PHOTOSYNTHESIS AND CONDUCTANCE OF SPRING-WHEAT LEAVES - FIELD RESPONSE TO CONTINUOUS FREE-AIR ATMOSPHERIC CO2 ENRICHMENT

Spring wheat was grown from emergence to grain maturity in two partial pressures of CO2 (pCO(2)): ambient air of nominally 37 Pa and air enr iched with CO2 to 55 Pa using a free-air CO2 enrichment (FACE) apparat us. This experiment was the first of its kind to be conducted within a cereal field without the modifications or disturbance of microclimate and rooting environment that accompanied previous studies, It provide d a unique opportunity to examine the hypothesis that continuous expos ure of wheat to elevated pCO(2) will lead to acclimatory loss of photo synthetic capacity. The diurnal courses of photosynthesis and conducta nce for upper canopy leaves were followed throughout the development o f the crop and compared to model-predicted rates of photosynthesis. Th e seasonal average of midday photosynthesis rates was 28% greater in p lants exposed to elevated pCO(2) than in contols and the seasonal aver age of the daily integrals of photosynthesis was 21% greater in elevat ed pCO(2) than in ambient air. The mean conductance at midday was redu ced by 36%. The observed enhancement of photosynthesis in elevated pCO (2) agreed closely with that predicted from a mechanistic biochemical model that assumed no acclimation of photosynthetic capacity. Measured values fell below predicted only in the flag leaves in the mid aftern oon before the onset of grain-filling and over the whole diurnal cours e at the end of grain-filling. The loss of enhancement at this final s tage was attributed to the earlier senescence of flag leaves in elevat ed pCO(2). In contrast to some controlled-environment and field-enclos ure studies, this field-scale study of wheat using free-air CO2 enrich ment found little evidence of acclimatory loss of photosynthetic capac ity with growth in elevated pCO(2) and a significant and substantial i ncrease in leaf photosynthesis throughout the life of the crop.