treatments, whereas, in both seasons, there was a strongly positive growth response to elevated CO2 in seedlings and cuttings in the high nutrient supply regimes, resulting in increases in the root/shoot ratio and in carbon allocation to the roots. In contrast, the protein content and activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco, EC 4.1.1.39) were down regulated in elevated CO2. The loss of Rubisco on an area basis in plants in the elevated CO2 treatments was compensated for at the canopy level by increased leaf area. The loss of Rubisco protein was accompanied by decreases in the contents of chlorophyll and the thylakoid membrane proteins D1, D2 and cytochrome f, which are involved in light harvesting and photo-electron transport. We conclude that, in the medium- to long-term, the initial stimulation of biomass production by elevated CO2 may be increasingly offset by a lower photosynthetic capacity per unit leaf area in perennial plants.