Abstract
The objectives of the present study were to examine (i) the effect of whole season CO 2 enrichment on seasonal radiation absorption and radiation use efficiency of above ground biomass production (RUE 1) of wheat, (ii) the relationship between daily radiation use efficiency and temperature, and (iii) the effect of CO 2 enrichment on this relationship in the period before and during grain filling when plant growth is assumed to be source and sink limited, respectively. During two consecutive years wheat ( Triticum aestivum L. cv. Minaret) was grown in open-top chambers at different plot sizes (1 and 3 m 2) at ambient and elevated CO 2 concentrations (ca. +280 ppm above ambient) with sufficient water and nutrient supply and analysed for final biomass and grain yield. In the 2nd year also light absorption by the green canopy and above ground biomass production were measured during the whole season. Canopy CO 2 exchange rates (CCER) were recorded on 50 days (2nd year) from stem elongation until canopy senescence with canopy chambers. CCER were used for the calculation of daily radiation use efficiency of the net CO 2 flux (dRUE 2) before and during grain filling. Daily net carbon assimilation was linearly related to absorbed photosynthetic active radiation. Mean seasonal RUE 2, which was calculated from this relationship, was increased by CO 2 enrichment. This corresponded to the findings for RUE 1. Seasonal light absorption was unaffected by the CO 2 treatment. Final biomass and grain yield were increased under CO 2 elevation by <11 and <13% in the 1st and 2nd year, respectively. Regression analysis yielded a significant negative relationship between dRUE 2 and temperature under ambient CO 2 in the period before and during grain filling. CO 2 enrichment mitigated this negative relationship in the period only before but not during grain filling. The present experimental findings support the theoretically expected decrease of RUE at ambient CO 2 and the increase of the CO 2 effect with temperature in the preanthesis phase. The results also indicate that the positive CO 2 x temperature interaction on canopy assimilation disappears during grain filling, which might be responsible for the decrease of the CO 2 effect on plant biomass between anthesis and grain maturity.
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