Discussion

The present studies indicate that cell death can be delayed by salt stress. Where the functional properties of aerenchyma (i.e., aeration) are concerned, this could be an important issue, since some rice plants are confronted both by salt and waterlogged conditions in the field. In mature rice plants, reduced aeration would lead to physiological distress. Responses to salinity stress are often expressed as anatomical and cellular changes.39

In the present work, compared to control roots the basic anatomical structure was retained in roots subjected to NaCl. However, higher concentrations of salt may be expected to cause irregularities such as lessening of root diameter and delaying of differentiation.40 This notion is in agreement with present observations. We examined changes in the dimensions of cortical cells from the cell division phase to the cell death phase. The measurements were confined to central cortical cells in the root zone, because cell death initiation occurred in this central cell portion.29 Hence, no significant difference was seen in final cell length between longitudinal cell files. As observed in other roots41 no clear demarcation was observed between the two phases (i.e., cell division and elongation); both cell division and elongation occur in the same zone. The limited number of cells observed in the cell proliferation phase under salt stress suggests that the zone of cell division was shortened. The zone of cell elongation began closer to the root tip, implying that there was a more rapid transition from cell division to cell elongation. Ishikawa and Evans41 named the region between the zones of cell division and cell elongation the "distal elongation zone." The cells in this zone react differently from cells in other zones in response to stimuli. Thus, salt stress seems to stimulate the transition phase from cell division to elongation, suggesting a shift in the elongation zone toward the root tip.

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