Progressive Cortical Senescence and Formation of Lysigenous Gas Space (Aerenchyma) Distinguished by Nuclear Staining in Adventitious Roots of Zea mays

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Abstract
The pattern of loss of nuclear integrity in the epidermis and cortex of maize adventitious roots was examined during (1) non-pathogenic cortical senescence associated with root ageing, and (2) lysigenous aerenchyma formation, to determine whether these phenomena are related. Nuclear integrity was estimated by counting the percentage of cells with nuclei detectable by acridine orange fluorescence. In roots of both soil-grown (90 d) and solution-grown (19 d) plants, nuclei were lost progressively, from the epidermis and from successively deeper cortical cell layers, with increasing distance behind the root tips; this occurred irrespective of the degree of aeration in solution culture, and independently of aerenchyma formation. Aerenchyma developed in soil-grown plants and in sub-ambient oxygen concentrations (<5 kPa partial pressure) in solution culture. It started to form in the middle cortex and coincided with a marked loss of nuclear staining in the inner cortex, especially in the innermost cortical cell layer next to the endodermis, but not in the remaining cells of the middle cortex. Two distinct patterns of nuclear deletion from the cortex were thus demonstrated; they occurred independently but simultaneously in some conditions. These findings are discussed in relation to mechanisms of cell death, and the metabolic status of root cortical cells participating in ion transport to the xylem.

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