RICE PLANT STRUCTURAL CHANGES BY ADDITION OF EXCESS MANGANESE

Abstract

Manganese (Mn) has not been considered to be a polluting metal, however soil conditions often present in acid and volcanic soils can lead to Mn reduction favoring Mn toxicity in natural and agricultural systems. As this problem has particular importance in rice paddy fields, with the aim of gaining fundamental insights into rice tolerance to Mn excess, the structural changes promoted by this metal accumulation in roots and shoots were investigated. Rice (Oryza sativa L. cv. Safari) plants were grown over 15- and 21-day periods in nutrient solutions containing Mn concentrations of 0.13, 8, and 32 mg/L. It was found that Mn accumulation increased significantly (15 and 21 days after germination, 3.9 and 8.1 fold in the roots and 2.7 and 17.7 fold in the shoots, respectively). The production of root hairs became inhibited with increasing Mn concentrations. In both experimental periods, it was further verified that electron dense deposits accumulated in the cell walls and/or at the outer boundaries of the root protoplasts. In the leaves, between the 15th and the 21st day after germination, stomata dimensions decreased to about 50% between the first and the last two treatments. At a subcellular level, the deposition of electron dense deposits within protoplasts did not occur when the plants were submitted to high Mn concentrations. It was concluded that in the early stages of growth, rice tolerance to excess Mn was closely related with this metal exclusion from root cells and with the physiological control of its translocation to the shoot in a process that implicates an alteration of stomata dimensions.

Keywords:

• Metals accumulation
• Oryza sativa L
• Ultrastructure of roots
• Ultrastructure of shoots

ACKNOWLEDGMENTS

The author thanks Dr. Conceição Santos (Plant Biology Department, Universidade de Aveiro, Portugal), Tech. Eng. Carlos Santiago de Carvalho (Physiology Department of the Estação Agronómica Nacional, Portugal), and Octávio Chaveiro (Microscopy Department of the Estação Agronómica Nacional, Portugal) for technical assistance.