Photosynthesis in willows (Salix × dasyclados) grown at different CO₂ concentrations and fertilization levels

Willows (Salix x dasyclados) were grown for 4 months in growth chambers at four nutrient and CO₂ levels, and photosynthesis measurements were made during the latter half of this period. Photosynthesis became saturated at lower light intensities at low CO₂ concentrations than at higher ones. The effect of CO₂ concentration on photosynthesis was greater at higher temperatures. The willows grown at the highest CO₂ concentration (1000 ppm) had a lower photosynthetic capacity than the others when measured at various concentrations. The effect of nutrient status on photosynthesis clearly increased with rising CO₂ concentrations. Although photosynthetic acclimation took place to a certain extent at higher CO₂ concentrations, photosynthesis still remained higher the higher the growth concentration was. At each CO₂ level photosynthesis increased contemporaneously with leaf nitrogen content, but at each fertilization level a rise in CO₂ concentration slightly increased photosynthesis and reduced the nitrogen content. The relative increase in photosynthesis achieved by a rise in CO₂ was greater than the corresponding increase in biomass growth, whereas the effect of fertilization was greater on biomass growth than on the rate of photosynthesis in the same willows.