Ozone (O3) is the most important air pollutant in rural areas in industrialized countries, and concentrations during the growing season are much higher than the threshold level for effects on vegetation (Fuhrer et al., 1997). Visible leaf injury, an indication of short-term effects, has been observed in a range of plant species, including white clover, a representative species of productive grasslands (Becker et al., 1989). Grass species found to be sensitive to ozone include orchard grass (Dactylis glomerata) and timothy (Phleum pratense) (see Mortensen, 1992). The effect of O3 depends largely on stomatal uptake and, thus, is influenced by environmental factors (such as VPD, atmospheric turbulence and soil moisture) that affect stomatal conductance.
Intraspecific differences in stomatal conductance may also be responsible for differences in O3 sensitivity of cultivars (Becker et al., 1989) and ecotypes (Nebel and Fuhrer, 1994). Long-term effects of O3 are caused by cumulative O3 exposure and consist of changes in growth, yield and reproduction (Fuhrer et al., 1997). A screening experiment revealed that growth reductions are least likely to occur in species characterized by a stress-tolerator strategy (i.e. S-strategy, sensu Grime, 1979) (Bungener et al., 1999). Physiological effects on individual leaves include reduced photosynthetic capacity, increased dark respiration and a reduced leaf duration. These effects can be associated with changes in biomass allocation of the whole plant. In graminoids, biomass is preferentially allocated to shoots at the expense of allocation to roots and seeds. In clover, biomass allocation to stolons and roots is reduced, which may in turn affect regrowth, winter survival (Blum et al., 1983) and N fixation (Montes et al., 1983). Rebbeck et al. (1988) observed a reduction in the starch content of roots but not of shoots of ladino clover with increasing O3; soluble-sugar contents were unaffected in both roots and shoots. In a 0.09 ppm O3 treatment, the root/shoot ratio was reduced by 24% in crimson clover and by 22% in annual ryegrass compared with an O3-free treatment (Bennett and Runeckles, 1977). In both species SLA and leaf area ratio (LAR) were lowered by O3, but increased net assimilation rates did not affect relative growth rate (RGR).
During growth and regrowth of pot-grown tall fescue, shoot dry weight was not affected by increasing O3 levels. However, under the same conditions, regrowth of ladino clover declined whether grown alone or in combination with fescue (Montes et al., 1982). In the same study, the total forage yield of the mixture was reduced, and the clover/fescue ratio declined. There is evidence that effects of O3 differ depending on whether plants are grown in monoculture or in mixtures. For instance, the total dry weight of crimson clover and annual ryegrass decreased less in a mixture. In a mixture of white clover and perennial ryegrass, constant elevation of O3 caused a decline in the clover fraction, which was partly compensated for by increased growth of ryegrass (Nussbaum et al., 1995). When the same total dose of O3 was applied episodically, growth of ryegrass and total forage yield declined. This indicates the importance of the effects of intermittent peak concentrations of O3 on the relatively O3-tolerant ryegrass.
The different effect of O3 on grasses and clover grown in potted mixtures was confirmed by experiments with field-grown plants. Two-year studies with timothy and red clover (Kohut et al., 1988) or tall fescue and ladino clover (Heagle et al., 1989) pastures showed clear increases in the grass/clover ratio, with increasing O3 causing a reduction in the clover fraction combined with a stable, or even larger, fraction of the grass. In the latter study, total forage yield was reduced by 10% at ambient levels of O3; there was no significant effect of reduced irrigation on the O3 response. In a 2-year experiment with a fescue and clover pasture, there was a negative effect of O3 on the fractional clover yield. The effect was much larger during the second year compared with the first year (Blum et al., 1983). In a 1-year experiment with two cutting frequencies, the effect of O3 on the ratio of ryegrass and the more sensitive clover varied from harvest to harvest, probably because of the time of cutting relative to the O3 episodes and differences in the phenology of the species (Wilbourn et al., 1995).
Two recent studies on the response of grass/clover mixtures to O3 have produced contrasting results. The study by Fuhrer et al. (1994), using a standard seed mixture, gave results very similar to those found by Heagle et al. (1989), i.e. no effect of O3 on the total 2-year yield of the grasses but a substantial decline in clover yield. In contrast, a Swedish study showed a linear decrease in forage yield with increasing O3, but no effect on the clover fraction (Pleijel et al., 1996). This suggests that the effect of O3 on the competitive balance depends on the ratio of the species and cultivar sensitivities. Overall, the available data suggest that the clover decline in grass and clover mixtures is accelerated by O3, but that the extent of the effect depends on the species and cultivars used (Fig. 12.1).
In ambient air, periods of elevated O3 occur at irregular intervals. Their effect on the competitive balance in plant mixtures depends not only on the impact during the O3 episodes, but also on the capacity of the species to recover between episodes. Wilbourn et al. (1995) observed quick recovery of a clover canopy after a period of elevated O3, but there was a persistent effect on stolon density, which may affect the long-term performance of clover. Experiments with ryegrass and clover mixtures confirmed that the overall effect on the clover proportion may be related to the cumulative exposure to O3 in all preceding growth periods (Nussbaum et al., 1995). In a frequently cut pasture, clover declined rapidly in plots exposed to elevated O3 during the first season but recovered during the second season when the same plots were exposed to a reduced concentration of O3 (Fuhrer et al., 1994). Therefore, the extent of negative effects of O3 on clover persistence in productive pastures could be counteracted by introducing tolerant lines of clover.
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