Histological observation of secondary aerenchyma formed immediately after flooding in Sesbania cannabina and S. rostrata

The effect of up to 48 h of flooding on the development of roots of Sesbania cannabina an0d S. rostrata seedlings was examined in a pot experiment. Light microscopy revealed that the outermost cells of the phellogen of the taproot of S. cannabina expanded and elongated during the first 12 h of flooding. After 18 h, the outermost of these regions was composed of cells that had expanded radially direction to form a spongy zone inside the endodermis. These elongated cells were radially connected to each other and formed the secondary aerenchyma surrounding the stele of taproot. While those histological alterations were not observed in S. rostrata, the number of layers of cells originating in the pericycle increased slightly, but elongation of the cells was not found during the first 18 h of flooding. After 36 h of flooding, cell elongation was also detected as outer layers of the phellogen. The delayed response to flooding in aerenchyma production in S. rostrata was compensated by immediate development of adventitious roots on submerged parts of the hypocotyl.