EARLY EFFECTS OF HYDROGEN-FLUORIDE ON WATER RELATIONS, PHOTOSYNTHESIS AND MEMBRANE INTEGRITY IN EASTERN WHITE-PINE (PINUS-STROBUS) SEEDLINGS

Eastern white pine (Pinus strobus L.) seedlings were treated with 0.5 and 2 ppb hydrogen fluoride (HF) for 1-28 days and water relations, photosynthesis and membrane integrity were studied to determine the mechanism of fluoride action on plants. The first symptoms of fluoride injury appeared on young needles as tip necrosis after 20 days of 0.5 ppb HF treatment, and 5 days of 2 ppb HF treatment. Both HF levels reduced transpiration rates in seedlings treated for 1, 7 and 27 days, but had little effect on net photosynthesis. The initial decline in transpiration rates was not accompanied by an increase in tissue fluoride levels. Reduced transpiration was a possible cause of higher water potentials and water contents in fluoride-treated seedlings compared with control plants. Changes in membrane permeability were among the earliest noticeable effects of HF and greater electrolyte leakage was observed from seedlings treated for 2 and 8 days with HF compared with the controls. The results of this study suggest that cell membranes are among the initial sites of fluoride action on plants.