Photosynthetic and growth responses of old and modern spring wheat cultivars to atmospheric CO2 enrichment

Cultivars of spring wheat (Triticum aestivum L.) introduced between 1890 and 1988 were cultivated in pots under optimal growth conditions and exposed during the whole growing season to normal (379 p.p.m.) and elevated CO2 concentrations (689 p.p.m.) in open-top chambers. CO2 effects were measured at anthesis on flag leaf composition (chlorophyll and protein) and photosynthetic parameters, and at maturity on plant growth and yield. CO2 enrichment did not affect light saturated rate of photosynthesis measured at 400 p.p.m. CO2 or protein, total chlorophyll and dry weight content per unit leaf area. However, single flag leaf area and fresh weight per leaf area were increased by CO2. This increase was possibly responsible for a significant decrease in the chlorophyll a/b ratio. Under normal atmospheric CO2 concentration, the total above-ground biomass, stem weight and height, and ear number were negatively correlated with the year of cultivar release. Despite no evidence of CO2 acclimation, i.e. changes in flag leaf composition, CO2 enrichment resulted in a greater growth stimulation of the older than the modern cultivars. This was due to a greater CO2 effect on those growth components that were altered during plant breeding of wheat in the past, i.e. stem weight and height, and ear number. The average CO2-related increase in biomass and grain yield amounted to ca 46% and 28% for the three old (1890–1943) and three modern cultivars (1965–1988), respectively. Differences in yield response to CO2 enrichment between old and modern cultivars could be mainly explained by changes in ear number.