The Effects of Priming and ‘Natural’ Differences in Quality amongst Onion Seed Lots on the Response of the Rate of Germination to Temperature and the Identification of the Characteristics under Genotypic Control

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Abstract
Ellis, R. H. and Butcher, P. D. 1988. The effects of priming and ‘natural’ differences in quality amongst onion seed lots on the response of the rate of germination to temperature and the identification of the characteristics under genotypic control —J. exp. Bot. 39: 935–950. A screening procedure was applied to define the response of the rate of seed germination to sub-and supra-optimal temperatures for different lots or sub-lots of two onion (Allium cepa L.)cultivars. Three sub-lots of the cultivar White Lisbon were derived from a control lot by osmotic priming (−1.4 MPa, 20 °C. 7 d) alone, by priming and drying and by priming, drying and subsequently storing the seeds for 7 weeks at 2–5 °C. The major effect of priming was to reduce the thermal time for germination at both sub- and supra-optimal temperatures. Priming alone also altered the distribution of thermal times at sub-optimal temperatures. A new equation is presented to describe this variation. In contrast, priming had no consistent effect on base temperature (Tb and little effect on the distribution of ceiling temperatures [Te(G)]. For the control lot of White Lisbon Tb was 4°C, whilst the best common estimate of Tb for all four sub-lots was 3.5°C. The mean estimate of Tc(50) for the control, primed and primed and dried sub-lots was 35.5°C.Comparison of three lots of the cultivar Senshyu Semi Globe Yellow of widely-differing viability showed substantial differences in the thermal time for germination at sub-optimal temperatures, but no significant differences in Tb (P>0.10), the common estimate being 4°C. There was a significant negative correlation between probit percentage viability and the logarithm of the thermal time for 50% germination at sub-optimal temperatures amongst the three lots (P<0.05). The work suggests that base temperature for germination is a genotypic characteristic which is unaffected by differences in seed quality. It also shows that the effect of priming, quantified as a reduction in thermal time requirements for germination, varies amongst the seeds within a lot.

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