ULTRASTRUCTURAL-CHANGES IN ROOT CAP CELLS OF 2 AUSTRALIAN NATIVE GRASS SPECIES FOLLOWING EXPOSURE TO ALUMINUM

Transmission electron microscopy was used to investigate ultrastructur al changes in root cap cells of two aluminium-tolerant native grass sp ecies, Danthonia linkii Kunth and Microlaena stipoides (Labill.) R.Br. , following exposure to Al. Quantitative differences in root cap cells and organelles in response to 0-10 ppm aluminium were determined usin g image analysis of electron micrographs Changes to the size of root c ap cells due to exposure to Al were similar in the two species with a low Al concentration (1-2 ppm) resulting in larger cells, while higher Al levels (5-10 ppm) reduced cell size. In peripheral cap cells of th e more Al-tolerant M. stipoides, the size of secretory vesicles was no t affected by exposure to Al, while in peripheral cap cells of the les s Al-tolerant D. linkii, exposure to Al resulted in significantly smal ler secretory vesicles being produced Central root cap cells from cont rol plants of M. stipoides contained 90% more dictyosomes and had 50% larger amyloplasts than D. linkii. Measurement of mucilage droplets sh owed that roots of M. stipoides produced much more mucilage than D. li nkii. Exposure of roots of M. stipoides to Al in the range 2-10 ppm ha d no effect on the size of mucilage droplets produced. The possible ro le of mucilage production in aluminium tolerance is discussed.