EFFECT OF PH AND NITROGEN-SOURCE ON ALUMINUM TOLERANCE OF RYE (SECALE-CEREALE L) AND YELLOW LUPIN (LUPINUS-LUTEUS L)

Soluble aluminium (Al) is a major factor limiting plant growth in acid mineral soils. Aluminium concentrations in soil solutions are mainly determined by soil pH. However, pH also affects the ratio between activities of protons and cationic Al species and the equilibrium between mono-and polynuclear hydroxy-Al species. The phytotoxicity of these species is not yet clear. The objective of the present study was to clarify the role of minor changes of pH in the rhizosphere on Al phytotoxicity in two Al-tolerant plant species by direct control of the pH in the nutrient solution (4.1, 4.3, 4.5) and in addition by varying the pH in the root apoplast using either nitrate or ammonium as N source. The plants were grown in solution culture at constant external pH. Whereas the Al-sensitive plant species barley and horse bean were damaged at very low Al supplies (1.85 μM and 9.3 μM respectively), 222 μM had to be applied to rye and yellw lupin for a comparable inhibition of root elongation. Yellow lupin was initially severely inhibited in root growth by Al, but then gradually recovered from this 'Al shock' within 3 days. In contrast to lupin, rye was hardly affected by Al initially, and it took about 16 h until maximum inhibition of root elongation. In the presence of nitrate, raising the pH from 4.1 to 4.5 aggravated root-growth depression by Al in rye and lupin. Whereas rye roots were severely damaged by ammonium especially at low pH, lupin was rather indifferent to the N source. Aluminium toxicity was less severe in presence of ammonium compared to nitrate N. This effect was less clear with rye at lower pH, because of it's higher proton sensitivity compared to lupin. Less Al injury at lower pH and in presence of ammonium was related to lower Al concentrations in the 1 cm root tips. The results are compatible with data showing high phytotoxicity of mononuclear and polynuclear hydroxy-Al species. However, they could also be interpreted in the light of proton amelioration of Al toxicity owing to competition for Al-sensitive binding sites in the root apoplast. © 1990 Kluwer Academic Publishers.