Plants and Drought
By: Thuận Sarzynski
Tags: Plants, Water, Drought, Evolution, Adaptation

Plants and Drought

When you need water but you cannot move: the big problem that all plants face.

A drought is a natural event in certain regions of the world where it is the consequence of the cycles of Earth around the sun.

Earth has an ellipsoidal rotation around the sun. An ellipsoid is not a perfect circle but rather an ellipse, shaped like an egg. Turning around the sun in an ellipsoid fashion, Earth does not receive the same amount of sunlight and energy depending on its position and time of the year. In its revolution around the sun, the Earth does not stand straight but it slightly leans on one side in a 23.5 degrees angle from vertical. We can describe this weird position as a tilt.

This tilt added to the imperfect rotation of the Earth around the sun creates seasons. We know them as winter, spring, summer and fall in temperate countries far from the equator, and as rainy and dry seasons in tropical countries around the equator.

When one side of the Earth is exposed to the sun, it receives a lot of heat and sunlight. Heat makes the water evaporate from the ground. If it doesn’t rain to compensate for this evaporation of the water, the ground will become dry.

A drought is a shortage of water in the ground often due to a lack of rainfall. Plants do not really enjoy droughts because as all living things, plants need water to stay alive. Every single chemical reaction which happens in a living thing happens in a water-based liquid, most commonly sap or blood. We, all living things, are completely dependent on water to live.

As a human being, I often walk to get drinkable water and quench my thirst. In some countries, I can drink tap water, in others I need to get to the supermarket to buy large water bottles. The point is that I can move. I have legs. I love them.

Other living organisms do not have legs and cannot move. For example: plants. Plants are rooted to one place. They live at the exact same place where their seeds fell and germinated. They cannot move to another place with water to drink. They must find solutions to survive during a drought and keep water flows in their bodies.

The first thing plants can do to resist a drought is to get rid of some of their leaves. Leaves are very important for plants since they allow them to photosynthesize or create sugar to ‘eat’. However, during a drought, sacrificing some leaves allows a plant to keep more water inside. Losing leaves allow a plant to reduce its surface of exchange with the atmosphere and thereby lose less water. One disadvantage of this strategy is that a plant will photosynthesize less with less leaves and therefore create less sugars. Less sugar can mean less growth as well as less fruit production. Us, human beings really care about plant growth and fruit production because we eat the leaves, fruits, and vegetables of plants, or we rely on them to feed our livestock.

Against drought, plants can also behave in a smart way to save water. A plant can close the small holes on his leaves called stomata to prevent transpiration or water loss. This way, the plant loses less water but again, it also reduces its photosynthesis. Stomata are the place on a leaf where all gas exchanges happen between a plant and its environment. Through its stomata, a plant can release oxygen and take carbon dioxide which will later be changed into sugars during photosynthesis.

A plant needs to make a choice between absorbing carbon dioxide with opened stomata or saving water with closed stomata. A plant species or variety adapted to drought will close its stomata during the dry time of the day and open them wide during fresh hours. Environmental stress like low soil humidity and low air humidity will trigger the closing of stomata. In a dry environment, resistant plants will close their stomata while sensitive plants will keep them open leading the latter to dry out and, in the worst case, die.

At the molecular scale, plants can also produce more sugars and other solutes like polyamines, polyols and other dissolvable substances. This in turn adjusts their osmotic level and helps them tolerate drought.

Water is naturally attracted toward medium with high solute concentration, therefore a plant richer in sugar will better attract and retain water. In addition, certain organic solutes protect the plant against oxidation. Oxidation is when extra oxygen atoms attach themselves to the plants molecules, changing their nature in the process. Oxidation occurs during water shortage and is very toxic to plants as it creates unwanted molecules. To cope with oxidation, plants produce dehydrins, molecules produced during dehydration, to reduce cell damages during a drought.

The molecules that are produced during a drought stress creates a cascade of response involving various genes which can release molecules such as osmoprotectants and antioxidants to increase drought resistance and to change the behaviour of the plant for example, abscisic acid which controls stomatal closure. The activation of certain genes can even transform the anatomy of the plant increasing the length of its roots, thickening its leaf cuticle or wax layers, and decreasing the number of stomata on its leaves.

Certain plant species and varieties are already adapted to drought. They either express genes or produce molecules for drought resistance that other varieties do not. These plant species and varieties are very valuables to us, plant-eaters, because they will allow us to breed new varieties productive in dry conditions. Wild relatives of edible plant species like wheat, corn, rice, or even coffee should be protected in their natural environment and studied to know whether they are drought resistant and why.

In the future, knowing how to design varieties which can produce more or the same amount of food with less water will be crucial for our food system. As we did a green revolution in the 70’s where we increased the world food production, now we need a blue revolution where we maintain our food production with less water.

This kind of targeted selection takes a long time. As you know plants depend on water in so many different ways and at so many different levels — from robust roots down to minute molecules. Selecting the correct genes for all these levels can seem like an impossible task!

Then again, we can look to the drought-resistant plants of today. Over the millennia, they have learnt to live with droughts, and have become something of experts at how to deal with them

For now, even humankind has to work with the plants produced by nature — the most diligent genetic engineer of them all.

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