A plan for all seasons.

Bluebells at Ripley, North Yorkshire. Credit: John Bryant.

John writes …
 
It’s all about the tilt.
As we have mentioned before, Earth is a planet with ‘added interest’, namely the existence of different seasons, caused by the tilt in its axis (the 'obliquity of the ecliptic'), relative to the orbit plane, as shown in the picture below. As we move from the Equator towards the poles, the greater are the inter-seasonal differences in temperature and daylength. Indeed, at the poles, daylength varies from total darkness around the winter solstice to 24 hours of daylight around the summer solstice. 

The Earth's rotational axis is not normal to the orbit plane, and it is this axial tilt that produces the seasonal variations. Credit: Source unknown.

​But we have a problem.
In northern Europe, Spring is the most spectacular season, with the very obvious environmental change from the mainly brown hues of winter to the vibrant greens of new leaves and the range of glorious colours of myriad Spring-flowering plants. The soundscape becomes punctuated by birdsong and there is a general impression that the natural world is waking up. But this typifies the problem alluded to in the sub-heading. We think of natural selection as the selection of genetic variants that are best suited to their environment, leading to greater reproductive success. However, the environment is not constant but is subject to the seasonal variations that I have already alluded to, albeit that those variations are regular in the annual cycle. Thus, organisms living in, for example Finland, are subject to very different climatic pressures from those living in, for example, Kenya (I do not intend to discuss the changes in distribution and location of landmasses over geological time – this adds another, albeit very long-term perspective to the discussion). In this post and in my next contribution (probably in July), I want to discuss some of the features of various plants and animals that enable them to flourish in regions with significant seasonal variations.

Spring greens at Dove Cottage, Grasmere. Credit: John Bryant.

Out in the cold.
Moving quickly from Spring to Autumn, think of a tree, growing in northern Europe, that has produced seeds in September or October. The weather is still warm, warm enough to support seed germination and early seedling growth. However, the seeds do not germinate even when conditions seem ideal. We say that the seeds are dormant. Now let us move forward to the start of the new year. The weather is cold, the temperature may be below 0°C (depending on where you are) and indeed, there may be snow on the ground. These are not ideal conditions for young seedlings and thus it is a good thing that those seeds did not germinate.

Norway Maple tree. Credit: British Hardwood Tree Nursery.

​Allow me now to introduce the Norway Maple (Acer platanoides), a beautiful tree that has been planted in many parks in the UK. There is a particularly fine specimen in Ashton Park, Bristol. Earlier in my career, I and Dr Robert Slater studied the regulation of genes in relation to dormancy and germination in this species. It was already known that in order to break the dormancy, seeds needed to be kept moist for about 100 days at temperatures of 5°C or lower (a process known as stratification). Only then are the seeds able to respond when conditions such as soil temperature become favourable for germination. This means that in many parts of its range, established after the most recent glaciation, the seeds of Norway Maple rarely or never germinate and that includes specimens planted as ornamentals in parks in southern Britain. Further, this problem (for the tree) is exacerbated by climate change.

‘Helicopter’ seeds of Norway Maple. Credit: Gardener’s Path (1).

One of the things that happens during stratification is a change in the ratio of the concentrations of growth inhibitory hormones to concentrations of growth-promoting hormones. The changing ratio is in effect a measure of the length of the cold exposure. One of our key findings is that the genes associated with germination growth are not active until an appropriate ratio of growth regulators has been reached; thus we saw a flurry of gene activity at the end of the period of stratification. I need to say that Norway Maple seeds are not unique in requiring exposure to low temperatures before they can germinate. The seeds of many plant species native to north temperate regions, including the region’s tree species, exhibit the same trait, although few require a cold exposure as long as that required by the Norway Maple.

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Further, it is not only seeds that require a cold exposure before becoming active. Think now of biennial plants, plants that flower in the second of their two years of life. The buds which give rise to flowers in year-two will only do so after the plant has gone through a cold period; exposure to cold thus primes the floral buds to become active, a process which is known as vernalisation. Thus in my garden, the Purple Sprouting Broccoli plants that I am growing will not produce their purple sprouts (i.e., flowering shoots) until next Spring.
 
In summary, the seeds and plants that I have described cannot respond to the increasing ambient temperatures in Spring until they have experienced (and ‘measured’ that experience) the harsher conditions of the previous winter. Natural selection has thus led to the development of mechanisms that faciltate flourishing in areas with marked seasonal variation. ‘For everything, there is a season’
 
John Bryant
 
Topsham, Devon
May 2024
 
(1)  https://gardenerspath.com/