6.2.3  Carbon dioxide

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Figure 6.10 Early growth of cucumber (Cucumis sativus) and wong bok (Brassica pekinensis) (b) is greatly enhanced in elevated CO2 (1250 ppm) compared with ambient controls (325 ppm). As shown here, that initial effect is still apparent after 52 d of greenhouse culture in nutrient rich potting mix. Scale bar = 10 cm. (Further details in Kriedmann and Wong (1984) and Table 6.4) (Photographs courtesy Maureen Whittaker)

 

Growth responses to elevated CO2 can be spectacular, especially during early exponential growth (Figure 6.10a, b) and derive largely from direct effects of increased CO2 partial pressure on photosynthesis. C3 plants will be most affected, and especially at high temperature where photorespiratory loss of carbon has the greatest impact on biomass accumulation.

Global atmospheric CO2 partial pressure is expected to reach 60–70 Pa (c. 600–700 ppm) by about 2050 so that growth response to a CO2 doubling compared with 1990s levels has received wide attention (e.g. Cure and Acock 1986; Poorter 1993). Instantaneous rates of CO2 assimilation by C3 leaves usually increase two- to three-fold but short-term response is rarely translated into biomass gain by whole plants where growth and reproductive development can be limited by low nutrients, low light, low temperature, physical restriction on root growth (especially pot experiments) or strength of sinks for photoassimilate. Given such constraints, photosynthetic acclimation commonly ensues (Chapter 13). Rates of CO2 assimilation (leaf area basis) by CO2-enriched plants, grown and measured under high CO2, will match rates measured on control plants at normal ambient levels.

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Figure 6.11 A survey of growth response to elevated CO2 (ratio of growth indices in 600-800 cf. 300-400 ppm CO2) in 63 different C3 species (a) and eight C4 species (b) reveals systematic differences in median values for growth indices that relate to photosynthetic mode. C3 plants show a positive response in NAR that results in slightly faster RGR despite some reduction in LAR. C4 plants reduce RGR under elevated CO2 due to diminished NAR. SLA of C3 plants is generally lower under elevated CO2, but increased somewhat in C4. LWR is essentially unchanged in either group (Based on Poorter et al. 1996)

Acclimation takes only days to set in, and because plant growth analysis commonly extends over a few weeks, CO2-driven responses in growth indices tend to be more con-servative compared with instantaneous responses during leaf gas exchange. Moreover, C4 plants will be less affected than C3 plants (for reasons discussed in Chapters 2 and 13) so that broad surveys need to distinguish between photosynthetic mode. For example, in Figure 6.11, average NAR for 63 different cases of C3 plants increased by 25–30% under 600–800 ppm CO2 compared with corresponding values under 300–400 ppm CO2. However, NAR increase was not matched by a commensurate response in RGR, and decreased LAR appears responsible. CO2-enriched plants were less leafy than controls (i.e. lower LAR), but not because less dry matter was allocated to foliage (LWR was on average unaltered). Rather, specific leaf area (SLA in Figure 6.11) decreased under high CO2 so that a given mass of foliage was presenting a smaller assimilatory surface for light interception and gas exchange. Accumulation of non-structural carbohydrate (mainly starch; Wong 1990) is commonly responsible for lower SLA in these cases, and in addition generally correlates with down-regulation of leaf photosynthesis.

By contrast, in C4 plants LWR was little affected by elevated CO2, but in this case SLA did show slight increase with some positive response in LAR. However, photosynthetic acclimation may have been more telling because NAR eased and RGR even diminished somewhat under elevated CO2.

Global change, with attendant increase in atmospheric CO2 over coming decades, thus carries implications for growth and development in present-day genotypes and especially the comparative abundance of C3 cf. C4 plants (Chapter 13), but elevated CO2 also has immediate relevance to greenhouse cropping. In production horticulture, both absolute yield and duration of cropping cycles are factors in profitability. Accordingly, CO2 effects on rate of growth as well as onset of reproductive development and subsequent development are of interest.

