Tolerance of rice to excess manganese in the early stages of vegetative growth. Characterisation of manganese accumulation

Abstract

Summary Rice (Oryza sativa L. cv. Safari) tolerance to excess Mn was investigated. Rice plants were grown over a 15-, 21-, and 28-day period in nutrient solutions containing Mn concentrations varying between 0.125 and 32 mg/L. Accumulation of Mn increased significantly over this range of treatments (15, 21, and 28 days after germination, 3.9-, 8.1-, and 7.9-fold in the roots and 2.7-, 17.7-, and 6.8-fold in the shoots, respectively). In 15-day-old roots, the extend of membrane permeability and proton extrusion did not vary significantly, but thereafter that process was associated with ca. 1.8-fold increase and 50 percnt; inhibition, respectively. In these tissues, Mn accumulation remained unchanged within the vacuoles during all the experimental periods, but from the 21st day after germination onwards, a significant decrease was found in the protoplasts. The transpiration rates remained unaffected, but showed a negative correlation with increasing time of exposure. During all the experimental periods, the electrolytic conductance of the shoot remained unchanged, but at the subcellular level, the deposition of Mn within the protoplasts and chloroplasts increased significantly when the plants were submitted to high Mn concentrations. It was concluded that rice is highly tolerant to excess Mn, and that this process is related to metal exclusion from root cells with a physiological control of its translocation to the shoot. These characteristics implicated: (i) the inhibition of the apoplasmic influx from the cortex towards the stele; (ii) after the end of the mobilisation of seed reserves, a decreased transpiration rate which limited the translocation process; (iii) a symplastic assimilation of Mn in the shoot protoplasts, the chloroplasts being a main target.