Alternating temperatures

Some seeds are even more specific about temperature requirements. Figure 6 (below) shows how some species, in their natural environment, experience a diurnal rhythm, where temperatures change from day to night. Seeds near the surface of the soil are most likely to experience diurnal variations in temperature, whereas seeds buried deep in the soil experience a more constant temperature.

Small seeds with very small food reserves, which germinate on the surface of soil, and the seeds of plants from wetland habitats are among those most likely to benefit from alternating temperatures in germination tests. Seeds that tend to be buried deep underground in nature are more likely to germinate at fixed temperatures, because temperatures deep down in the soil do not alternate as much.

The image illustrates different patters of germination at different depths in the soil. On the left side of the image, there is a soil profile showing the soil surface, topsoil and subsoil. On the right side, there is a graph illustrating how temperature varies at each of these levels. At the soil surface, there is a regular curve, oscillating from high temperatures during the day to low temperatures at night, repeating the next day, and so on. In the topsoil, there is still an oscillation between day and night, but the difference between the daytime peaks and the night-time troughs is smaller – the curve is shallower. In the subsoil, the curve has flattened out, so the differences between daytime and night time temperatures are barely detectable.
Figure 6: diurnal temperature rhythms in the soil

If you are working with species that are adapted to diurnal rhythms, and you mimic this pattern of alternating temperatures, germination tests may be more successful. For instance, watermelon seeds will germinate best when temperatures alternate between 20˚C for 16 hours per day (or 24 hour period) and 30°C for 8 hours.

Germination requirements: temperature

Germination requirements: light