No abstract

ABSTRACIPea (Pisum sativum L. cv Alsweet) plants were exposed to mixtures ofozone plus sulfur dioxide at different times of the day. InJury, evaluated either as necrosis or chlorophyll, was greatest at midday when stomatal conductance was greatest. Abscisic acid levels were similar over the day, and showed no relation to stomatal conductance.Investigations of pollutant exposure at different times of the day have been undertaken with 03 and SO2 singly (5). As early as 1949, Katz (10) showed that barley and alfalfa grown in fields were less sensitive to SO2 early and late than during the middle of the day. Such variations in diurnal injury responses to single pollutants have been confirmed in controlled environments with other crops (7, 22), including peas (18). No diurnal studies with mixtures of pollutants were found.The reduced injury early and late in the day has been related closely to stomatal response during the period of exposure (18). Plants exposed early and late in the day exhibited greater stomatal closure than plants exposed at midday (18).Mechanisms regulating the changes in stomatal conductance of plants exposed to pollutants at different times of the day are not understood. Olszyk and Tibbitts (18) suggested that ABA might be involved in controlling stomatal responses. This suggestion is supported by the fact that when ABA content of leaves was increased by foliar applications or by water stress, stomata closed, resulting in decreased 03 uptake and hence less injury to plants (4, 9). Also, ABA concentrations were higher in SO2-resistant plants which had greater stomatal closure during exposure than S02-sensitive plants (12,13 at different times of the day have been reported, although patterns over the day have not been consistent (3,8,14,17, 20,23). No reported studies of fluctuations in ABA concentrations of plants exposed to pollutants at different times of the day were found.Plants were exposed for 2 h to a mixture of 03 plus SO2, two pollutants commonly occurring together in the ambient environment. Exposures were undertaken for 2 h to simulate the length of ground level S02 exposures around coal-fired electrical generating stations (21). To

Pea (Pisum sativum L. ‘Alsweet’) plants were grown in a controlled environment and exposed to ozone, sulfur dioxide, or a mixture of the 2 pollutants. Plants were preconditioned with low (36%) or high (77%) relative humidity (RH) for 6 days and then exposed to the pollutants for 2 hr in low (31%) or high (67%) RH. Injury was evaluated as both necrosis and chlorophyll concentrations. A high RH prior to exposure to air pollutants greatly increased subsequent plant injury from pollutants. The increased injury with high RH prior to exposure to pollutants was associated with increased stomatal conductance. Relative humidity during exposure to air pollutants had little effect on plant injury.

An attempt has been made here to condense the great volume of literature for many different air pollutants and from many different plant systems. Only those responses that have been reported for several species are emphasized and our discussion is limited to responses obtained with intact plants.