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.