PHYSIOLOGICAL-RESPONSE OF YELLOW-POPLAR SEEDLINGS TO SIMULATED ACID-RAIN, OZONE FUMIGATION, AND DROUGHT

One-year-old containerized seedlings of yellow poplar (Liriodendron tulipifera L.) were exposed to simulated acid rain (pH 3.0, 4.0, 5.5), ozone (O3) fumigation (0, 0.05, 0.1, 0.2 ppm), and drought (1, 2, or 3 one-week drying cycles) from late May until mid-October. Measurements of net photosynthesis (Pn), leaf stomatal conductance (Gs), and xylem water-potential (ψx) were made on seedlings from each treatment during June, July and August. Seedlings treated with simulated acid rain at pH 3.0 showed reductions in Pn (29%), Gs (33%), and ψx (17%) when compared with similar seedlings treated with rain at pH 5.5. For seedlings fumigated with O3, there were no significant differences in Pn or Gs between the controls and the other O3 treatments. For seedlings subjected to drought,Pn increasing stress but ψx declined. Interactions involving drought and O3 together had a greater impact on ψx than did either factor alone. For certain stress combinations, particularl those involving drought, it appears that one factor may exert a dominant role over another in determining the overall physiological response. © 1990.