Photosynthesis and conductance of spring‐wheat leaves: field response to continuous free‐air atmospheric CO₂ enrichment

Abstract

Spring wheat was grown from emergence to grain maturity in two partial pressures of CO 2 (pCO 2 ): ambient air of nominally 37 Pa and air enriched with CO 2 to 55 Pa using a free-air CO 2 enrichment (FACE) apparatus. 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 provided a unique opportunity to examine the hypothesis that continuous exposure of wheat to elevated pCO 2 will lead to acclimatory loss of photosynthetic capacity. The diurnal courses of photosynthesis and conductance for upper canopy leaves were followed throughout the development of the crop and compared to model-predicted rates of photosynthesis. The seasonal average of midday photosynthesis rates was 28% greater in plants exposed to elevated pCO 2 than in contols and the seasonal average of the daily integrals of photosynthesis was 21% greater in elevated pCO 2 than in ambient air. The mean conductance at midday was reduced 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 afternoon before the onset of grain-filling and over the whole diurnal course at the end of grain-filling. The loss of enhancement at this final stage was attributed to the earlier senescence of flag leaves in elevated pCO 2 . In contrast to some controlled-environment and field-enclosure studies, this field-scale study of wheat using free-air CO 2 enrichment found little evidence of acclimatory loss of photosynthetic capacity with growth in elevated pCO 2 and a significant and substantial increase in leaf photosynthesis throughout the life of the crop.

Carbon Assimilation, Biomass Partitioning and Productivity in Grasses

“…However, this is strongly species-specific; in loblolly pine (Pinus taeda L.) saplings fed nitrate as their sole N source, CO2 enrichment led to a suppression of growth compared to plants fed ammonium, while in the same study, neither elevated CO2 levels nor nitrogen form had much of an effect on wheat (Triticum aestivum L.) growth [24]. While some studies have demonstrated acclimatory decreases in leaf photosynthesis as a result of elevated CO2 levels [25], others have noted increases in leaf photosynthetic rates [26], which may be linked to decreased stomatal conductance and improved water use efficiency. Clearly there remain significant questions about the photosynthetic and growth responses in C3 grasses, and the importance of elevated CO2 to future productivity.…”

Photosynthesis in a CO2-Rich Atmosphere

“…The magnitude of the stomatal response generally does not diminish with time in long-term experiments (Lee et al, 2001;Gunderson et al, 2002;Ainsworth et al, 2003). However, it does vary on hourly and daily timescales (Garcia et al, 1998;Rogers et al, 2004). On four dates during 2003, the varying effect of elevated [CO 2 ] on g s in soybean fumigated by FACE technology in the field could be predicted with the Ball et al (1987) model (Leakey et al, 2006a).…”

The growth of soybean under free air [CO2] enrichment (FACE) stimulates photosynthesis while decreasing in vivo Rubisco capacity

“…The use of open-top chambers provides unlimited rooting volume under field conditions. However, these chambers disrupt the coupling between the canopy and atmosphere, reduce exposure to wind, and alter precipitation and light environments of the canopy, thereby causing an effect of similar magnitude to, or even greater than, the CO 2 treatment itself (Allen et al 1992;Garcia et al 1998). The use of free air [CO 2 ] enrichment (FACE) technology ensures that plant growth and development occur in an environment that minimizes the potentially confounding effects of enclosures (Hendrey et al 1993;Kimball et al 1997;Garcia et al 1998;McLeod and Long 1999).…”