Interacting effects of photosynthetic photon flux density and temperature on canopy CO2 exchange rate of spring wheat under different CO2-concentrations

The objective of the present study was to quantify the interaction between photosynthetic photon flux density (PPFD) and temperature on canopy CO2 exchange rate (CCER) of wheat at ambient and elevated CO2-concentrations. Spring wheat (Triticum aestivum L. cv. Minaret) was grown from emergence to maturity in open-top chambers under ambient (360 ppm) and elevated CO2-concentrations (680 ppm). CCER was measured using a specifically designed open system consisting of two canopy chambers (ca. 1.26m(3) each) and a monitoring unit. Air temperature and light intensity were measured directly above the canopy. CCER was measured from the first node stage until anthesis when leaf area index ranged between 2-3. PPFD ranged from 0 - ca. 1,100 mu mol m(-2) s(-1) and temperature from 7.5 degrees-39 degrees C. CCER increased with increasing: temperature and PPFD and was light-satured at 800 mu mol m(-2) s(-1) under ambient CO2-concentration. CO2-enrichment stimulated CCER by up to 50 % but there was no complete light saturation. Under high PPFD conditions (>600 mu mol m(-2) s(-1)) and at low temperatures (<18 degrees C) as well as at high temperatures (>30 degrees C) and low PPFD conditions (<300 mu mol m(-2) s(-1)) the positive CO2-effect on CCER was reduced nearly to zero. Thus, temperature optima of the stimulation of CCER by elevated CO2-concentrations were found. The optimum temperature of CCER increased with increasing light intensity and was shifted by the CO2-enrichment to higher temperatures by ca. 0.5 degrees C (at low PPFD) - 2 degrees C (at high PPFD). The results were compared with the predicitions from recent mechanistic models of CCER.