Factors for the formation of deep-root-system that improves the drought resistance in the view of cell elongation and cell-wall expansibility

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In this study, we first established the method using maize plants to investigate the relationships between the elongation of seminal roots and cell elongation and cell division under different water conditions. Not the low osmotic potential as pseudo-drought stress but the real drought stress using autoclaved vermiculite was treated on seedlings. Closs sections of the apical part of the seminal roots were made by a freezing microsilcer and observed by a fluorescent microscope to measure the cell length of the cortex cells. The cell length of cortex cells in the apical 5cm of seminal roots in rice cultivars with different drought resistance was measured with this method. The drought resistant varieties showed remarkable increase of cell length in the root elongation zone under low water content of the medium, suggesting the close relation of the cell elongation pattern in the elongation zone to the varietal differences in root elongation and the root response to the drought. The cell wall thickness in roots (in particular, in endodermis) was large in drought resistant varieties. The final cell length, however, did not showed difference among the varieties, suggesting the large contribution of the cell division in root apex and the root growth angle in the determination of the vertical distribution of roots. The observation of the root caps of maize plants indicated that the root growth angle gets closer to vertical with the longitudinal development of columella tissue and the enlargement of amyloplasts. Varietal difference in the accumulation of silicon on the cell wall of the root endodermis was also investigated in relation to the drought resistance.