By the way, as always it turns out that things are not so simple as just adding a more efficient photosynthesis pathway to a life-form and everything will be better. There are complex trade-offs between the different photosynthesis pathways, and some results are counter-intuitive. Despite having evolved independently multiple times, practically no tree species with the C4 pathway seem to have evolved: only
"grasses, herbs, bushes and shrubberies"[0]. According to the summary on Wikipedia[1], C4 has an advantage in high temperatures and low rainfall environments, and needs more sunlight. Furthermore, as a linked article on the researchgate discussion[3] mentions:
> While growth and photosynthetic potentials are typically less in the woody species relative to the C4 plants, the woody seedlings often tolerate low light within the C4 canopy, and will steadily grow until they overtop the C4 plants, unless they are reduced or killed by an episodic disturbance (Bond and Midgley, 2000; Wedin, 2004; Bond, 2008)
So there is a reason the trees win in the long-run if left undisturbed, and that could mean that adding C4 to other plants isn't a panacea as it is less robust under poor sunlight conditions. And in another article[3] comparing a C3 tree with one of the few known C4 trees:
> The results show that the carbon-gaining capacity of E. forbesii is comparable to that of a C3 species in a moderately cool, shaded forest environment. There appears to be no particular advantage or disadvantage associated with the C4 photosynthetic pathway of E. forbesii in this environment.
Perhaps most disturbing is this opening sentence in the abstract of a paper from 2006:
> Plants with the C4 photosynthetic pathway dominate today's tropical savannahs and grasslands, and account for some 30% of global terrestrial carbon fixation. Their success stems from a physiological CO2-concentrating pump, which leads to high photosynthetic efficiency in warm climates and low atmospheric CO2 concentrations.
Since "low atmospheric CO2 concentrations" are not exactly our biggest concern at the moment, does C4 actually help then?
So as always, improving on nature is complicated.
[0] https://www.researchgate.net/post/Do_trees_with_C4_photosynt...
[1] https://en.wikipedia.org/wiki/Evolution_of_photosynthesis#Wh...
[2] https://www.researchgate.net/profile/Arvind_Singh56/post/Do_...
[3] https://link.springer.com/article/10.1007/BF00384538
[4] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1626541/