EARLY SENESCENCE OF THE ROOT CORTEX OF AGRICULTURAL GRASSES, AND OF WHEAT FOLLOWING ROOT AMPUTATION OR INFECTION BY THE TAKE‐ALL FUNGUS

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Abstract
Nuclear fluorescence following acridine orange staining was used to assess patterns and rates of death of the root cortex (RCD) in Lolium perenne L., L. .times. hybridum Hausskn. and Dactylis glomerata L. grown in pathogen-free soil in a glasshouse. The pattern of RCD was as previously described for cereals. The rate of RCD differed significantly between grasses and between cultivars of L. perenne, but in most instances the cortex was anucleate, except for the innermost cell layer next to the endodermis, in 26 to 27 d old regions of seminal root axes. Root impedance caused by a nylon gauze barrier in soil significantly increased the rate of RCD in one tested cultivar of L. perenne. RCD was more rapid in wheat than in grasses. In 8 d old regions of wheat seminal root axes the cortex contained only 54% of the nuclei initially present. Infection of wheat roots by the take-all fungus, Gaeumannomyces graminis (Sacc.) Arx and Olivier var, tritici Walker, caused vascular disruption followed by more rapid RCD than in uninfected roots. Similarly rapid RCD occurred in amputated roots, though in all instances the pattern of RCD was unchanged. The rate of RCD was slower in long than in short lengths of amputated root, perhaps because of the remobilization of nutrients from dying cells. Nuclei persisted for at least 10 d in the inner cortex of 2 cm lengths of young roots of wheat, barley and L. perenne buried in soil at 20.degree.C. the pattern of nuclear deletion from the root pieces was the same as in whole roots, and the rate of RCD was faster in wheat than in barley, as also found for roots attached to plants. All these results demonstrate a consistent pattern of cortical senescence in graminaceous roots and suggests that it is a programmed phenomenon.