|Seed of Spinifex sp. by Avenue - Own work, CC BY-SA 3.0|
The Poaceae as a whole are about equally divided between those that use C3 photosynthesis, and those that have the C4 version of photosynthesis, which is more energetically costly, but which allows the plants to be more productive in hot and arid environments.
In C3 plants, which are the vast majority of plants in the world, CO2 is directly fixed into a 3-Carbon compound by the enzyme RuBisCo in the Calvin-Benson cycle. Unfortunately, RuBisCo can use both O2 and CO2, and when a C3 plant closes its stomata in order to conserve water in hot dry environments, more O2 is used, which leads to a net loss of carbon in the plant. In fact, some calculations show that C3 plants could end up wasting up to 40% of absorbed energy due to photorespiration!
C4 plants solve this problem through a carbon-concentrating mechanism. They have a modified leaf anatomy wherein CO2 is first fixed onto a 4-carbon compound in outer mesophyll cells by an enzyme not affected by O2, before being shunted to inner bundle sheath cells, where it is converted back to CO2 for use by RuBisCo in those cells.
|CAM process by YikrazuulDerivative: Ed (Edgar181)|
CAM plants are also adapted to hot and dry environments, and they also initially fix CO2 into a 4-carbon compound. In their case, they keep their stomata close during the heat of day, then open them at night. The CO2 that comes in when the stomata are open are fixed into a 4-carbon compound by the same enzyme used by C4 plants, then shunted into a nearby vacuole, where they remain until needed to complete carbon fixation during the daytime. So in the case of CAM plants, they concentrate carbon by separating the process temporally, instead of spatially like C4 plants do.
The Poaceae recently became one of only 8 families of plants that have members using all 3 different photosynthetic methods, with the single grass species that exhibits CAM metabolism able to use both C4 and CAM.
Spinifex littoreus is a halophyte with thick, stiff leaves that is found on coastal sand dunes in East Asia. It has been reported as a C4 plant, with all the concomitant structural modifications for this pathway, but a relatively new study showed evidence that it may also be undergoing CAM metabolism.
Measurements confirmed that the leaves of this species are succulent, which is usually correlated with CAM plants. In addition, the researchers found diel fluctuations in leaf acidity, with acidity rising during night, another clue that it is using CAM photosynthesis.
If true, S. littoreus is the first grass ever exhibiting CAM, and it will be joining members of the non-grass genus Portulaca as the only plants that can do C4/CAM cycling.
|From Flowers of India by Prashant Awale|
Che-Ling Ho, Jyh-Min Chiang, Teng-Chiu Lin, Craig E. Martin (2019). First report of C4/CAM-cycling photosynthetic pathway in a succulent grass, Spinifex littoreus (Brum. f.) Merr., in coastal regions of Taiwan, Flora,Volume 254, Pages 194-202, ISSN 0367-2530, https://doi.org/10.1016/j.flora.2018.08.005.