Let's take another look at growing rooftop food gardens. Rooftop permaculture is a rapidly growing movement across the world. Highly productive vegetable gardens can be found on green roofs in nearly every city.
Choosing vegetables that possess C4 photosynthesis capabilities can help ensure rooftop garden success.
Examples of C4 plants include;
Vegetables & Flowers -
- Mustard Greens
- Cress & Watercress
- Most lettuces
- Jerusalem artichoke
- Tagetes marigold
- Goldenrod, and more
The beauty of C4 plants is their ability to withstand drought and the harsher environment of many rooftops.
Generally, C4 plants loose only one third the amount of water to evapotranspiration as C3 plants, giving them a great advantage on the roof.
Remember, we've discussed C$, C3 and CAM photosynthesis processes in prior posts.
Photosynthesis occurs in most C3 plants by:
- 'Breathing' in CO2 and combining the CO2 with a 5-carbon sugar
- The reaction is facilitated by an enzyme, Ribulose Bisphosphate Carboxylase Oxygenase or RUBISCO abbreviated
- Now there are 6-carbons
- The 6-carbon chain breaks into 2 molecules of a glyceric acid called PGA - 3-phosphoglyceric acid
- The completed reaction is called the Calvin cycle and the PGA is the base for glucose synthesis, along with other carbohydrates
However, some plants have an supplemental or alternative method of implementing the above food production cycle.
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
CAM plants are different from C4 plants in several ways such as CAM plants taking in CO2 only at night.
Interestingly, many C4 plants have developed the capabilities to switch back and forth between C3's 'full-steam ahead' photosynthesis on the leaf's surface (where the process can be interrupted by environmental conditions) to the C4 process where the photosynthesis required compounds are protected by layers of plant cells.
The process of switching back and forth between C3 and C4 gives a plant the opportunity to grow quickly, a survival advantage, during good weather and the switch into energy conservation survival mode when extreme and damaging weather arrives.
C4 plants, since the plant's photosynthesis process is protected deeper under the surface of the leaf, are more resilient to drought and extreme temperatures.
Therefore, under certain conditions and when placed in certain roof locations, make very good green roof plants.
Realize however, green roof plant selection is modeled on many inputs, C3, C4 or CAM type just being one factor.
However the green roof plant designer must understand the difference in the photosynthesis processes taking place in the plants being specified on any green roof.
There are many interesting discussions about C4 and C3 plants and climate change. We will reserve a future discussion on insights into whether C3 or C4 or CAM plants will have an evolutionary advantage in the future with increased levels of CO2. Surprisingly - no - not so surprisingly, 'Mother Nature' always steps in and provides adaptation mechanisms. We will discuss these adaptations both C3 and C4 plants implement to survive either higher or lower levels of CO2.
Having moved through some heavy technical facts on C3, C4 and CAM plants we hope the discussion has not been too boring.
The real photosynthesis processes are much more complicated than we've talked about in these articles. Understanding the basic differences between C3, C4 and CAM plants will allow the green roof plant designer to greatly increase survival opportunities for their green roof design.
As always, email your comments or questions.
Happy Green Roofing!