Do open‐top chambers overestimate the effects of rising CO<sub>2</sub> on plants? An analysis using spring wheat

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<jats:title>Abstract</jats:title><jats:p>The microclimate in facilities for studying effects of elevated CO<jats:sub>2</jats:sub> on crops differs from ambient conditions. Open‐top chambers (OTCs) increase temperature by 1–3 °C. If temperature and CO<jats:sub>2</jats:sub> interact in their effect on crops, this would limit the value of OTC experiments. Furthermore, interaction of CO<jats:sub>2</jats:sub> and temperature deserves study because increases in atmospheric CO<jats:sub>2</jats:sub> concentration are expected to cause global warming.</jats:p><jats:p>This paper describes two experiments in which a recently developed cooling system for OTCs was used to analyse the effects of temperature on photosynthesis, growth and yield of spring wheat (<jats:italic>Triticum aestivum</jats:italic> L., cv. Minaret). Two levels of CO<jats:sub>2</jats:sub> were used (350 and 700 ppm), and two levels of temperature, with cooled OTCs being 1.6–2.4 °C colder than noncooled OTCs.</jats:p><jats:p>Photosynthetic rates were increased by elevated CO<jats:sub>2</jats:sub>, but no effect of temperature was found. Cross‐switching CO<jats:sub>2</jats:sub> concentrations as well as determination of <jats:italic>A–C</jats:italic><jats:sub><jats:italic>i</jats:italic></jats:sub> curves showed that plant photosynthetic capacity after anthesis acclimated to elevated CO<jats:sub>2</jats:sub>. The acclimation may be related to the effects of CO<jats:sub>2</jats:sub> on tissue composition: elevated CO<jats:sub>2</jats:sub> decreased leaf nitrogen concentrations and increased sugar content. Calculations of the seasonal mean crop light‐use efficiency (LUE) were consistent with the photosynthesis data in that CO<jats:sub>2</jats:sub> increased LUE by 20% on average whereas temperature had no effect. Both elevating CO<jats:sub>2</jats:sub> and cooling increased grain yield, by an average of 11% and 23%, respectively. CO<jats:sub>2</jats:sub> and temperature stimulated yield via different mechanisms: CO<jats:sub>2</jats:sub> increased photosynthetic rate, but decreased crop light interception capacity (LAI), whereas cooling increased grain yield by increasing LAI and extending the growing season with 10 days. The effects of CO<jats:sub>2</jats:sub> and temperature were not additive: the CO<jats:sub>2</jats:sub> effect was about doubled in the noncooled open‐top chambers. In most cases, effects on yield were mediated through increased grain density rather than increased individual grain weights.</jats:p><jats:p>The higher growth response to elevated CO<jats:sub>2</jats:sub> in noncooled vs. cooled OTCs shows that a cooling system may remove a bias towards overestimating crop growth response to CO<jats:sub>2</jats:sub> in open‐top chambers.</jats:p>

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