Air Pollution
Winter heating units can produce carbon dioxide, sulfur dioxide and nitrogen oxides. Carbon dioxide is an input to photosynthesis and increased levels of carbon dioxide can stimulate plant growth. Sulfur dioxide and nitrogen oxides, on the other hand, are detrimental to crop growth. These products are unavoidable even if clean fuel and a ‘low emissions’ burner are used.
Nitrogen Oxides or NOx
These substances include NO and NO2.
Plant response to NOx depends on the light irradiance as well as the nitrogen status of the soil. Different plant types are more/less sensitive to NOx. Tomatoes, for example, will show tissue necrosis at concentrations where lettuce will not.
Ambient concentrations in England are ~ 0.02 ppm (Caporn, 1989). Mathematical models have determined that even a low NOx blower will produce concentrations above the damage threshold depending on the outside temperature and wind velocity. For example, an outside temperature of 32F (0 C) and wind velocity of 9 mph (4 m/s) could produce a concentration that is a bit over 0.4 ppm, which exceeded the crop damage threshold of 0.2 ppm (Kiel, 1990).
Much of the research that has been done on the effects of NOx on plant growth has included elevated carbon dioxide levels in order to simulate conditions in a greenhouse where the NOx is produced by heaters that are located and vented into the greenhouse. This makes it extremely difficult to separate the detrimental effects of NOx and the beneficial effects of CO2. These studies are further complicated by the fact that the ambient air temperature was often much lower (as was the custom at the time) than air temperatures that are currently used in the winter for plant production. Since air temperature affects enzyme reaction rates and thus overall growth, another confounding layer is added to the evaluation and subsequent application of experimental results.
Short-term (1 hour or less at 2ppm) exposure to NOx has been demonstrated to have a reversible effect on plant growth decreasing photosynthesis to 88% (higher temperature) or 50% (lower temperature) of normal during exposure and recovering fully after about 20 minutes(Caporn, 1989; Caporn et al., 1994). Exposure to NOx can negate the positive effects of increased CO2 concentration (1500 ppm).
Growth of hydroponic lettuce under winter conditions (low light, low temperature and CO2 enriched) was reduced by 47% after 6 weeks of exposure to Nox at concentrations of 0.45 ppm which is typical of a propane burner used for heating and CO2 enrichment (Hufton et al., 1996). Additional long-term exposure to NOx demonstrated unexpected results. In 1996, Hufton et al. demonstrated that initial growth depressions caused by increased concentrations of NOx were reversed as the final harvest showed larger lettuce than control plants at the end of 10 weeks of continuous exposure.
Ozone
Ozone(O3) is a photochemical oxidant that occurs everywhere but most notably in city settings. Ozone is created by sunlight (primarily in the ultraviolet range) acting on air pollutants from automobile exhaust and, thus, may be in highest concentrations in the downwind suburbs of metropolitan areas. It damages plant membranes, lowers photosynthesis and induces premature senescence (plant biochem book). Recent work seeks to elucidate the effect of ozone on photosynthetic efficiency and determine specific biochemical pathways that are damaged (Calatayud et al., 2002). Yield reductions have been demonstrated in field production of soybean that ranged from 8-41% (Kohut et al., 1986). The National Park Service (US Dept of Interior) reports a 30-50% reduction in the growth of trees in Smoky Mountain National Park during high ozone years. Damage to plants has been confirmed at concentrations of 4 ppm.
Sulfur Dioxide
One study has examined the effect of sulfur dioxide on winter-grown lettuce. There were 47 total days of exposure from 0-0.15 ppm. There was a slight growth stimulation at 0.03 and 0.07 ppm but slight growth inhibition at 0.15 ppm (25% lower fresh weight than at 0.7 ppm but no significant difference from ambient control conditions). This study was conducted outdoors in both a contained air-exclusion chamber and open-top chamber that controlled the air mixture to the plants. The year the study was conducted was slightly warmer than typical years, so the authors’ hypothesized that this elevated temperature may have reduced the sensitivity of the plants to the pollutant.
References
Calatayud A., J. W. Alvarado, E. Barreno. (2002). Similar effects of ozone on four cultivars of lettuce in open top chambers during winter. Photosynthetica 40,195-200.
Caporn S. J. M. (1989). The effects of oxides of nitrogen and carbon dioxide enrichment on photosynthesis and growth of lettuce (Lactuca sativa L.). New Phytol 111,473-481.
Hufton C. A., R. T. Besford, A. R. Wellburn. (1996). Effects of NO (+NO2) pollution on growth, nitrate reductase activities and associated protein contents in glasshouse lettuce grown hydroponically in winter with CO2 enrichment. New Phytol 133,495-501.
Kiel A. (1990). CO2 enrichment with natural gas fired hot-air heaters. Acta Hort 268,111-120.
Kohut R. J., R. G. Amundson, J. A. Laurence. (1986). Evaluation of growth and yield of soybean exposed to ozone in the field. Environmental Pollution Series A, Ecological and Biological 41,219-234.
Mortensen L. M. (1992). Effects of ozone concentration on growth of tomato at various light, air humidity and carbon dioxide levels. Scientia Hortic 49,17-24.
Olszyk D. M., A. Bytnerowicz, G. Kats, P. J. Dawson, J. Wolf, C. R. Thompson. (1986). Effects of sulfur dioxide and ambient ozone on winter wheat and lettuce. J Environ Qual 15,363-369.
http://www.nature.nps.gov/air/studies/ecoOzone.cfm. Accessed 9/24/10
