Why Are Aloes Water-Wise

Aloe plants are succulents, so they don’t need a lot of water. As a succulent special water-storing tissues called parenchyma in its leaves that help it survive drought.  This allows the aloe to store water within its tissues, as a means of coping with prolonged dry periods in those areas where it grows naturally.   The thick epidermis of an aloe is important for water retention a sit prevents excessive transpiration.

Cross-section of an aloe leaf

Aloes are successful because of several important adaptations to their environments.  They use a special kind of photosynthesis called CAM (Crassulacean Acid Metabolism) that minimizes water loss that would occur with standard photosynthesis in hot climates.  CAM photosynthesis occurs in nearly all other succulent, drought-adapted plants.  In “normal” plants, the pores (called stomata) act as a vent that takes in carbon dioxide as well as releases water for cooling (transpiration). 

On a very hot, dry day, the amount of water that can escape through the stomata would cause a desert plant to dry out, but it needs to keep these pores open to collect the carbon dioxide needed for photosynthesis.  In CAM photosynthesis, the stomata only open at night.  This way, the plant gets as much carbon dioxide as possible while minimizing water loss through the stomata. 

The carbon dioxide is stored overnight as an acid (malic acid) until the next day, when sunlight provides the cue to release the carbon dioxide, which the plant then converts into sugars, the energy currency of plant cells, to use and store for later use.  Another adaptation is an aloe’s ability to store relatively large amounts of water in the leaves, so in times of drought they will have a supply to draw upon.  A waxy coating on the surface of the leaves of aloes also helps with moisture loss by minimizing evaporation of water from cells on the surface of the leaves and limiting water loss to the stomata.  The wax can also reflect excess light away from the plant.  This wax can be seen on the leaves of most aloes as a bluish colour cast.

How do I know when to water

Lack of water causes drought stress, which causes the aloe leaves to curl inwards.   The aloe does this to reduce the amount of water stored in the leaves from being lost.

Not getting enough water, the leaves will often become wrinkled and may shrivel up and turn a little yellow or brown on the ends.

Overwatering and its leaves may turn pale or yellowish and may ultimately become mushy and brown, causing a bacterial rot.

All Aloe species follow the Crassulacean acid metabolism (CAM). CAM plants can fix carbon dioxide at night and photosynthesize with closed stomata during the day, thus minimizing water loss. This, together with their succulent leaves and stems and the presence of a thick cuticle, makes them well adapted to dry conditions.