Current interest in seed hydration treattnents for the improvement of level, rate and uniformity of gennination or field emergence has revealed how little is Imown of the physiology of getmination control under water stress. This review surveys briefly the responses of seeds held at different hydration levels, from normal germination in a free water supply to seed activation without germination under slight moisture stress, seed deterioration at greater moisture stress and the damage that can be caused to seeds in very dry conditions, as well as the responses to subsequent dehydration. Inhibition of germination, though not of seed activation, at certain levels of water stress is likened to various forms of dormancy, and the mechanism governing the initiation of cell elongation is suggested as the possible key to control over germination. Several lines of evidence on cell membrane integrity and action, and their responses to external factors, point to tlie role that the membrane may play in cell elongation (and hence germination) eontrol, and membrane integrity may also be associated with the transition between seed deterioration at one hydration level and seed activation and repair at slightly higher hydration levels. Seed activation witliout germination is also considered in an ecological context.

SUMMARYSeeds of calabrese and carrot were held at 10°C or 20°C for periods of up to 21 days in water or polyethylene glycol '6000' solutions of from -5 to -20 bars. Seeds were subsequently placed to germinate in a favourable environment or were first dehydrated before transfer to this environment. The seeds' ability to germinate was virtually unaffected by any of the temperature, water potential or duration treatments. Increasing water stress progressively reduced the levels of water content and oxygen uptake rates after initial imbibition, and delayed the onset of germination with the accompanying increase in these factors. At -15 and -20 bars, water content remained approximately constant throughout the experiment but oxygen uptake rates fell after day 6. The results have a direct bearing both on the seed 'priming' treatment used to accelerate seed germination and on the tolerance to drought of seeds in the field.

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The heat resistance of a wild type and nalidixic acid resistant strain of Yersinia enterocolitica, and Listeria monocytogenes, was measured in meat (minced beef and minced beef homogenate) and potato substrates over the temperature range 50–60C. Comparisons of heat resistance were determined using D‐values calculated using a linear survival model. The results showed that the wild‐type strain of Y. enterocolitica was more heat resistant than the mutant (p<0.05). Under most conditions, the use of a nonselective/overlay recovery medium resulted in higher D‐values compared to a selective recovery medium (p<0.05). Analysis of the data using a nonlinear survival model (D1 and D2 ‐values) suggested the presence of heat resistant subpopulations and was particularly evident for the mutant strain, and in potatoes compared to minced beef.