David C. MacLean

Field tests of a linear gradient system for exposure of row crops to S02 and HF

An exposure system for exposing plants to gradients of gaseous air pollutants in the field was tested using sulfur dioxide SO2 and hydrogen fluoride (HF). Well defined linear gradients of SO2 concentration and HF flux were easily produced and were repeatable from exposure to exposure. The gradients were altered by wind speed and direction, but in tightly closed canopies, the alterations were minor. This system has many advantages: a graded series of exposures can be conducted in a small area, plants may be grown using accepted cultural practices, the cost of the apparatus is low, and more than one pollutant can be used, either concurrently or countercurrently. Disadvantages include the requirement for an intensive air monitoring network and an unnatural vertical pollutant profile in the canopy. The gradient system should be used as a supplement to open-top chambers, and not as a replacement for them.

Response of sugar maple to multiple year exposures to ozone and simulated acidic precipitation

Potted sugar maple seedlings were exposed to ozone and acidic precipitation in open-top chambers for three consecutive growing seasons. Periodic measurements of photosynthesis, dark respiration, through-fall and soil solution chemistry, and annual measurements of the weight of plant parts were made. Experimental treatments caused few and minor effects on above- or below-ground growth of the seedlings, even after three growing seasons. There were trends for reduced photosynthesis in trees exposed to elevated concentrations of ozone and increased photosynthesis in those exposed to the lowest pH simulated rain treatment. The chemistries of soil-solutions and through-fall were not altered significantly by treatment. Although major effects were not observed, sugar maple may respond to exposures that take place over a significant part of its life cycle.

Effects of fluoride on chlorophyll a fluorescence in spinach

Chlorophyll a (chl a) fluorescence was used to determine the effects of treatments with gaseous HF or aqueous solutions of NaF on the photosynthetic apparatus of spinach prior to the appearance of visible injury. Placing the petioles in 2 mM NaF for 3 h resulted in the accumulation of 240 ppm F in leaf blades. The second oldest leaves of spinach plants accumulated similar concentrations (270 ppm F) when the plants were exposed to gaseous HF at 5 microg F m(-3) for 6 days. These NaF and HF treatments did not result in visible injury nor did they affect Fo, Fm or Fv/Fm. However, during the slow (>2 s) induction kinetics, fluorescence quenching in fluoride-treated leaves increased during the P to S phase and the M peak was no longer resolved. This was due, in part, to increased photochemical quenching. The results are consistent with a reduced ability to develop or maintain a trans-thylakoid proton gradient in chloroplasts containing elevated concentrations of F.

Effects of simulated acid rain on uptake, accumulation, and retention of fluoride in forage crops

Forage crops accumulate F from exposures to the air pollutant HF and the rate and amount taken up can be affected by a number of external factors, one of which is precipitation. To assess how precipitation, including acidic precipitation, alters F uptake and retention in forage, alfalfa (Medicago sativa L. var. Saranac) and tall fescue (Festuca arundinacea Schreb. var. Kentucky 31) were subjected to extended exposures to HF and were treated periodically with various solutions (pH 5.6, 4.0, and 3.0) supplied as simulated rain or, for comparisons, as soil amendments. None of the treatments affected growth, but precipitation treatments significantly reduced the F content of both species relative to plants that received the same volumes of the same solutions added to the soil. Analyses of washed and unwashed foliage indicated that this loss of F was primarily due to the removal of F from foliar surfaces. There was no effect of pH of rain on the F content of tall fescue, but for alfalfa an increase in acidity from pH 4.0 to 3.0 resulted in a further decrease in the F content of foliage, suggesting that in addition to removing superficial F, the more acidic simulated rain resulted in the leaching of F from within foliage was well.

Joint action of ozone and hydrogen fluoride on foliar senescence in maize.

Maize (Zea mays, L.) plants were exposed intermittently to O(3), HF or both pollutants and the progression of foliar senescence was followed by measuring chlorophyll loss, membrane breakdown and changes in stomatal conductance. At concentrations insufficient to cause foliar symptoms (0.06 microl O(3) litre(-1) and 1.0 microg Fm(-3)), exposures to HF had little or no effect, whereas O(3) exposures accelerated the rate of senescence. The rapid rate of senescence produced by O(3) was moderated if the plants were also exposed to HF. Topical application of 6-benzyladenine (BA) prior to pollutant exposures delayed senescence in all plants and completely prevented the O(3)-induced acceleration of senescence.

Responses of two deciduous plants to hydrogen fluoride during winter dormancy

Results from continuous exposure of Norway maple and lilac for 12 days to high concentrations of HF indicate that significant concentrations of F may be absorbed (or adsorbed) during the winter by tissues exposed directly to the pollutant (bark, buds, etc.). Only a small fraction of the F accumulated during dormancy is recovered in new growth that emerges in the spring, and these concentrations are usually too low to cause direct effects on the plant. We conclude that deleterious direct effects from exposures of these two, and possibly, many other deciduous species to realistic doses of HF during winter dormancy are unlikely. Whether or not they lead to indirect effects, however, is uncertain. Exposures to HF during dormancy may increase the susceptibility of plants to spring frosts and should be tested experimentally. 10 references, 1 table.