Comparative Responses of Crops and Weeds

15.2.1 Climatic change and photosynthetic pathway

Almost all crop and weed species fall into two of the three major photo-synthetic pathways: C3 and C4. Plants with the CAM pathway, such as pineapple and cacti, will not be discussed here (see Chapter 14, this volume). Numerous observations of the response of growth of C3 and C4 species to elevated [CO2] support the expectation, discussed earlier, that C3 species are more responsive than C4 species. In a survey of responses of crop species, Kimball (1983) found that increases in biomass in response to [CO2] doubling from 350 to 700 |mmol mol-1 averaged 40% and 11% in C3 and C4 species, respectively. Very similar percentages resulted from the analysis of Cure and Acock (1986). However, studies that include non-crop C4 species (reviewed in Poorter, 1993 and Patterson, 1995a) indicate substantial responses in some C4 species. Ziska and Bunce (1997a) reported larger stimulation in both photosynthesis and biomass with [CO2] doubling among weedy C4 species as a group than among C4 crop species. Reasons for this have not been identified. Among C3 species, crop and weed species tend to have similar stimulation of growth by elevated [CO2] (e.g. Poorter, 1993; Bunce, 1997), probably because they have similar, high relative growth rates, which enhance the stimulation of biomass compared with slower growing species (Hunt et al., 1993; Poorter, 1993; Bunce, 1997). Not surprisingly, studies in which C3 and C4 species were grown in direct competition have almost invariably found that elevated [CO2] favoured the C3 species (reviewed in Patterson and Flint, 1990; see also Table 15.4). An exception was Amaranthus retroflexus competing with Abutilon theophrasti (Bazzaz et al., 1989). Amaranthus retroflexus is one of the C4 species most responsive to increased [CO2] (Ziska and Bunce, 1997a). Because the range of responses of biomass stimulation for C3 and C4 species overlap (Poorter, 1993), such exceptions will occur.

It is generally thought that global warming would favour C4 species relative to C3 species. This is based on the observation that C4 species are distributed in warmer environments and generally have higher optimum temperatures for photosynthesis and growth, and because their higher intrinsic water-use efficiency (the ratio of photosynthesis to transpiration) might better adapt them to the greater evaporative demand that would result from warming. However, because warmer temperatures are only likely to occur if

[CO2] is also high, the relevant question is whether the combination of increased temperature and elevated [CO2] would favour C3 or C4 plants.

At high [CO2], the optimum temperature for photosynthesis of many C3 plants increases and becomes similar to that of C4 plants (e.g. Osmond et al., 1980; cf. Leegood and Edwards, 1996). This is expected since the main function of the C4 cycle is to concentrate CO2 at the same enzyme used by C3 plants. Differences among species in the optimum temperature for photosynthesis at high [CO2] are probably related to membrane stability (Bjorkman et al., 1980), although this may be manifest as enzyme deactivation (e.g. Kobza and Edwards, 1987). It might be expected, from the increase in the optimum temperature for photosynthesis at elevated [CO2], that the stimulation of growth of C3 species by elevated [CO2] would be greater at warm than at cool temperatures (Long, 1991). This pattern is commonly observed (Imai and Murata, 1979; Idso et al., 1987), although there are numerous exceptions, both for photosynthetic stimulation (Greer et al., 1995) and for growth (e.g. Patterson et al., 1988; Tremmel and Patterson, 1993; Ziska and Bunce, 1997b). Nevertheless, for the modest increases in temperature expected from global warming (i.e. 1.5-4.5°C), the stimulation of growth by the combination of elevated [CO2] and elevated temperature will probably be greater in C3 than in C4 species. This is consistent with some of the limited experimental data (Patterson et al., 1988; Coleman and Bazzaz, 1992; Tremmel and Patterson, 1993). Exceptions have been observed when large temperature increases have been imposed (Read and Morgan, 1996) or critical threshold temperatures for reproductive damage in the C3 species have been exceeded (Alberto et al., 1996). High temperatures sometimes limit reproductive development and global warming may decrease reproductive output in such situations despite an increase in [CO2] (e.g. Wheeler et al., 1996; Ziska et al., 1997). It is unclear whether this is more likely to occur in C3 than C4 species, but if it were, it could alter weed community composition and affect crop/weed interactions. In the field, mean temperatures in temperate regions are usually substantially below the optimum temperature for vegetative growth even for C3 species -especially early in the season, when competitive outcomes are often determined (e.g. Kropff and Spitters, 1991). This is probably true everywhere except in warm arid regions, since mean temperatures even in humid tropical areas are usually below 27°C. In temperate zones, it is likely that planting times would be adjusted such that crops and weeds would be exposed to the same temperatures during the establishment phase as they are now, even if global warming occurs.

