It’s not just animals and people that feel the effects of stress. In many ways, plants are even more vulnerable than we are, lacking our option to move away from whatever is causing us difficulties. Plants have little alternative other than to hunker down and get on with life in the place where they find themselves, if they want to survive at all.
Of course this hunkering down can take many forms; if the threat is just making life uncomfortable, rather than impossible, a species may be able to adapt by slow genetic change over a number of generations until it better fits its environment. However sometimes that is not enough, and a plant has to be able to acclimatise quickly by changing its structure or function to survive at all. Such temporary changes to physiology or morphology, not passed on from an individual to its offspring, are known as phenotypic plasticity. Phenotypic plasticity is particularly important in letting plants respond to a rapidly changing environment but often the two processes act together to contribute to plants’ tolerance of extreme environments.
So what causes plants stress? Factors such as a lack of water (or occasionally too much water), extremes of temperature, too much or too little light and soil conditions are among the most important abiotic influences on plant growth (those resulting from the environment rather than other organisms). The first three of these (water shortage, temperature extremes and too much light) are likely to be particularly significant at the sites we visit in Ladakh and, of course, they all interact with one another to exacerbate the effect. At 3000 to 5000 m above sea level, the altitude has an important effect on temperature, light and water availability.
Of these factors, water availability is probably the single most important in determining whether or not plants can grow. It is no coincidence that one of the most botanically diverse places we visited in Ladakh was below the Thajiwas glacier at around 2800 m.
Thajiwas glacier, Sonamarg
Here, meltwater seeping out from beneath the snow pocket allowed the growth of an amazing variety of plants amongst the rocks. Many of these were the type of plants that, at home, we would associate with damp, shady areas, despite the aridity of the surrounding landscape.
Ferns need water for reproduction so are generally a good indicator of a habitat where water is not in short supply and buttercups, forget-me-nots and eyebrights are all plants associated with damp meadows in the UK. Their presence indicates a microhabitat here quite different to what we might expect – in much of Ladakh, where annual rainfall averages around 10 cm, drought is a serious problem.
Much of the water taken up by plants where water supplies are adequate (up to 97 %) is ‘wasted’ in the process of transpiration – the evaporation of water through pores called stomata on the leaf surface. Only around two percent is used to drive cell expansion and one percent for metabolic activity. The role of the stomata is to allow plant leaves to take up the carbon dioxide gas needed for photosynthesis but the loss of water through the open pores is an inevitable consequence. Of course this water is not really being wasted – water entering the plant through the roots brings with it dissolved minerals which the plant needs to survive. The water itself then passes up the plant through a pipe-like system of xylem vessels in the stem and out through the leaves. In the process it keeps cells fully hydrated or turgid – necessary both for growth and to act as a hydraulic skeleton to support the plant. Normally the stomata are open during the day, when photosynthesis occurs, and closed at night. This allows the plant a chance to rehydrate itself at night.
Leaf surface showing three stomatal pores (false colour). Image: AJC1 via Creative Commons
A low relative humidity in the atmosphere (as found throughout Ladakh for most of the year) creates a problem for plants because water may evaporate more rapidly from the leaves than it can be replaced, rather as if we are sweating heavily but unable to drink. When this happens, a plant is said to be in water deficit – a situation which it can tolerate for a short while but which will, eventually, lead to permanent wilting and death. The problem is exacerbated, in very dry soil, by the fact that water is held so tightly by the soil particles that roots cannot take it up at night to rehydrate the plant.
Those plants not lucky enough to have their own private water supply can respond to drought and other stresses in a variety of ways including temporary metabolic adjustments, changes in organ shape, plant architecture and life cycle, to either avoid or tolerate stress. Many of these changes are triggered by a signalling pathway within the plant which switches on or off the particular genes needed to produce the appropriate response. More on this to come…