A Transmission and Cryo-Scanning Electron Microscopy Study of the Formation of Aerenchyma (Cortical Gas-Filled Space) in Adventitious Roots of Rice (Oryza sativa)

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Abstract
The initiation and development ofaerenchyma in adventitious roots of rice (Oryza sativa) was studied in tissuc up to approximately 36-h-old using transmission electron microscopy (TEM) and cryoscanning electron microscopy (cryo-SEM). Aerenchyma resulting from selective cortical cell collapse is a naturally occurring feature of rice roots. Evidence of some cortical cell disruption was noticeable by TEM in cells that were approximately 6-h-old; it became more advanced as the cells aged. At 12 h, early stages of cell wall breakdown and loss of cell turgidity were seen. Complete collapse of columns of cells had occurred by 24 h. In tissue that was 36-h-old, the senescent cytoplasm of many remaining cells began to disperse. The visual evidence suggests that cell collapse was the result of autolysis. This pattern of cortical degeneration in rice was dissimilar to that reported elsewhere for Zea roots grown in an oxygen depleted environment. Cryo-SEM revealed the occurrence of small structures within the cortex with the external appearance of miniature, intact cells which are not preserved during conventional SEM preparative procedures.