Higher temperatures would also create increased evaporative demand. In some respects, with its high water-use efficiency and CO2-saturated photosynthesis, C4 metabolism is better adapted to high evaporative demand (e.g. Bunce, 1983). However, this is another situation where, if increased [CO2] occurs in combination with higher temperatures, it becomes unclear whether C4 species would be favoured. This is because C3 species show at least as large an increase in water-use efficiency at elevated [CO2] as do C4 species. In addition to the reduced stomatal conductance that usually occurs in both photosynthetic types, C3 species also have increased photosynthesis at elevated [CO2]. This distinction is probably even more important in the field, where transpiration is much less sensitive to changes in stomatal conductance (McNaughton and Jarvis, 1991; Bunce et al., 1997), and photosynthetic responses would dominate changes in water-use efficiency at elevated [CO2].

15.2.2 Crop/weed competition

Many studies in which weed and crop species have been included in the same [CO2] treatments have been interpreted in terms of crop/weed competition even though the plants did not compete with each other. For example, weed/crop biomass ratios were decreased at elevated [CO2] in itchgrass (C4)/soybean (C3) comparisons (Patterson and Flint, 1980), and barnyardgrass, goosegrass and southern crabgrass (all C4)/soybean (C3) comparisons (Patterson, 1986). Weed/crop ratios were increased in itchgrass (C4)/maize (C4), velvetleaf (C3)/maize (C4), and velvetleaf (C3)/soybean (C3) comparisons (Patterson and Flint, 1980). Many additional such comparisons could be gleaned from the literature, but they are only suggestive of how crop/weed interactions might change with increased [CO2], because the outcome of competition is notoriously difficult to predict from comparisons of the growth of species in isolation. This is illustrated by the work of Bazzaz and co-workers (Bazzaz and Carlson, 1984; Zangerl and Bazzaz, 1984; Bazzaz and Garbutt, 1988), who measured species responses to [CO2] when plants were grown alone or in competition and found little relationship between the relative responses of isolated and competing plants.

The few studies in which crop and weed species have been grown in competition with each other at different [CO2] can be divided into those in which the crops and weeds differed in photosynthetic pathway and those in which the pathway was the same. All studies in which the photosynthetic pathway differed have compared C3 crops with C4 weed species. Increasing [CO2] increased the crop/weed (i.e. the C3/C4) ratio in all of these studies (Table 15.4). This pattern is consistent with the greater response of photosynthesis and growth to elevated [CO2] in C3 than in C4 plants. Similarly, in rangelands of the southwestern USA, the increase in [CO2] from preindustrial to the current values has strongly favoured C3 over C4 species (Johnson et al., 1993; also see Chapter 14, this volume), although in that system the C3 species are often considered weeds.

Only a very few studies have examined crops and weeds grown in competition with each other at different [CO2] when both species had the same photosynthetic type (Table 15.4). Several studies, including a few field studies, have indicated that the species dominance in grassland or pasture mixtures may change with [CO2] (e.g. Owensby et al., 1993; Luscher et al., 1998; see also Chapter 12, this volume). Two such studies of pasture mixtures in which the 'weed' species were identified are included in Table 15.4, and in both, the weed species were favoured by elevated [CO2]. In the study involving Chenopodium album and sugarbeet (Table 15.4), the competitive advantage of sugarbeet at elevated [CO2] was attributed to late emergence in the weed

Table 15.4. Studies in which crop and weed species were grown in competition as a function of [CO2].

