THOMAS A. DAY, CHRISTOPHER T. RUHLAND, and FUSHENG XIONG, Department of Plant Biology, Arizona State University, Tempe, Arizona 85287-1601
The Antarctic Peninsula provides a unique opportunity to examine the influence of climate change on plants. Stratospheric ozone depletion events over the continent during spring and early summer lead to well-documented enhanced levels of ultraviolet-B (UV-B) radiation [280-320 nanometers (nm); UV-B] levels (Booth et al. 1994; Madronich et al. 1995). In addition, mean summer air temperatures along the peninsula have risen more than 1°C in the last 45 years (Smith 1994; Smith, Stammerjohn, and Baker 1996).
The 1996-1997 field season (November to March) was the second year of our main field experiment on Stepping Stones Island, near Palmer Station, Antarctic Peninsula. We are using filters to manipulate UV levels and temperatures around naturally growing plants of Deschampsia antarctica (antarctic hair grass) and Colobanthus quitensis (antarctic pearlwort), the only vascular plant species native to Antarctica. The treatments involve reducing different components of UV radiation [UV-B and/or ultraviolet-A radiation (UV-A; 320-400 nm)] in combination with passively increasing temperatures around plants. Additionally, in some treatments, we supplement soil nutrients or water.
We assessed the performance of Deschampsia and Colobanthus under each frame by monitoring leaf photosynthetic rates, as well as more integrated measures such as plant growth rates and reproductive success. Reducing ambient UV radiation levels with filters did not appear to have any large effects on field net photosynthetic rates (Pn). Pn of both species, however, was usually higher under warming treatments than under ambient treatments. The notable exception was on warm, sunny days (canopy air temperature >20°), when Pn of plants under all treatments were negligible. Further laboratory experiments at Palmer Station and Arizona State University have confirmed that high temperatures are responsible for the depressions in Pn we observe in the field on warm days. Both species are quite sensitive to higher temperatures, and Pn begin to decline abruptly at temperatures above their photosynthetic temperature optima of 12°. The main reason for this sensitivity to supraoptimal temperatures is that these species have high rates of temperature-enhanced respiration; at higher temperatures, photosynthesis or carbon dioxide (CO2) assimilation is offset by high rates of respiration or CO2 evolution. Additionally, key enzymes in the photosynthetic Calvin Cycle of these species appear sensitive to higher temperatures and further depress photosynthetic rates.
Because of the sensitivity of the photosynthetic apparatus to higher temperatures in these species, continued regional warming might prove detrimental to their performance on the peninsula, but an assessment of their performance under rising temperatures also depends on
With respect to acclimation, when we grew both species under contrasting temperature regimes (ranging from 7 to 20°) in growth chambers at Arizona State University, their photosynthetic temperature optima changed very little (<2°C), suggesting that these species have a very limited ability to acclimate photosynthetically to warmer temperatures. Although little acclimation of the photosynthetic apparatus to warmer growing temperature regimes was apparent, somewhat surprisingly, plants grown at warmer temperatures (20°) had higher growth rates and produced more biomass than those grown at their photosynthetic temperature optima (12°). Thus, Pn may not be a straightforward predictor of plant growth rate and overall performance.
We did not detect any large changes in growth rates under our field warming treatments, although the slow growth rates exhibited by these species under field conditions may make such changes difficult to detect in only two seasons of field manipulations. In contrast, we found that leaf elongation rates of both species during the second field season were improved when we reduced ambient UV-B levels with filters. This finding suggests that enhanced levels of UV-B could be stunting leaf elongation and the growth of these species. Although the mechanism for this stunting is not clear, it does not appear to involve UV-induced reductions in Pn.
Our warming treatments had very strong effects on sexual reproduction of both species. Under warming, the reproductive structures of both species were more developed or mature throughout both of the first two growing seasons. In addition, under warming, Colobanthus produced more seeds per reproductive structure or capsule. However, these seeds were not heavier than seeds from ambient-temperature treatments, and they were no more viable than their ambient-temperature counterparts, based on seed germination studies.
Taken collectively, our preliminary results suggest that with continued regional warming and a greater prevalence of warm days during the growing season, Pn and carbon uptake of these species may be reduced, but growth may improve with these warmer temperatures. Indeed, increases in the number of individuals and populations of Colobanthus and Deschampsia have been documented over the past 5 years along the peninsula, and these increases have been attributed to rising temperatures (Fowbert and Smith 1994; Smith 1994; Grobe, Ruhland, and Day 1997). Results from this past field season also suggest that enhanced UV-B levels may stunt leaf elongation and growth in these species. Thus, enhanced UV-B associated with ozone depletion events could potentially offset some of the improvements in plant growth brought about by rising air temperatures.
We thank Erin Vining and William Karl for technical assistance and Antarctic Support Associates personnel for logistical support. This research was supported by National Science Foundation grants OPP 95-96188 and OPP 96-15268.
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