EFFECTS OF FREE-AIR CO2 ENRICHMENT ON THE DEVELOPMENT OF THE PHOTOSYNTHETIC APPARATUS IN WHEAT, AS INDICATED BY CHANGES IN LEAF PROTEINS

A spring wheat crop was grown at ambient and elevated (550 mu mol mol( -1)) CO2 concentrations under free-air CO2 enrichment (FACE) in the fi eld, Four experimental blocks, each comprising 21-m-diameter FACE and control experimental areas, were used, CO2 elevation was maintained da y and night from crop emergence to final grain harvest, This experimen t provided a unique opportunity to examine the hypothesis that CO2 ele vation in the field would lead to acclimatory changes within the photo synthetic apparatus under open field conditions and to assess whether acclimation was affected by crop developmental stage, leaf ontogeny an d leaf age, Change in the photosynthetic apparatus was assessed by mea suring changes in the composition of total leaf and thylakoid polypept ides separated by SDS-PAGE, For leaves at completion of emergence of t he blade, growth at the elevated CO2 concentration had no apparent eff ect on the amount of any of the major proteins of the photosynthetic a pparatus regardless of the leaf examined, Leaf 5 on the main stem was in full sunlight at emergence, but then became shaded progressively as 3-4 further leaves formed above with continued development of the cro p, By 35 d following completion of blade emergence, leaf 5 was in shad e, At this point, the chlorophyll alb ratio had declined by 26% both i n plants grown at the control CO2 concentration and in those grown at the elevated CO2 concentration, which is indicative of shade acclimati on. The ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) cont ent declined by 45% in the control leaves, but by 60% in the leaves gr own at the elevated CO2 concentration, The light- harvesting complex o f photosystem II (LHCII) and the chlorophyll content showed no decreas e and no difference between treatments, indicating that the decrease i n Rubisco was not an effect of earlier senescence in the leaves at the elevated CO2 concentration. Following completion of the emergence of the flag-leaf blade, the elevated-CO2 treatment inhibited the further accumulation of Rubisco which was apparent in control leaves over the subsequent 14 d, From this point onwards, the flag leaves from both tr eatments showed a loss of Rubisco, which was far more pronounced in th e elevated-CO2 treatment, so that by 36 d the Rubisco content of these leaves was just 70% of that of the controls and by 52 d it was only 2 0%, At 36 d, there was no decline in chlorophyll, LHCII or the chlorop last ATPase coupling factor (CFI) in the elevated CO2 concentration tr eatment relative to the control, By 52 d, all of these proteins showed a significant decline relative to the control, This indicates that th e decreased concentration of Rubisco at this final stage probably refl ected earlier senescence in the elevated-CO2 treatment, but that this was preceded by a CO2-concentration-dependent decline in Rubisco.