Saturday, October 23, 2021

Which plant makes it drought tolerant? | Real dirt

You’ve often seen these terms: “drought tolerant” and “water-based.” For water conservation and sustainable gardening, master gardeners recommend plants that adapt to the long, hot, dry summers of the Mediterranean climate. But what are the qualifications of plants for membership in a drought tolerant society?

The beauty of evolution is its reliance on judgment and error or adaptation. The climatic conditions of the seven Mediterranean regions of the planet include five to seven months of zero rainfall and several consecutive days with high temperatures of 100 degrees Fahrenheit. These are challenging situations for any organism. Yet different types of plants have devised clever techniques to survive in these long, dry, hot periods.

‘The Real Dart’ is a column of various local master gardeners who are part of the UC Master Gardener in Butt County.

Generally speaking, three main strategies are used by plants to survive the annual drought situation: absorption tolerance, drought avoidance and drought tolerance. These techniques have evolved over millions of years of adaptation and are endlessly fascinating in their ingenuity. (Please note that the survival strategies described below, the results of complex chemical and molecular biological processes, are simplified for this article.)

Exploitation tolerance: When something dries, it dries well. Drought tolerance gives a plant an extraordinary ability to survive almost complete dehydration. This technique is employed by moss and ferns. In short, plants of this class have the ability to enter and recover from anhydrobiosis by shutting down metabolic activity as a result of low intracellular water content. The next time you hike to Bidwell Park or the foothills above in the dry months, you can see this phenomenon for yourself. Find a patch of rusty, wrinkled dry algae on a rock and gently pour a small amount of water over it. In seconds, what seemed to be a completely dead plant would turn green and tender.

Avoid drought: Many succulents use drought prevention techniques. The most common of these techniques is the Crassulacian acid metabolism, hence the name because this type of metabolism was first studied in plants of the Crassulaceae family. To avoid complete dehydration due to heat and drought, the leaf stomata are closed during the day to reduce the pressure of evaporation, but are open at night to collect carbon dioxide, from where they are able to complete the photosynthesis process in daylight.

Another way to avoid drought is to drop the leaves in advance. A good example of this is the bakai (esculus), which occupies a unique environmental niche by becoming one of the first leaves and flowering shrubs in early spring, and the first one to lose its leaves before the onset of attack is the heat and drought of summer. The leaves hold valuable nutrients and energy needs and can conserve these resources without them. In dry years, and during periods of high temperatures, our valley oak and blue oak lighten their metabolic burden by dropping some leaves more than usual.

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The main example of avoiding the annual drought. When they find the weather too unpleasant, they shrink and die and let their seeds out until the situation improves with nutritious rainfall and mild temperatures. (Interesting fact: California has significantly more local annuals than anywhere else in the world.)

Which plant makes it drought tolerant? | Real dirt
The point at the foot of the Blue Oaks California coastal range. (Laura Lucas / Contribution)

Drought tolerance: Finally, this catch-all phrase has. Plants of this class are better at working in annual drought conditions, due to many creative adaptations.

This type of plant is also called xerophytes – literally “dry plant.” These remain green all year round, but can often conserve or store water through structural (usually leaf) morphology.

The general structural adaptations for water conservation are:

  • Waxed skin leaves, waxed cuticles, which perform the dual function of reducing water loss and reflecting heat away from the plant. A prime example of our native cyanothas (California lilac).
  • Small, thin leaves that effectively reduce the surface area from which water can be lost. The small but highly fragrant leaves of Santolina mark this adaptation.
  • Submerged stomata peat, which traps moist air and reduces water loss. Pine needles use this technique (as well as smaller and thinner).
  • Hairy leaves, such as those found in Cyprus ironwort (Cideritis cypria) or Lamb’s Years (Stachis Byzantina), provide shade to the stomata and reduce contact with hot air, protecting the plant from light and temperature extremes.

Redundancy is a strategy for species survival (think of two kidneys in humans); And most plants use multiple methods to kill the hot dry summers of the Mediterranean climate. Now that you know what to look for, see how many simple biological adaptations you can see.

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