In vitro characterization of manganese toxicity in relation to phosphorus nutrition in potato (Solarium tuberosum L.)

This study investigated the effect of P (0.0, 2.5 and 6.25 mM) on potato response to excess Mn (0.1, 1.0, 2.5 and 5.0mM) at two pH (4.0 and 5.8) conditions in vitro using (KH2PO4)-P-32 and (MnSO4)-Mn-54-H2O as dual isotopic tracers. Toxicity due to excess Mn supply resulted in stem streak necrosis with significant inhibition in microplant rooting and growth. Addition of P had a significant beneficial effect in controlling the severity and progression of Mn toxicity symptoms. This beneficial effect of P in relation to Mn supply was, however, pH-dependent and genotype-specific with its effect more prominent at pH 4.0 than 5.8. Mn-54 uptake in microplants was much higher at pH 5.8 than 4.0 due to interference by H+ ions under low pH. P enhanced the uptake of Mn-54 under an excess supply of Mn up to 2.5 mM, but this increased concentration was not accompanied with any concomitant increase in severity of toxicity symptoms, suggesting that P exerted its synergistic effect by inactivating Mn within the plant. Mn at a very high level (5.0 mM) depressed the P-32 concentration in microplants and tissue content of total inorganic P, and was, therefore, irreversibly toxic even in the presence of high levels of external P supply. Restricted P accumulation inside the plant could be the reason as to why, in general, P was unable to check the progression of toxicity symptoms under excess Mn supply beyond a critical level. (C) 2004 Elsevier Ireland Ltd. All rights reserved.