Germination requirements: temperature

Seed growth only occurs within a certain range of temperatures. If the temperature is too high, the enzymes needed for germination are destroyed. If the temperature is too low, molecules inside the seed move too slowly for chemical reactions to get started. For any seed, there are three key temperatures, known as the cardinal temperatures, which describe the effect of temperature on germination:

  • Tmin is the minimum temperature, below which none of the seeds from a sample will germinate.
  • Topt is the optimal temperature, where there is high and fast germination.
  • Tmax is the maximum temperature, beyond which none of the seeds from a sample will germinate.

  

Figure 5 (below) shows the results of an experiment where germination rates were compared at different temperatures. The successfully germinated seeds were counted and plotted early in the experiment (the red curve). Then, when germination appeared to have finished, successfully germinated seeds were counted and plotted again (the black curve).

The graph shows the germination rates of seeds at different temperatures. The x axis is temperature, from zero to forty-five degrees Celsius. The y axis is the percentage of seedlings that have germinated. There are two separate curves, corresponding to the first and final counts of germinated seedlings. In the first count, a few seedlings that had been kept between twenty and twenty-five degrees had started to germinate, but no seedlings had germinated at other temperatures. In the final count, there is a bell-shaped curve, with the best germination rates occurring in seeds germinating at temperatures between twenty and thirty degrees. Between these two temperatures, the germination rate was above eighty-five percent. At a temperature of ten degrees, only forty percent of the seedlings germinated, and at a temperature of thirty-five degrees, only about sixty-two percent germinated. There was no germination at temperatures below seven degrees or above thirty-seven degrees.
Figure 5: impact of temperature on germination

Activity 1

Allow 5 minutes for this activity

Using Figure 5, estimate Tmin, Topt and Tmax for this particular species. Write your answers down in the boxes before checking our answers.

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Discussion

You should have read off temperatures from the final germination (black) curve, although you may have found the first results (red) curve informative. Depending on what sort of screen you are viewing the graph on, your answers should be somewhere in the region of:

  • Tmin = 7°C
  • Topt = 25°C
  • Tmax = 38°C

Different species of seed have different optimal temperatures. If you are dealing with a seed you have not encountered before, there are clues in the climate and conditions in the type of habitat the plant originates from. As a rule of thumb, for accessions of temperate origin, try conducting germination tests at constant temperatures between 15°C and 20°C. For accessions of tropical origin, consider constant temperatures between 20°C and 25°C.

Germination requirements: gaseous environment

Alternating temperatures