NITROGEN-SOURCE EFFECTS ON AL-TOXICITY IN NONMYCORRHIZAL AND MYCORRHIZAL PITCH PINE (PINUS-RIGIDA) SEEDLINGS .2. NITRATE REDUCTION AND NO3-UPTAKE

Nitrogen source utilization, mediated by the ectomycorrhizal symbiont Pisolithus tinctorius, may modulate Al toxicity in pitch pine (Pinus rigida) seedlings. Nitrate reduction, occurring primarily in the roots of pitch pine seedlings, represents a critical metabolic pathway that may be directly sensitive to Al or indirectly affected by changes in NO3- availability associated with Al exposure or mycorrhizal infection. To investigate these possibilities, pitch pine seedlings were grown in sand culture with NO3-, NH4NO3, or NH4+ and exposed to 0 or 200-mu-M Al for 6 weeks. Foliar N concentrations, root nitrate reductase (NR) activity, and Al inhibition of NR activity were highly dependent on the proportion of NO3- of the nutrient solution. The association of Pisolithus tinctorius with seedling roots reduced both root and foliar NR activity compared with non-inoculated controls, suggesting that NO3- uptake and translocation to foliage was reduced by the symbiont. This was confirmed by using (ClO3)Cl-36 to measure unidirectional plasma membrane NO3- fluxes. Mycorrhizal root tips absorbed 50% less NO3- than nonmycorrhizal root tips. Preferential use of NH4+ by ectomycorrhizal roots may thus result in reduced movement of Al into root tissue and amelioration of Al toxicity.