EFFECTS OF CARBON-DIOXIDE ENRICHMENT ON RESPONSE OF MYCORRHIZAL PITCHPINE (PINUS-RIGIDA) TO ALUMINUM - GROWTH AND MINERAL-NUTRITION

Carbon dioxide enrichment may increase the Al tolerance of trees by in creasing root growth, root exudation and/or mycorrhizal colonization. The effect of elevated CO2 on the response of mycorrhizal pitch pine ( Pinus rigida Mill.) seedlings to Al was determined in two experiments with different levels of nutrients, 0.1- or 0.2-strength Clark solutio n. During each experiment, seedlings inoculated with the ectomycorrhiz al fungus Pisolithus tinctorius (Pers.) Coker & Couch were grown 13 we eks in sand irrigated with nutrient solution (pH 3.8) containing 0, 6. 25, 12.5, or 25 mg/l Al (0, 232, 463, or 927 mu M Al) in growth chambe rs fumigated with 350 (ambient) or 700 (elevated) mu l/l CO2. At ambie nt CO2 in the absence of Al, mean total dry weights (DW) of seedlings at the high nutrient level were 164% higher than those at the low leve l. Total DW at elevated CO2, in the absence of Al, was significantly g reater than that in ambient CO2 at the low (+34%) and high (+16%) nutr ient levels. Root and shoot DW at both nutrient levels decreased with increasing Al concentrations with Al reducing root growth more than sh oot growth. Although visible symptoms of Al toxicity in roots and need les were reduced by CO2 enrichment, there were no significant CO2 x Al interactions for shoot or root DW. The percentage of seedling roots t hat became mycorrhizal was negatively related to nutrient level and wa s greater at elevated than at ambient CO2 levels. Generally, elevated CO2 had little effect on concentration of mineral nutrients in roots a nd needles. Aluminum reduced concentrations of most nutrients by inhib iting uptake.