Growth Response to Acid Rain, Mg Deficiency and Al Surplus, and Amelioration of Al Toxicity by Humic Substances in Pitch Pine Seedlings

The individual and combined effects of acidic rain, Mg deficiency (-Mg) and Al surplus (＋Al) on the growth of shoots and roots of pitch pine seedlings and the effect of humic substances (Lit) on Al toxicity were investigated. The growth of height and dry matter were not significantly less for pitch pine seedlings sprayed with simulated acid rain (SAR) of pH 3.5 than for those sprayed with SAR of pH 5.6. But treatments of Al and ＋Al-Mg in soil solution reduced the growth of seedlings in terms of height of shoots, and dry matter of shoots or roots. Effect of Mg deficiency on the growth of seedlings was apparent only when Al was treated simutaneously. The growth of seedlings, regardless of rain pH, decreased in the following order: control=-Mg>Lit＋Al>＋Al>＋Al-Mg. Treatments of Al and ＋Al-Mg in soil solution reduced the total length of secondary and teritary roots of seedlings regardless of rain pH, and decreased in the following order: the primary root<the secondary root<the tertiary root. Total length of tertiary roots were the lowest in treatment of ＋Al-Mg. Total length of tertiary roots treated with Al was 47% of those of control. But total length of tertiary roots treated with Lit＋Al was 90% of control. Decrease in the growth of seedlings by Al toxicity was significantly ameliorated by addition of humic substances such as litter extract.