I’m glad we are summer visitors to Ladakh with Rocks, Routes and Shoots given that average temperatures, even in the valley bottoms, fall well below zero at this time of year. I do have a bit of a yen to visit in the winter and maybe see a snow leopard but am not sure I could cope with the possibility of being stranded for days in sub-zero temperatures if the flights were cancelled, as they often are.
Plants, of course, don’t have the luxury of choosing when to be in Ladakh – it’s a clear case of ‘adapt or die’. In ‘What about the Cold?’, I looked at how plants in temperate regions undergo biochemical changes to prepare for falling temperatures in autumn. In the same way, winter dormancy is one important strategy for plants in the Himalayas; just like hibernating animals, plants enter a state in which they can survive prolonged periods of inactivity. Dormancy can occur either in the proto-plant or embryo within seeds, which can lie dormant in the ground during a cold winter, or in the growing parts of the plant itself, as over-wintering buds. This dormancy needs to be broken by a specific stimulus in order for normal life to resume when conditions improve. Maybe it’s time for a bit of a diversion into exactly how plants grow.…
One of the main differences between plants and animals is that animals, including ourselves, grow in a determinate fashion; we end up a particular shape and size determined by a mixture of genetics and the environment in which we find ourselves. Animals don’t change much in size and shape once they reach adulthood, assuming they don’t indulge in too many pies! Plants, however, grow in a very different way, with a flexible pattern of growth which continues throughout their life and allows them to adapt to conditions which may vary over time.
As with all things biological there is a trade-off at play here; mobility allows animals to move around and find the optimum conditions in which to grow, but this comes at a cost. We must invest in complex anatomical structures (such as a skeleton and musculature) to allow movement. Plants employ a different strategy – they have a relatively rigid structure, where each cell is firmly attached to its neighbours by a shared cellulose cell wall. They cannot just move away if conditions become unfavourable but, instead, must adapt their growth form to obtain the light, water and minerals they need. Their rigid anatomy means that plants have to grow in a particular fashion, by the division and then elongation of cells only found those parts of the body without thick cell walls. These tissues, known as meristems, are typically found at the very tip of roots and shoots.
The first meristems in a new plant are those at the tips of the root (radicle) and shoot (plumule) of the tiny embryo which lies inside its seed.
A runner bean seed split open; the radicle is emerging at the top and the plumule sits on top of the large cotyledon, one of a pair which fill a runner bean seed
Most of the space inside a seed is taken up by food to sustain the seedling until it is able to photosynthesise for itself. The meristems lie dormant until the seed absorbs water and starts to swell, bursting its tough outer seed coat. Often the water which causes the seed to swell is also responsible for washing away growth inhibitors which keep it in a dormant state. Once dormancy has been broken, first the radicle and then the plumule will start to grow rapidly. At first cells expand simply by taking up water but soon those at the tip must also start to divide to sustain the growth. The energy for growth comes from the breakdown of materials stored inside the seed, a process controlled by the embryo via plant hormones. In the case of the runner beans I’ll be planting soon, the food is stored in the pair of large seed leaves (cotyledons) which occupy most of its volume (see above).
In beans, the seed leaves remain in the ground as the seedling starts to grow rapidly, sending its radicle downwards and plumule upwards.
The plumule elongates into an axis or hypocotyl, which bears the shoot apical meristem and, within a few days, the first ‘true’ leaves unfurl.
Beans belong to a loose grouping known as dicotyledonous plants because they have a pair of ‘seed leaves’. The other major group of flowering plants, the monocotyledonous plants (monocots), have just a single tiny seed leaf and store foodstuffs for their developing seedling in starchy endosperm material rather than in this seed leaf. This group includes grasses and all our staple cereals – rice, wheat, maize and so on.
Of course all plants manufacture sugar and starch as their initial energy storage material but in dicot seeds an extra stage occurs before the seeds mature. The starchy endosperm is consumed during seed development and used to produce the large cotyledons, which are rich in storage proteins. These seeds contain around twice as much protein as cereal seeds, hence the importance of legumes and pulses in a vegetarian diet.
When a dicot seed starts to germinate, the seed proteins are broken down rapidly into their constituent amino acids and transported to the growing plumule and radicle, giving the seedling a head start with the production of key proteins such as rubisco, essential for photosynthesis. In plants such as my pumpkin and courgette seedlings where the seed leaves emerge above the ground, the advantage is even greater – pre-synthesised rubisco is ready and waiting to catch the first rays of light.
Young courgette plant – the seed leaves are easily distinguished from the first ‘true’ leaves by their rounded shape
Not many wild cucurbits grow in Ladakh, of course, though there are plenty of legumes, so I’ll have to come back to dormancy in seeds and buds in another blog…
Oxytropis microphylla, a member of the pea/bean family