| What are C4 Plants?
Plants with the C4 photosynthetic pathway
rank among the world's most important crops and noxious weeds.
Maize, Sorghum and Millet are staple foods throughout the
tropics, Sugarcane is traded globally, and 14 out of the world's
18 worst weeds are C4 plants.
Less than 4% of plant
species use the C4 pathway, but on a global scale
they annually account for 20% of plant growth (net primary
production). Yet this tremendous worldwide significance has
been reached in a short stretch of geological time, with the
evolution of C4 species from C3 predecessors
occurring only within the last 25 Million years or so.
|
|
What is C4 photosynthesis, and how does it work?
The C4 pathway is a fuel injection system for photosynthesis that increases the rate of leaf sugar production in hot climates.
Most plants on Earth use the C3 photosynthetic pathway, which fixes carbon dioxide (CO2) from the atmosphere using the enzyme Rubisco. The C3 cycle uses this fixed CO2 and energy from sunlight to manufacture sugars. The process is inefficient because Rubisco is not saturated and not very specific, which means that it also fixes atmospheric oxygen. This inefficiency increases at high temperatures and low CO2 concentrations.
C4 plants overcome the inefficiency of C3 photosynthesis using a combination of anatomical and physiological tricks. First, the C3 cycle is isolated from the atmosphere within a leaf compartment. Secondly, the C4 cycle pumps CO2 into this compartment, filling it with CO2 and ensuring that Rubisco fixes nothing else. The pump is powered by energy from sunlight, and works by using the enzyme PEPc to fix carbon in the form of bicarbonate (HCO3).
|
|
|
How did C4 plants evolve?
C4 plants have the photosynthetic edge over their C3 contemporaries when atmospheric CO2 is low, and light and temperature are high. Current evolutionary theory therefore suggests that C4 photosynthesis raises the likelihood of plant survival under low CO2 and hot conditions, resulting in C4 evolution by natural selection.
The factors leading to C4 plant evolution are undoubtedly strong, because the photosynthetic mechanism has appeared independently more than 30 times in different plant groups. This striking example of convergent evolution is remarkable because the C4 pathway is complex, and requires the coordinated expression of more than 30 genes. It raises the important question - how did this complex trait evolve so many times?
New geological evidence suggests that the early success of C4 species occurred against a background of relative constant atmospheric CO2, suggesting that CO2 was not the only trigger for the rise of these plants. In support of this finding, recent experiments suggest that life history and water balance may be as important as photosynthetic rate in mediating the effects of CO2 on plant fitness. These findings raise another important question – what factors are responsible for the ecological success of C4 plants, both in the modern world and the geological past?
|
| What determines the modern distribution of C4 plants?
The global distribution of C4 plants in today's world is mathematically modelled using current understanding of plant relationships with CO2, climate and soils (Map - Ian Woodward and Mark Lomas). C4 grasslands (orange) have evolved in the tropics and warm temperate regions where forests (green) are excluded by seasonal drought or fire. C3 plants (yellow) remain dominant in cool temperate grasslands because C4 grasses are less productive at low temperatures.
|
 |
|
 |
Research overview
My research investigates the evolution and consequences of C4 photosynthesis. Over the past three years, the emphasis of this work has been on the model species Alloteropsis semialata – a grass with C3 and C4 subspecies. How has the photosynthetic pathway evolved in this species? And what are the implications of this evolution for its ecology?
The aims of this work are twofold. First, to build up a detailed picture of the physiological ecology of this species, using an experimental approach. Secondly, to use key experimental findings to formulate general hypotheses about C4 plants.
See my main page for links to each of these projects. |
|
|
Home
