Green Roof Plant Design - Understand Heliotropism and Paraheliotropism

Understanding photosynthesis processes in green roof plants is crucial to the green roof designer and we have discussed the difference between C3, C4 and CAM photosynthesis processes in prior notes, including;

• CAM Plants for Green Roofs, Effects of Heat and of Humidity
• Adaptation and Green Roof Plants

• Green Roof Vegetables - A List of Recommended C4 Plants

   

   

   

Under Intense Sunlight Leaves Fold to Prevent Desiccation

Yet nature is complicated and does not limit herself in resisting environmental stressor conditions to just the above three types of photosynthesis processes.  In fact, some plants have evolved other survival mechanisms to help prevent dessication and to conserve water, especially in arid and hot climates.

Some plants, and many of these are excellent species to use on Green Roofs, can move, open or close their leaves to prevent dessication.  This is commonly referred to heliotropism and paraheliotropism.

Cowpeas and other plants for example possess the ability to adjust their leaves position to either increase or decrease the amount of leaf surface area receiving direct sunlight.

Under optimal Sunlight C3 Leaves Open Widely

Under Optimal Sunlight Conditions C3 Leaves Open Widely

Heliotropism is the term used to describe a plant's actions when it orients leaves to receive more sunlight and thereby increasing photosynthesis capabilities, i.e. - more CO2 fixation.

Diaheliotropism describes a plant whose leaves actually track the sun'd path, maximizing solar contact.

Paraheliotropsim denotes a plants actions similar to the above photographs.  During paraheliopropsim, plants may fold or move their leaves to either;

• Minimize solar contact, or
• Minimize total leaf surface area,

either way reducing water loss and preventing desiccation.

Though C3 plants such as the beans - and most other of the world's food plants except the grasses, maize, and sorghum - do not possess as complicated a multi-cellular Calvin Cycle as C4 plants and are more susceptible to drought, they - through Heliotropism biomechanisms they do possess their own unique desiccation prevention mechanisms.

Green Roof designers take note!