More thirsty plants

So how do plants which live in the drier areas of Ladakh cope?

Some desert plants make the most of water when it is available (mostly July and August in Ladakh) by having an annual life cycle and a particularly short growing season. They germinate at the first hint of warmth and moisture in the soil and grow quickly to the point of flowering and setting seed. Seeds dry out naturally as they ripen and, once dehydrated, can live in the soil for many years in a sort of suspended animation until conditions are suitable for a new plant to grow. Some seeds carry an added layer of protection in the form of dormancy. In a climate such as that of Ladakh, where winters are bitterly cold, seeds may require a period at a low temperature (known as vernalisation) to break this dormancy. This provides a safety net to ensure than the seeds do not germinate in a particularly warm or damp autumn, only to be killed by the winter weather.

Amongst the most dramatic of these annual plants are many poppies, including perhaps the best known, Papaver somniferum. When the opium poppy seed capsule reaches maturity, it contains thousands of the tiny black seeds we know as poppy seeds, though anyone looking for an opiate hit from these will be disappointed. Opium is produced from the latex harvested by making incisions in the green seed capsules. When the capsule dries out, the disc at the top pulls away from the base leaving a series of holes, out of which the seeds are shaken, like pepper from a pot. At least a few of these are likely to survive long enough to grow into new plants.

Opium poppy Bowburn Opium poppy

Papaver somniferum

Other plants adopt different strategies to allow them to survive in this hostile environment. One common strategy is to modify the leaf structure in some way. When growing where there is plenty of water the outer layer, or epidermis, of plant leaves is punctuated by many of the tiny stomatal pores we looked at in my previous post (Plants get stressed too!). These serve the dual purpose of allowing gas exchange and helping to cool the leaves by evaporation when temperatures are high. However when water is in short supply, the density of these is much reduced. This is easy to show by preparing a stomatal peel using clear nail varnish painted onto the leaf surface.

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Leaf peel from lower epidermis of Ficus sp.

Many plants growing in dry areas reduce the size of their leaves, to minimise the area over which water can be lost; this can either be a short term acclimation or longer term adaptation. Leaf growth is, of course, a function of both cell growth and cell division. The rate at which cells divide is reduced in plants under water stress when one of the signalling cascades I mentioned previously slows down replication of the genetic material, DNA, which is necessary for the cell cycle to continue. Cell expansion is also reduced because cells will be less turgid under mild water stress so the cell contents cannot exert the outward pressure on the cell wall which is one of the driving forces for cell growth.

Whilst a reduction in stomatal density will reduce a plant’s ability to cool itself by evaporation, small leaves have a reduced boundary layer effect (air cannot flow so freely over them) so can lose heat to the air more rapidly to compensate for this. Dividing a leaf into lobes has a similar effect and you can sometimes see this example of phenotypic plasticity in different locations on same plant, dependent on the microhabitat experienced by each leaf.

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Leaves of an Indian Chinar tree (Platanus orientalis) in Dachigam National Park, Srinagar, where annual rainfall is around 250 mm

Some plants take this to extremes – all that remains of leaves in the Ephedra gerardiana we found growing near Leh are tiny brown scales at the joints of slender green branches. Here the branches have to take over the leaves’ photosynthetic role.

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Ephedra gerardiana, growning near Leh (average rainfall 100 mm)

Unappetising as it may look to us, Ephedra is an important winter food for goats and yak in Ladakh and is also the source of the alkaloid ephedrine.

Another strategy employed by plants is to have hair like projections known as trichomes on the leaf surface, giving it a silvery grey colour. Trichomes help to trap moist air near the surface of the leaf and, at the same time, reflect sunlight to keep the leaves cooler. Some species produce leaves without trichomes in spring, to maximise the amount of photosynthesis which can take place at cooler times of the year.

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Nepeta floccosa (Catmint) growing on scree at Shang Sumdo

Of course many plants in the most hostile environments combine several of these adaptations, alongside metabolic adaptations such as Crassulacean Acid Metabolism, which we’ll come to next.

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One thought on “More thirsty plants

  1. Pingback: How did plants evolve stomata? | heatherkellyblog

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