Weed

Crop

High CO2

favours Environment Reference

Differing photosynthetic pathway

Sorghum halepense (C4) Sorghum halepense (C4) Echinochloa glabrescens

Paspalum dilatatum (C4) Various grasses (C4)

Same photosynthetic pathway

Chenopodium album (C3) Sugarbeet

Meadow fescue

Crop

Glasshouse

Carter and Peterson, 1983

Soybean

Crop

Chamber

Patterson et al, 1984

Rice

Crop

Glasshouse

Alberto et al., 1996

Various grasses

Crop

Chamber

Newton et al, 1996

Lucerne

Crop

Field

Chamber Houghton and Thomas, 1996

Field Bunce, 1995

Chamber Newton et al., 1996 Field Potvin and Vasseur, 1997

Crop

Taraxacum officinale (C3) Plantago lanceolata (C3) Taraxacum and Plantago

Lucerne Weed

Various grasses Weed Various grasses Weed

Chamber Houghton and Thomas, 1996

Field Bunce, 1995

Chamber Newton et al., 1996 Field Potvin and Vasseur, 1997

species in this particular experiment (Houghton and Thomas, 1996). The authors noted that competitive outcome in this system, as in many others, depends critically on the timing of emergence, which could be influenced by climatic change. In the lucerne study (Bunce, 1995), the competitive advantage of Taraxacum officinale probably occurred because lucerne growth was not stimulated by elevated [CO2] in the long term. The outcome of crop/weed competition at elevated [CO2] within a photosynthetic type will thus probably be determined more by factors affecting plant development than by differences in photosynthetic responses.

15.2.3 Elevated [CO2] and plant development

As has been recognized for many years by crop physiologists (e.g. Potter and Jones, 1977; Evans, 1993), differences in photosynthate partitioning or allocation (i.e. allometric relationships) are often more important in determining plant growth rates than are differences in single-leaf photosynthetic rates. This probably also applies to comparative responses to environmental change. For example, in a study of ten C3 species, differences among species in leaf photosynthetic response to elevated [CO2] provided a good prediction of the stimulation of whole-plant photosynthetic rate, but the stimulation of photosynthesis was a poor predictor of the increase in biomass at elevated [CO2] (Bunce, 1997). The increase in biomass was more related to species differences in relative growth rate (Bunce, 1997), which is strongly influenced by patterns of allocation of photosynthate. Differences in allocation are probably even more crucial in determining competitive outcomes, where the difference between success and failure may sometimes simply depend on leaf height. Elevated [CO2] sometimes increases height growth (e.g. Patterson and Flint, 1982), but it is unknown whether there is a differential response for crops and weeds grown in competition.

Many aspects of plant development are affected by [CO2] - for example, rates of germination, leaf initiation, tillering, branching, flowering and senescence, all of which could affect crop/weed interactions. One potentially important effect which has been documented under field conditions is the more rapid emergence of weed seedlings at elevated [CO2] (Ziska and Bunce, 1993). More rapid emergence at elevated [CO2] occurred more often in species with small rather than large seeds, suggesting that weeds may be more affected than most crops.

Elevated [CO2] can also alter the time of flower initiation or the rate of floral development. Flowering can be faster, slower or unchanged at elevated [CO2], depending on species (reviewed in Patterson, 1995a), but no clear patterns have emerged relative to crop/weed interactions. Reekie et al. (1994) reported that elevated [CO2] delayed flowering in four short-day species and hastened it in four long-day species, but these responses are not universal. Little is known concerning how increasing [CO2] could affect seed quality or viability in weeds. In some crops (Conroy, 1992), there is a reduction in nitrogen or protein content in seeds developed at elevated [CO2]. However, the impact of increasing [CO2] on seed viability for either crops or weeds has not been investigated. None of the studies to date in which crop/weed competition was compared at different [CO2] with annual crop species has been of long enough duration to include more than one generation of weeds. In one long-term field study with lucerne, elevated [CO2] increased the dominance of one weed species relative to the other weeds (Bunce, 1995). Because weed species vary in their responses to global climatic change, the composition of weed communities will undoubtedly change (Zangerl and Bazzaz, 1984).

