Winter wheat (Triticum aestivum L., cv. Mercia) was grown at two different atmospheric CO2 concentrations (350 and 700 μmol mol−1), two temperatures [ambient temperature (i.e. tracking the open air) and ambient +4°C] and two rates of nitrogen supply (equivalent to 489 kg ha−1 and 87 kg ha−1). Leaves grown at 700 μmol mol−1 CO2 had slightly greater photosynthetic capacity (10% mean increase over the experiment) than those grown at ambient CO2 concentration, but there were no differences in carboxylation efficiency or apparent quantum yield. The amounts of chlorophyll, soluble protein and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco) per unit leaf area did not change with long‐term exposure to elevated CO2 concentration. Thus winter wheat, grown under simulated field conditions, for which total biomass was large compared to normal field production, did not experience loss of components of the photosynthetic system or loss of photosynthetic competence with elevated CO2 concentration. However, nitrogen supply and temperature had large effects on photosynthetic characteristics but did not interact with elevated CO2 concentration. Nitrogen deficiency resulted in decreases in the contents of protein, including Rubisco, and chlorophyll, and decreased photosynthetic capacity and carboxylation efficiency. An increase in temperature also reduced these components and shortened the effective life of the leaves, reducing the duration of high photosynthetic capacity.

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Attempts to select C3 plants with slow rates of photo-respiration and increased rates of net photosynthesis have met with little success. This review analyses the properties of mutant genotypes of tobacco (Nicotiana tabacum L. cv. Wisconsin), derived from selection of haploid plants (produced by in vitro mutagenesis of anthers) which survived in CO2 concentrations close to the compensation point. Survivors were diploidized and doubled-haploid plants were self-pollinated to obtain seeds (the selected genotypes). Several glasshouse and field experiments showed that the method of selection at low CO2 concentrations gave genotypes with increased capacity for total dry matter accumulation; increases were similar (mean 24%; range 14–36%) in different conditions for two selected genotypes (SP422 and SP451) when compared to the parental genotype Wisconsin-38. This increase was related to a greater leaf area per plant (mean increase 19%; range 9–43%), to faster photosynthetic rates in mature and old leaves and to similar rates of dark respiration per unit leaf area, but smaller rates per unit dry matter. These changes were related to a greater number of mesophyll cells of smaller size in the selected genotypes. However, the increased productivity could not be related to reduced photorespiration rate or CO2 compensation point nor to improved Rubisco properties (e.g. increased specificity factor) which the selection method was designed to achieve. Selection by survival at low CO2 produced genotypes able to invest more assimilate in growing larger leaves and to maintain a better leaf carbon balance than the parent genotype. These features improved light capture and carbon accumulation and thus increased dry matter production.

The requirements for the experimental study of the effects of global climate change conditions on plants are outlined. A semi-controlled plant growth facility is described which allows the study of elevated CO2 and temperature, and their interaction on the growth of plants under radiation and temperature conditions similar to the field. During an experiment on winter wheat (cv. Mercia), which ran from December 1990 through to August 1991, the facility maintained mean daytime CO2 concentrations of 363 and 692 cmm~^ for targets of 350 and 700 cm^ m~^ respectively. Temperatures were set to follow outside ambient or outside ambient +4°C, and hourly means were within 0-5 °C of the target for 92% of the time for target temperatures greater than 6°C. Total photosynthetically active radiation incident on the crop (solar radiation supplemented by artifical light with natural photoperiod) was 2% greater than the total measured outside over the same period.

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