Cell-wall structure and chemical components in the root endodermis and exodermis of cereal roots as an environmental response and their functional meanings.

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Anatomical response of roots to drought, water-logging and salinity in the root of cereal crops was investigated. Rice roots exhibited cell wall thickening of root endodermis in drought, in particular in drought-tolerant upland rice varieties. Besides, the drought-tolerant upland rice varieties had higher content of silicon in the thickened cell walls than the lowland rice variety. On the other hand, the thickening of endodermal cell wall was limited in all the rice varieties under flooded conditions. In wheat, an upland crop, the cell wall of root endodermis showed similar or rather larger thickness under flooded condition than in aerobic condition. Moreover, presence of ammonium as the nitrogen source in the growth medium enhanced the cell wall thickening and lignification of endodermal cell wall under flooded condition in wheat and maize. Those enhancement of thickening and lignification of endodermal cell wall in upland cereal crops may contribute to decline the infiltration of harmful components in reduced soil into the central tissues of roots. Salinity stress decreased the distance of the position of Casparian band formation from the root apex in rice seminal roots. To clarify if the onset of Casparian band formation is really accelerated by the stress, a new experimental system to grow rice seedlings with treating only one side of the seminal root with high osmotic pressure caused by mannitol was developed. A cross section of the seminal roots grown with this method had treated side and control side with the same age. The observation of the cross sections suggested that the Casparian band formation was not hastened but the thickening of the suberin lamellae was accelerated by the higher osmotic pressure.