Rice Adaptation to Excess Manganese: Nutrient Accumulation and Implications of the Quality of Crops

The nature of the adaptation of rice (Oryza sativa L. cv. Safari) to toxic Mn contents was characterised at the end of the reproductive phases. Plants were grown over a l70-day period in nutrient solutions containing 0.5, 2 and 32 mg/L. It was found that root and shoot growth was significantly inhibited in the highest Mn treatment. Up to the last treatment, the concentrations of Mn also increased significantly in both root and shoot tissues, but the activity of the root ATPase proton pump decreased. In both root and shoot tissues, the release of electrolytes followed a similar pattern to that of Mn accumulation. Additionally, increasing Mn concentrations triggered a general synergistic effect on Ca, Mg, Na, P, Cu and Fe net uptake, but displayed an antagonistic action on K and Zn. The net translocation rate of Fe was also inhibited. Concerning crops, increasing Mn concentrations delayed production, the levels of nutrient accumulation also being changed. However, the concentration of potentially toxic metals in the grain was much lower than that in vegetative plant tissues. The concentrations of Ca, K, Na, P and Zn interacted with increasing Mn concentrations mostly in a manner similar to that of the shoot, but a different pattern was found for Mg, Cu and Fe. In these crops, increasing concentrations of Mn did not affect the contents of starch, soluble sugars and oils, but significantly decreased the concentration of protein. Moreover, the amino acid composition showed a significant increase in the concentration of Tyr, but sharply decreased levels of all of the basic amino acids positively charged at physiological pH values. The effects of increasing Mn concentrations on nutrient net uptake and translocation were related to effects on crop production. It was concluded that at the end of the reproductive phases, the adaptation of the cvs. Safari to excess Mn is associated to a selective uptake that implicates a diminished crop production. The protein patterns of the grain also showed a decreased value.