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Separating Cardinal Temperatures for Seed Dormancy and Germination in Temperate, Sub-Tropical and Tropical Species

Germination of Liparia villosa (Leguminosae) seed

This project is one of a series in the theme ‘Ecophysiology and Morphology’.

An assessment of the proportion of dormancy in a seed lot is primarily achieved by measuring the failure of viable seed to respond in a germination test. However, such assessments are highly dependent on germination test times. At temperatures very close to the minimum (base, Tb) temperatures for germination rate, exceedingly slow germination progress would be expected, which could be misconstrued as dormancy. One way to attempt to separate these two physiological states (dormant/ non-germinating and germinating) with greater certainty is to characterise cardinal temperatures for both states. Seed dormancy can be separated into many different classes and types, including physiological dormancy of which Type II is the most common. Seeds with such dormancy respond to cold stratification (chilling) by widening the range of temperatures conducive for germination towards lower temperatures. Using over 50 different treatments, each varying in time and temperature of stratification and germination to a total test time of over three years, we have developed a predictive model for dormancy release and germination in Aesculus hippocastanum. Seed germination could be predicted in relation to thermal time accumulation above a gradually reducing Tb. This type of response may be characteristic of summer annuals, suggesting future applications for this approach in seed ecology studies. We are also continuing to investigate inter-specific differences in Tb in a range of Cactaceae species collected from across the Americas.

At a species-specific high temperature (ceiling temperature) above the optimum for germination rate, germination will be completely inhibited. However, such temperatures can be conducive for germination when ‘sensed’ for only a short period. We have shown that desiccation induced dormancy in Carica papaya is released by heat shock at 35°C for 4 hours, allowing germination to then progress at 10°C lower. Our more recent studies have revealed that heat shock causes protein changes in 75 kDa protein that closely matches HSP70 of Chlamydomonas reinhardii. Such rapid responses to heat shock fits with the weedy, gap-invading habit of this sub-tropical species. 

In tropical dryland environments the frequency of species with physically dormant hardseeds (i.e. water impermeable) is high and these seeds too have temperature thresholds for dormancy loss. We have used c. 80°C temperatures, close to those reached in the soil layer during flash fires, to break physical dormancy in Apeiba tibourbou and other species.   See Annex 1 for the five publications from this project.

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