Effects of [CO2 on biomass and grain yield

Biomass results from accumulation of carbon in plant products as the difference between photosynthesis and respiration, plus accumulation of minerals. Increasing biomass might, therefore, be expected to parallel stimulation of instantaneous Pn. However, in practice there are many complicating factors; for example, stimulation of root or canopy growth may allow additional resource capture. Conversely, increased C assimilation may result in nutrients being more limiting for growth, thus necessitating increased fertilizer applications, which may increase lodging and disease. At low temperatures, wheat growth is probably not limited by assimilation, but rather by sink capacity. These factors may explain the increases in wheat biomass that range from 0 to 40% in response to doubling [CO2]. Early field studies (reviewed in Lawlor and Mitchell, 1991) indicated responses ranging from 30% increase for a doubling of [CO2] to only a 20% increase for a quadrupling of [CO2]. Biomass of winter wheat in several experiments simulating field temperatures in the UK gave a 15-27% response to doubling [CO2] (Mitchell et al., 1993, 1995; Batts et al., 1997, 1998b), and a 6-34% increase when [CO2] was doubled in temperature gradient tunnels. Rawson (1995), in a similar Australian system, obtained increases from 7 to 36%, depending on temperature. In 25 experiments on spring wheat in OTCs at nine European sites, stimulation of biomass with [CO2] varied from 10% with a 320 |J.mol mol-1 enrichment to 75% with a 246 mmol mol-1 enrichment (Bender et al., 1999). Regression analyses using all the data indicated a 13% increase in above-ground biomass resulting from a 100 |J.mol mol-1 enrichment. Under the FACE conditions, biomass increased by 10 and 15% in 2 years, with a 200 |J.mol mol-1 increase in [CO2] (Kimball et al., 1997a,b; Pinter et al., 1996, 1997).

Response of wheat to extra C assimilate is flexible, and growth of all plant parts can be stimulated. Main stems are usually least affected. Tiller production and survival and root growth are stimulated most, but leaf size, grains per ear and grain size can all be increased, depending on environment and cultivar. Grain yield increase resulting from elevated [CO2] is often similar to that of biomass, but not always (Mitchell et al., 1995; Batts et al., 1997). Grain yield is more sensitive to biomass production during the reproductive phase of growth, particularly around anthesis, than during the vegetative growth phase. It has been shown that effects of elevated [CO2] on harvest index can be explained in terms of the relative increase of biomass in these different periods (Mitchell et al., 1996). In multiple-site OTC experiments using spring wheat, cv. Minaret, the average increased grain yield was 11% per 100 |mmol mol-1 enrichment (Bender et al., 1999), which is similar to that from an experiment on winter wheat cv. Mercia (Mitchell et al., 1995). The stimulation measured by Batts et al. (1997) varied greatly between seasons for a given temperature but was, on average, equivalent to 17% per 100 |mmol mol-1 enrichment. All these increases are greater than the c. 7% from FACE experiments (Pinter et al., 1996, 1997).

It would be expected (see arguments above on photosynthetic capacity) that yield increases resulting from elevated [CO2] are less with smaller N supply. While this was the direction of the interaction in both FACE and the multiple-site OTC experiments, it was not significant. Since CO2 stimulates root growth (Chaudhuri et al., 1990b; Wall et al., 1996), it may enable plants to capture more N in the field. When the total amount of N is strictly controlled, however, there is a strong interaction; for example, increased spring wheat grain yields resulting from elevated CO2 in controlled environment experiments were 5, 10 and 18% with N supplies of 4, 9 and 24 g m-2, respectively (Theobald et al., unpublished). Theory also predicts that the degree of stimulation will be less at lower light intensities, but this has not been explicitly tested in wheat. Interactions with temperature and water status are more complex and are discussed below.

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