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Table 6.4

Young seedlings in their early exponential growth phase are typically most responsive to elevated CO2, so that pro-duction of leafy vegetables can be greatly enhanced. This response is widely exploited in northern hemisphere green-house culture (e.g. Wittwer and Robb 1964) and was put to good effect in ‘Head Start’ programs at Beltsville (Krizek et al. 1974). In commercial operations, ambient CO2 is often raised three- to four-fold so that growth responses can be spectacular (Figure 6.10a, b) but tend to be short lived (Table 6.4) as accelerated early growth gives way to lower RGR. During each cycle of growth and development, annual plants show a sigmoidal increase in biomass where an initial exponential phase gives way to a linear phase, eventually approaching an asymptote as reproductive structures mature. If CO2 enrichment hastens this progression, a stage is soon reached where RGR is lower under elevated CO2 due to accelerated ontogeny (see Gifford et al. 1996).

For example, wong bok (Brassica pekinensis in Figure 6.10b) is a highly productive autumn and winter vegetable that serves as ‘spring greens’ and is especially responsive to CO2 during early growth. In present trials (Table 6.4) RGRA at c. 330 ppm CO2 was initially 0.230 d–1 compared with 0.960 d–1 at c. 1350 ppm CO2, but by 40–52 d, RGRA had fallen to 0.061 and 0.020 d–1 for control and CO2 enriched, respectively.

CO2-driven response in NAR and RGR also diminished with age, and especially where these larger individuals failed to sustain higher RGR past 18 d (Table 6.4). Nevertheless, a response in NAR was maintained for a further two intervals so that a CO2 effect on plant size was maintained (Figure 6.10b).

Intensive greenhouse fruit crops such as tomato and cucumber are also raised under elevated CO2, and as noted above for cucumber and leafy greens, young plants are especially responsive (and in tomato, even at low light; Hurd 1968; Hurd and Thornley 1974). Marketable yield of fruit is also increased with CO2-enriched plants commonly flowering earlier and producing about 30% more crop over a whole season with early cycles of reproductive development typically more responsive (50% increase; Madsen 1974). Photosynthetic acclimation in CO2-enriched plants contributes to this diminished response over time, and has led to a management practice where CO2-enriched greenhouses gradually revert to ambient as cropping seasons progress. An alternative strategy might be to ‘pulse’ greenhouses with CO2 rather than enrich continuously, thereby forestalling photosynthetic acclimation. A duty cycle of 2 d enriched followed by 1 d ambient has been suggested (Kriedemann and Wong 1984).

Potato (Solamum tuberosum L.) offers an interesting variant in CO2 effects on growth indices where differentiation of tubers provides sinks that can sustain NAR response to CO2 (Table 6.5). In this experiment, over 400 potato plants were established in large containers of potting soil and held in a greenhouse (sunlight plus daylength extension to 15 h) under either ambient (300–370 ppm CO2) or enriched conditions (600–700 ppm CO2) from emergence to bloom (early en-rich-ment 0–55 d; phase 1) or from bloom to final harvest (late enrichment 55–110 d; phase 2). Tuber yields at 55 d were increased significantly from 5.5 g plant–1 in control to 10.9 g plant–1 under CO2 enrichment. Tuber number per plant was not significantly increased. By final harvest, tuber weight had increased to 17.5 and 22.0 g plant–1 for control and early enrichment respectively, but reached 30.5 g plant–1 in response to late enrichment (phase 2).

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Table 6.5

Moreover, plants receiving late enrichment also sustained their NAR at 3.49 g m–2 d–1 during phase 2 compared with 1.77 in early-enriched plants and 1.91 in controls (Table 6.5). Presumably, photoassimilate generated by leaves during late enrichment with CO2 was directed to tubers rather than accumulating in leaves and suppressing further assimilation. A strong ontogenetic progression was none the less evident in canopy development where relative rate of increase in leaf area per plant (RGRA) dropped by an order of magnitude between phase 1 and phase 2, and also became insensitive to elevated CO2.

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