Implications for Agriculture, Forests and other Ecosystems
The general procedure in such field experiments is to supplement ambient sunlight with special fluorescent UV lamps filtered to supply either extra UV-B radiation (treatment) or with the UV-B removed (control). The methodology has continuously been improved, e.g., by introduction of automatic systems that change the lamp output to more realistically simulate the UV-B supplement with proper balance with the existing sunlight. Therefore, older experiments, and especially those performed in glasshouse or growth chamber conditions, are presently considered to be less reliable.
The compilation of harvestable yield
in field experiments in Figure 3.4 indicates how variably different varieties
responded and also that many varieties did not respond in a significant
manner (statistically speaking) and a very few were even stimulated in
production. From the entire population of studies, there is a tendency
toward more negative effects.
|Fig. 3.4. Relative changes in yield (seed production) of four crops evaluated for UV-B radiation responsiveness in 49 field trials with UV-B supplementation from lamps. Each bar represents results obtained with one variety in one field experiment in which ozone depletion was simulated (usually ca. 20% depletion). Soybean data from Teramura and Murali (1986), Sinclair et al. (1990), Teramura et al. (1990), D’Surney et al. (1993), and Miller et al. (1994); rice data from Nouchi et al. (1997) and Olszyk et al. (1996); pea data from Mepsted et al. (1996); and mustard data from Conner and Zangori (1997). Most effects smaller than 10% were not statistically significant, but small sample sizes and other environmental factors may have obscured differences.|
In addition to quantitative changes in crop yield, evidence exists for qualitative changes as well. For instance, in the study mentioned above, UV-B radiation also resulted in small changes on the order of 1 to 5% in the protein and oil content of the soybean seed (Teramura et al., 1990).
Because of the broad range of response patterns in crop species, plant breeding and genetic engineering for UV tolerance is an important aspect to consider in order to avoid significant crop production losses. There may, however, be some qualitative changes in seed or foliage characteristics that accompany the development or use of more UV-B-tolerant varieties. This remains to be explored. Other agroecosystem consequences of elevated UV-B radiation are likely to be more important, such as changes in insect or pathogen susceptibility of crops.
Fortunately, there is some information for mid-temperate latitude tree species. Because they are long-lived, trees present the opportunity to observe the longer-term cumulative effects of UV-B exposure over several years for the same individuals. These cannot be explored in annual crop species. In a field study using loblolly pine (Sullivan and Teramura, 1992), seedlings from several different geographic regions were grown for three consecutive years under UV-B lamps in a field experiment. Seedlings were exposed to either ambient solar UV-B or ambient levels supplemented with the UV-B from lamps, similar to studies with soybean yield (Teramura et al., 1990). After the first year of UV-B exposure, reductions were observed in biomass of seedlings derived from several geographic areas. By the end of the third year, these biomass reductions were several-fold larger in one variety. These overall growth reductions were generally associated with small decreases in both roots and shoots, but not necessarily accompanied by reductions in photosynthesis. This may be due to changes in needle growth or shifts in allocation of biomass as has been found for some crop species. These results suggested that the effects of UV-B radiation may accumulate in long-lived plants such as trees.
The fact that decreases in conifer needle biomass and needle length and leaf area of broadleaf trees were not accompanied by sizeable reductions of photosynthesis (Sullivan and Teramura, 1992; Dillenburg et al., 1995; Sullivan et al., 1996) may be due to the very low penetration of UV-B radiation into older foliage. It appears that the decreased growth of leaves and conifer needles upon exposure to enhanced levels of UV-B radiation may be due, in part, to epidermal cell wall thickening. This might prevent cell wall extension and, thereby, growth of these cells (Liu and McClure, 1995; Sullivan et al., 1996). Thus, changes at the level of the epidermis, the first leaf cell layer to receive the incident radiation, can have other important consequences.
Although terrestrial ecosystems at
high latitudes are not highly productive for grazing, timber production,
etc., the influence of ozone reduction on these systems may be important
for several reasons. Carbon sequestration is generally quite high in these
ecosystems, including the extensive peat formations which are also being
studied in the Swedish subarctic and southern Argentinean systems. Compared
with other locations, these ecosystems are under the greatest ozone depletion,
especially in the Southern Hemisphere, and they also experience the greatest
warming as the global greenhouse effect intensifies. Thus, they are sensitive
indicators of several features of climate change. These high latitude ecosystems
are also very important for the survival of indigenous ethnic groups in
the Northern Hemisphere.