Thursday, February 24, 2011

Green Roof Design - Plant Selection and Integration, Photosynthesis Basics cont.

Plants with C4 photosynthesis processes are now the focus of bioengineering efforts of the worlds foremost plant industry experts.

The Symposium abstract pdf offers significant insight into the interest of C4 plants' capabilities to survive where C3 plants may not.  Of course, most of the interest now lies in areas of food production (think rooftop permaculture), however horticultural paths, including green roof plant development will soon become a part of the C4 genetic engineering efforts.

As a biodiversity purist I emphatically prefer and recommend use of open pollinated native plant species on green roofs and open pollinated seeds for rooftop permaculture.  But we must take note that the industrial agriculture world understands the benefits a C4 plant offers and we need watch advances in genetic engineering, tracking potential impacts to native plant species and ultimately biodiversity issues.  Keeping abreast of industry trends is important to ensure continuation of biodiversity practices too.

One very clear fact is this:  C4 plants are much more hardier than C3 plants against wind desiccation and dehydration.

Brown, dead Poeaceae C3 species and green, alive Alliaceae C4 species subjected to 48 hr. 1.75 M/S winds

The photo here is of a trial green roof panel, removed from the roof after being subjected to 48 hours of low humidity (approximately 40% relative humidity water vapor) 1.75 meter per second winds, and approximately 12 degrees C average temperature.

The panel is planted primarily with a cool season annual Poaceae C3 species.  There are also several Alliaceae family C4 photosynthesis plugs int he panel.

Remember, the C4 plants, because of their internal structure, loose on average only 1/3 the amount of water during the Calvin Cycle and photosynthesis as C3 plants do.  C4 plants are much more resistant to dessiccation from strong winds.  This is because C4 plants generally conduct photosynthesis by:

  • Absorbing CO2 into the mesophyll - 
  • Once the CO2 is in the mesophyll it combines with a 3-carbon compound called PEP, phosphoenolpyruvic acid to form a 4-carbon substance called oxaloacetic acid - hence the C4
  • The C4 is moved into the deep bundle sheath layer of cells away from the leaf surface
  • Inside the bundle sheath the C4 breaks down into the C3 base and eventually PEP for the Calvin Cycle food production
Because the oxaloacetic acid (C4) is moved into the deep bundle of sheath layer cells away from the leaf surface, the C4 plants loose less to transpiration activities from wind and sun action.

Yet there are many additional considerations to integration of C4 and C3 plants on a green roof, including growth rates.  Because C3 plants conduct photosynthesis much more rapidly (remember the analogy between the C3 and C4 plants as a comparison of a large V8 engine vs. a smaller hybrid engine), C3 plants will provide green roof coverage quicker.

Understanding how to integrate C3 and C4 and then CAM species is key to a successful green roof design.  Knowing how green roof plants utilize photosynthesis is key to a successful green roof.

Too many times we in the green roof industry have focused on stone and soil media, plastics and drainage, pumped up additional irrigation and added more and more fertilizers when all we really needed to do is put the right plant in the right place.

Green Roofs should really be about the plants and biodiversity.

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