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

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 solut ions containing Mn concentrations varying between 0.125 and 32mg/L. Accumul ation 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 signi ficantly, but thereafter that process was associated with ca. 1.8-fold incr ease and 50 % inhibition, respectively. In these tissues, Mn accumulation r emained unchanged within the vacuoles during all the experimental periods, but from the 21(st) day after germination onwards, a significant decrease w as found in the protoplasts. The transpiration rates remained unaffected, b ut showed a negative correlation with increasing time of exposure. During a ll the experimental periods, the electrolytic conductance of the shoot rema ined unchanged, but at the subcellular level, the deposition of Mn within t he protoplasts and chloroplasts increased significantly when the plants wer e 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 s hoot. These characteristics implicated: (i) the inhibition of the apoplasmi c influx from the cortex towards the stele; (ii) after the end of the mobil isation of seed reserves, a decreased transpiration rate which limited the translocation process; (iii) a symplastic assimilation of Mn in the shoot p rotoplasts, the chloroplasts being a main target.