Carbon Dioxide Exchange of Spring-wheat in Relation to Age and Photon-flux Density at Different Growth Temperatures

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Abstract
CO2-exchange rates (CER) of the sixth and the flag leaves of two spring-wheat varieties, Kolibri and Famos, were compared using an open-circuit infrared gas analysing system. Measurements were repeated every two weeks starting when leaf blades were fully expanded. Single plants were grown in a controlled environment having a photopuiod of 15 h and a day/night temperature of 24/19°C (H), 18/13 °C (M), and 12/7 °C (L) respectively until approx. 2 weeks after anthesis and at 18/13 °C until maturity. The photosynthetic photon-flux density (PPFD) at the top of the plants was 500 μE m−2 sec−1. During the measurements PPFD was gradually reduced from 2000 to 0 μE m−2 sec−1 whereas the temperature was maintained at the respctive growth-temperatures during the light period. The CER of the sixth leaf declined fairly similarly for both varieties, except for Kolibri where a faster decline was observed during the first two weeks after full leaf expansion. The CER of the flag leaf declined more slowly than that of the sixth leaf. With the flag leaf of Famos, the decline was nearly linear, whereas with Kolibri it was very slow during the first few weeks but rapid as the leaves further senesced. This pattern became more pronounced as the growth temperature decreased. The decline in relation to leaf age was much smaller at low PPFD than at high PPFD during the same period. At full leaf expansion Kolibri reached higher maximum CER than Famos except at H. As the PPFD was reduced the difference became smaller and at very low PPFD such as 50 μE m−2 sec−1 was reversed for the sixth leaf. Under optimum growth conditions maximum values of CER were greater than 50mg CO2 dm−2h−1 and PPFD for light saturation was close to 2000 μE m−2 sec−1. A comparison between the actual CER and a fitted curve widely used, PN=(a+b/l)−1−DR, showed that the goodness of fit strongly depends on cultivar, treatment and leaf age as well as on the number and the level of PPFD from which data for calculations are taken.