The perennial grasses can be classified as either C3 or C4 plants. Also known as cool season and warm season grasses. As a generalisation, C3 plants are more temperate, preferring cooler, moister conditions – winter active. C4 plants grow better in warmer, dryer conditions. The terms C3 and C4 refer to the different pathways that plants use to capture carbon dioxide during photosynthesis. All species have the more primitive C3 pathway, but the additional C4 pathway evolved in species in the wet and dry tropics. The first product of carbon fixation in C3 plants involves a 3-carbon molecule, whilst C4 plants initially produce a 4-carbon molecule that then enters the C3 cycle.
Leaves of warm season grasses have thick walls around the vascular cells, these ‘walls’ are known as bundle sheath, these cells are absent from cool season grasses. These cells are packed with chloroplasts, mitochondria and other organelles to ensure that their overall rate of photosynthesis is rapid. In warm season grasses, C4 compounds are passed quickly though these cells where they release CO2. This CO2 is then utilized just as it is in the cool season grasses, and the products can be shunted directly into the vascular cells for transport to those parts of the plant where they are needed for growth.
Why are these differences important?
These differences are important because the two pathways are also associated with different growth requirements. C3 plants are adapted to cool season establishment and growth in either wet or dry environments. On the other hand, C4 plants are more adapted to warm or hot seasonal conditions under moist or dry environments. A feature of C3 grasses is their greater tolerance of frost compared to C4 grasses. C3 species also tend to generate less bulk than C4 species; however, feed quality is often higher than C4 grasses.
The presence of both C3 and C4 species can be desirable in a pasture as they can occupy different niches (e.g. C3 species are often more abundant in the shade of trees and on southerly aspects, while C4 species often dominate full-sun conditions and northerly aspects) and thereby provide greater groundcover across a range of conditions. It is not uncommon to find both C3 and C4 species in one paddock.
This has advantages in providing a broader spread of production throughout the year for both grazing enterprises and native animals. You can find a mixture of C3 and C4 grasses during the transition of winter to summer or summer to winter. Most commonly in places like Australia and along the ‘transition zone’ in the USA, where winters are too cold for warm season grasses and summer is too warm for cool season grasses.