Sulfur Dioxide and Nitrogen Growth, Gas Exchange, and Potato Plants

The effects of SO 2 and NO2, singly and in combination, on the growth and physiology of nontuberizing Solarium tuberosum L. 'Russet Burbank' plants were studied in controlled conditions. Plants were exposed to 0.11 µl SO2 and/or 0.11 µl NO 2/liter for 24 hours a day up to 10 days. Statistically significant effects were observed mainly in the SO2 + NO 2 treatments compared with the control plants. Leaf area was reduced from day 2 onward, and root fresh and dry weights were reduced from day 4 onward. Significant reductions in leaf and stem dry weights occurred on day 6. Net CO2 exchange rates were reduced for SO2 exposed compared with control plants beginning on day 3, while water loss rates were increased with SO 2 + NO 2 beginning on day 3. The increases in water loss rate were possibly due to the development of cuticular injury observed as abaxial glazing on the upper and middle canopy leaves. Leaf osmotic potential (π) of plants with SO2 + NO 2 became more negative within the first 24 hours of the exposure. This reduction was accompanied by an increase in reducing sugar concentration. Xylem water potential was reduced in the mature and expanding leaflets by day 2 of the SO 2 + NO 2 exposure. The most sensitive aspect of the action of SO 2 + NO 2 appeared to be the increase in reducing sugars that affected osmotic potential in the leaves. Considering the retardation of root growth, these data suggest that the pollutant gases may have interfered with partitioning of dry matter from the leaves to the roots. Potato plants demonstrate various responses to SO 2 and NO2. Sulfur dioxide exposure causes reductions in tuber yield and mean tuber size (Foster et al., 1983), while chronic NO 2 ex- posures throughout the growing season cause reductions in tuber number and fresh weight (Sinn and Pen, 1984). Sulfur dioxide + NO 2 alters potato plant water status and interferes with the partitioning of dry matter from the leaves to the roots (Petitte and Ormrod, 1988). At low doses, air pollutants may alter biochemical or physi- ological cell processes (e.g., stomatal function in photosyn- thesis) resulting in lowered plant productivity without visible injury (Heath, 1980). These alterations can be the result of sev- eral factors. Exposure of soybean (Amundson and Weinstein, 1981) and garden bean (Ashenden, 1979b) to SO2 and NO2 caused parallel decreases in transpiration and photosynthesis rates due to increased stomatal resistance. However, Sinn and Pen (1984) demonstrated that 'Kennebec' and 'Atlantic' potato plants chronically exposed to levels of NO2 ranging from 0.12 to 0.34 µl·liter -1 did not experience significant increases in leaf dif- fusive resistance. Reinert (1984) stated that the reductions in photosynthesis rates, as a result of plant exposure to some com- binations of SO2 and NO2, suggest an increase in respiration. Carlson (1983) believed that where reductions in photosynthesi s were not the result of increases in respiration (Black and Un- sworth, 1979), the reduction was due to increased internal dif- fusive resistances. A reduction in photosynthesi s also may be caused by the actions of pollutant by-products formed in the leaves. Malhotra and Khan (1984) state that SO 2 can affect