Sequential Effects of Acidic Precipitation and Drought on Water Relations of Pinus radiata Seedlings

Summary In this work we investigated the sequential effects of simulated acid rain and drought on water relations of radiata pine ( Pinus radiata D. Don) seedlings. Whole seedlings were firstly subjected during 1 month to acid rain, pH 3.0, applied 5 times a week at 3 mm precipitation equivalent per day. Afterwards, drought treatment was applied by withholding water for 20 days and subsequent rewatering for 6 days to analyze the sequential effect of acid precipitation and drought. Water status parameters were determined before the initiation of acid rain treatment and within 3 weeks after the end of treatment. Acid rain treatment in well-watered plants did not affect either water potential or relative water content, whereas a marked effect on electrolyte leakage from the needles and on instantaneous transpiration was recorded. Drought treatment had great effects on leaf water potential (-2.5 MPa), RWC (50 % diminution), membrane permeability (340 % increase) and transpiration rate (25 % inhibition). Interactions involving acid precipitation and drought led to much greater impacts on all of the parameters analyzed. We conclude that although acid rain had slight, if any, direct effects on the water relations of well-watered radiata pine, it altered the drought tolerance of this tree species. Under conditions of soil-water deficit, plants exposed previously to acid rain are more sensitive to drought and desiccation.