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SUMMARYNuclear staining with acridine orange was used to assess cell viability in the cortex of wheat and barley seminal roots from glasshouse and field experiments. Results from this method correlated well with nuclear assessments made in unstained or Feulgen-stained roots, and other evidence is presented to support the validity of the method.The pattern of root cortex death (RCD) was similar in wheat and barley and consistent over a wide range of conditions. Behind the extending root tip and zone of nucleate root hairs, nuclei disappeared progressively from the outer five (of six) cortical cell layers of the root axes, starting in the epidermis. Stainable nuclei remained in the sixth cell layer, next to the endodermis, and in most cell layers around the bases of root laterals and in a small region immediately below the grain. The onset of cell death was apparently related more to the age of a root region than to its distance behind the root tip, and it was not closely correlated with endodermal or stelar development assessed by staining with phloroglucinol/HC1.The rate of RCD was much faster in wheat than barley in both glasshouse and field conditions, and faster in some spring wheat cultivars than in others in the glasshouse. RCD occurred in sterile vermiculite and perlite and was not enhanced by the presence of soil microorganisms; nor was it enhanced in soil by the addition of the non-pathogenic fungal parasites Phialophora radicicola var. 9raminicola or Microdochium bolleyi.

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von Broembsen, S. L., and Deacon, J. W. 1997. Calcium interference with zoospore biology and infectivity of Phytophthora parasitica in nutrient irrigation solutions. Phytopathology 87:522-528.Calcium, applied as either CaCl 2 or Ca(NO 3 ) 2 to water or calcium-free soluble fertilizer solution (Peters 20-10-20 Peat Lite Special), affected several important stages of Phytophthora parasitica zoospore behavior relevant to infection and disease spread. Release of zoospores from sporangia was suppressed by Ca 2+ concentrations in the range of 10 to 50 meq. These concentrations also curtailed zoospore motility; 20 meq of Ca 2+ in fertilizer solution caused all zoospores to encyst within 4 h, whereas 94% of zoospores remained motile in unamended solution. In addition, Ca 2+ in the range of 10 to 30 meq stimulated zoospore cysts to germinate in the absence of an organic nutrient trigger, while suppressing

The pre-penetration sequence involving zoospore taxis, encystment, cyst germination and germ-tube orientation was studied by video microscopy when roots of wheat, cress or tomato were placed in suspensions of zoospores of Pythium aphanidermatum (Edson) Fitzp. The sequence was also studied in vitro, to identify the responsible factors.Several amino acids, sugars or volatile compounds elicited one of more stages of the sequence, but only glutamic acid and aspartic acid elicited all stages and thus could account for the observed behaviour on roots. Polyuronates of root surface slime also may be needed for encystment and orientation of spores, because germ-tubes emerged towards roots from a fixed point near the former position of the water-expulsion vacuole. However, identical patterns of zoospore accumulation and encystment on untreated roots and roots encased in alginate gel indicated that neither polyuronates nor specific sugar residues of root slime caused localized encystment on roots.Based on these results, a 'simplest case' model of the pre-penetration sequence on roots is proposed.

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