INTERACTIVE EFFECTS OF ELEVATED CO2 AND MINERAL-NUTRITION ON GROWTH AND CO2 EXCHANGE OF SWEET CHESTNUT SEEDLINGS (CASTANEA-SATIVA)

The effects of elevated atmospheric CO2 (700 mumol mol-1) and fertilization were investigated on 2-year-old sweet chestnut (Castanea sativa Mill.) seedlings grown outdoors in pots in constantly ventilated open-sided chambers. Plants were divided into four groups: fertilized controls (+F/-CO2), unfertilized controls (-F/-CO2), fertilized + CO2-treated plants (+F/+CO2) and unfertilized + CO2-treated plants (-F/+CO2). Dry matter accumulation and allocation were measured after one growing season and CO2 exchange of whole shoots was measured throughout the growing season. Shoot growth and total leaf area of unfertilized plants were not affected by elevated CO2, whereas both parameters were enhanced by elevated CO2 in fertilized plants. Elevated CO2 increased total biomass by about 20% in both fertilized and unfertilized plants; however, biomass partitioning differed. In unfertilized plants, elevated CO2 caused an increase in root growth, whereas in fertilized plants, it stimulated aboveground growth. At the whole-shoot and leaf levels, photosynthetic activity of both fertilized and unfertilized plants increased in response to elevated CO2, but the seasonal pattern of this enhancement varied with nutrient treatment. In unfertilized plants, a downward acclimation of photosynthesis was observed early in the season (June), and was related to reductions in nitrogen and chlorophyll content and to starch accumulation. The decrease in the slope of the A/Ci curve suggested a decrease in Rubisco activity. In both fertilized and unfertilized plants, shoot respiration decreased during the night in response to elevated CO2 until mid-July. The decrease was not related to changes in sugar concentration.