Research Projects

Project at a Glance

Project Status: complete

Title: Interaction of humidity and air pollutants on vegetation

Principal Investigator / Author(s): Thompson, C. R.

Contractor: Statewide Air Pollution Research Center, University of California, Riverside

Contract Number: A5-160-33


Research Program Area: Ecosystem & Multimedia Effects

Topic Areas: Ecosystem Impacts


Abstract:

The primary objective of this study was to determine the extent to which relative humidity affects plant responses to air pollutants in the field. This objective was addressed using a unique field humidification system which adds dry steam to large open-top chambers on the campus of the University of California at Riverside. The system was expanded to supply humidity to a total of six chambers. The objective was addressed in three sequential studies evaluating: the response of plants to ambient ozone and / or added humidity in the late summer and early fall of 1986; the response of five winter crops exposed to sulfur dioxide and / or added humidity in the winter of 1986-87; and the response of five tree and herbaceous species exposed to ambient ozone and / or added humidity in the spring and summer of 1987.

In the fall oxidant x humidity study there were five treatment: ambient air (high ozone) and ambient (dry) humidity, ambient air and added (35% above ambient between 1100 and 1600) humidity, filtered (low ozone) and ambient humidity, filtered air and added humidity and outside check plots. The study used tomatoes (Lycoopersion esculentum).

In the winter sulfur dioxide x humidity study there were five treatments: ambient air (no sulfur dioxide) and ambient (dry) humidity, ambient air and added (40% above ambient between 1100 and 1600) humidity, ambient air plus 0.12 ppm sulfur dioxide) and ambient humidity, 0.12 ppm sulfur dioxide and added humidity, and outside check plots. The study used wheat (Triticum aestivum), lettuce (Lactuca sativa), carrots (Daucus carota), and onions (Allium cepa).

In the spring oxidant x humidity study there were five treatments: ambient air (high ozone) and ambient (dry) humidity, ambient air and added (25% above ambient between 1100 and 1600) humidity, filtered (low ozone) and ambient humidity, filtered air and added humidity and outside check plots. The study used beans (Phaseolus vulgaris), melons (Cucumis melo), almonds (Prunus dulcis), ponderosa pine (Pinus ponderosa) and Douglas fir (Pseudotsuga menziesii).

Overall this study indicated that there is a definite interaction between humidity and air pollution on leaf injury, with increasing humidity greatly increasing the amount of visible leaf necrosis and senescence from ozone. However, this injury interaction was not associated with any general interaction in terms of crop yield. There were a few statistically significant humidity x air pollutant interactions for growth and biomass production of plants, with increased ozone effect in humid chambers in the Spring ozone study and decreased sulfur dioxide effect in the Winter study.

Ozone alone caused visible injury to tomatoes, almonds, beans, and melons. It also resulted in significant reduction in yield, growth, and biomass production for tomatoes and beans, and reductions in physiological processes (stomatal conductance, photosynthesis, and transpiration) for tomatoes, beans, and almonds. Sulfur dioxide by itself reduced growth and biomass production for wheat and lettuce and yield (weight / ear) for wheat. Humidity, in general, increased plant growth and biomass production, for tomatoes, carrots, onions, lettuce and beans. Added humidity resulted in increased yield for carrots, onions, lettuce, but decreased yield in beans and possibly tomatoes. Humidification also resulted in increases in physiological process rates for tomatoes, wheat, lettuce, onions, almonds and beans.

Overall, this study indicated that there is no general synergism resulting in greater yield losses from air pollutants for plants exposed at higher humidity levels. In fact for wheat and lettuce, sulfur dioxide reduced yields more in dry chambers compared to humid chambers. Evidently, the increase in stomatal conductance indicates the potential for greater pollutant effects at higher humidities, but the plant's greater growth in humid air compensates for these effects.


 

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