15.2.4 Interaction of [CO2] with water and nutrients

Potential increases in global temperature may be accompanied by changes in precipitation patterns and increased frequency of drought. Even when water is limited, elevated [CO2] can stimulate plant growth (e.g. Patterson, 1986; Choudhuri et al., 1990). Plants with C4 photosynthetic metabolism sometimes only have increased photosynthesis and growth at elevated [CO2] under dry conditions (e.g. Patterson, 1986; Knapp et al., 1993), when elevated [CO2] slows the development of stress. Whether dry conditions would alter the competitive relationships between C3 and C4 species at elevated [CO2] is not known. Within a photosynthetic pathway, crops and weeds have reasonably similar responses to drought (Patterson, 1995b), although the impact of weeds on crop production may decrease because of reduced growth of both weeds and crops (Patterson, 1995b). Studies which have examined the interactive effects of increasing [CO2] and water stress on annual weeds and crops competing with each other are unavailable. In a pasture mixture (Newton et al., 1996), the proportion of weed biomass increased with [CO2] to a similar extent in wet and dry treatments.

Under extreme nutrient deficiencies, there may be no response of biomass to elevated [CO2]; under moderate limitations more relevant to agricultural situations, the increase in biomass may be reduced but the relative stimulation by elevated [CO2] is often similar (e.g. Wong, 1979; Rogers et al., 1993; Seneweera et al., 1994). In some species even moderate phosphate deficiencies eliminate biomass stimulation (e.g. Goudriaan and de Ruiter, 1983). As in the case of water stress, reductions in growth caused by nutrient deficiency may reduce the impact of weeds on crop production (Patterson, 1995b), since smaller plants interfere less with each other. In some C4 species, the growth response to elevated [CO2] may only occur under nutrient-deficient conditions (Wong and Osmond, 1991). However, as with the interactive effects of [CO2] and water availability, we are not aware of any study that has directly examined the interaction between increasing [CO2] and nutrient availability when crops and weeds were competing with each other. In a study of a pasture mixture, Schenk et al. (1997) presented data suggesting that elevated [CO2] may affect how the outcome of competition between ryegrass and white clover varies with the nitrogen supply.

15.2.5 Uncertainties and limitations

To date, the majority of the data concerning crop/weed interactions with increasing [CO2] and changing climate are based on studies in controlled environment chambers or glasshouses. This is a substantial limitation, since extrapolation of results from such environments to field conditions is highly uncertain. For example, Ghannoum et al. (1997) found that the biomass of a C4 grass species increased substantially at elevated [CO2] under field conditions, but did not respond in a controlled environment chamber. This was attributed to a photomorphogenic effect of the artificial light, which limited stem extension in the chamber. It is understandable that crop scientists doing field work on global change have tried to eliminate weeds as a variable in their experiments, but we urge that some field work be directed toward understanding the interactive responses of weeds as well as crops. Increasing [CO2] could alter the production of secondary compounds, which could in turn affect allelopathy between crops and weeds, but this has not been examined to date. Most studies of crop/weed interactions and global change have dealt only with the separate responses of annual crops and their associated weeds. A largely different set of weed species are associated with perennial and woody crops (Holm et al., 1977) and these have received almost no experimental attention with respect to global change. No information is available on how long-term exposure to increased [CO2] by itself or in conjunction with other environmental changes (e.g. water, temperature and nutrients) will affect crop/weed interactions in the field. Yet, such information is necessary to predict how changes in [CO2] and climate may alter weed populations and crop losses due to weeds.

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