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Water is essential to plant life. Even plants that live in arid deserts need moisture and have developed sophisticated techniques to capture and retain moisture.
How Plants Use Water
Plants pull water from the soil into the plant; the water is in the form of a 'soup' that contains the nutrients that the plants use to grow. The plant draws the water from the roots and, in a process called 'mass flow', it moves up through the plant to the leaves.
The water drawn into the plant is lost through the leaves in a process called transpiration. Transpiration occurs when the water reaches the leaves and escapes through tiny holes called stomata.
Water Loss
The amount of water each plant loses varies; some plants have developed ways to reduce water loss while others, such as tropical plants, typically lose great quantities of water.
Most plants, however, lose over 95% of the water they take up through transpiration. The average tomato plant will transpire about 136 litres of water in a growing season.
Cold Deserts
 We often assume that dry equals hot. Often dry climates are also be extremely cold. Dry rainless winter temperatures can fall well below freezing. One of the driest regions on the earth is also the coldest - Antarctica.
Not every dry climate plant will survive cold temperatures, and for gardeners in colder regions, looking at a plant's natural habitat is a key to success.
Many dry climate plants come from light sandy soils that drain quickly and are almost as dry in cold winter months as summer. Plants from these regions will not survive cold, waterlogged soil. Other plants have adapted to handle the wet, damp and cold soils of winter along with the dry soils of summer - many of these plants are winter dormant for this reason. (For more see Coping with Winter Cold)
Wind - Compounding the Problem
Wind dries moisture on leaves, desiccating plants, and it dries out the soil. Plants in exposed, windy situations often face a double dose of drying climatic conditions.
Adapting with Dry Conditions
There are lots of plants that wilt as soon as the weather becomes hot and/or dry, and the wind makes them even thirstier. Drying winds, coupled with low rainfall, creates extremely dry conditions for plants.
In parched climates, hot or cold, plants only the resilient survive. Foliage, root, stem and life cycle adaptations all enable plants to tough out and survive in arid conditions.
Foliage Adaptations
 One of the most obvious adaptations to dry climates can be seen in the foliage of plants that cope with dry periods without turning a proverbial hair.
Plants loose water through their leaves in transpiration and on hot days a significant amounts of water can be lost, but in dry climates plants cannot replace lost water easily and in low-rainfall areas plants and have adapted to reduce transpiration water loss.
The most water-efficient plants combine a number of water conserving strategies.
- Fewer stomata, the pores in the epidermis or skin of the leaf on the leaf can reduce water loss
- In hot dry climates many plants open their stomata only in the cool of the evening.
- Small leaves (totora, lavender) have a reduced surface area to enable plants to conserve water, as do feathery, filigree leaves (romneya, eschscholzia or Californian poppy).
- Needle-like foliage (grasses, pine trees) has a very small surface area that looses very little water. Some of these plants can roll their leaves inwards, further reducing surface areas and water loss.
- Leafless plants, such as brooms, have lost their leaves altogether with stems undertaking essential photosynthesis and food production.
- Grey or light coloured foliage reflects light, reducing heat and thus water loss from transpiration
- Thicker cuticles, the outer waxy coating on leaves, slows transpiration; many coastal plants have a thick glossy coating on the leaves, reducing water loss and also protecting from salt-burn.
- Silver hairs coating leaves reflect light and help to lower temperatures inside the leaf, as well as reducing the effect of drying winds.
- Thick fleshy leaves can store water; many succulents have thick leaves that, when bruised or broken, contain a great deal of moisture.
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Yuccas and tough shrubs in arid northern Texas, USA
Transpiration
Plants absorb water from the soil in a 'soup' containing needed nutrients. The moisture is drawn up through the plant stem and lost through tiny pores, stomata, in the leaves.
In hot weather, evaporating moisture on the leaf surface serves to cool and protect leaf tissues. Plants with soft, lush foliage will lose huge amounts on very hot days - that's why tropical forests feel so muggy and humid, all the water expelled by the plants makes for a very damp atmosphere indeed.
Plants have different densities and size of stomata, releasing more or less moisture depending on their habitat. Other foliage adaptations serve to cool leaf tissues, reducing the need for transpiration.
Dry soil, hot air, low humidity and wind can all cause stomata to close. Some plants, adapting to survive hot dry climates, open their stomata only in the cool of the evening when the need for transpiration is at its lowest. |
Pachystegia insignis, the Marlborough Rock Daisy, leathery leaves and dense tomentum covering on new leaves
Corokia leaves are small, reducing surface area, and reflect light
Grasses such as Carex comans have narrow leaves with minimal surface area
Echeveria - thick fleshy leaves hold moisture and pale colours reflect light
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