Grasses in my head and how a C4 grass learned to love the shade

When do you know you think about something way too much? It's when you start dreaming about it!

In my case, I dreamed repeatedly of Microstegium vimineum (Japanese Stiltgrass) last night.

In my dream, there was a hill covered in the small bamboo-like grass, and I was teaching someone how to identify the species by showing them a leaf.

In M. vimineum, a distinctive mark is the silvery midvein on the upper side of the leaves, which you can see by tilting a leaf every which way until the light catches the line.

This species is not only interesting because it is a notorious invasive in the East Coast of the USA, but because it is a C4 grass that is shade tolerant, and it's relatively rare that a C4 grass is efficient in shady conditions.

The C4 photosynthetic pathway allows efficient photosynthesis under hot and arid conditions, but the structural changes needed in C4 grasses can make them less phenotypically pliable, such as when placed in shade. 

In this situation, some are unable to maintain high quantum yields when grown in low-light conditions. Quantum yield refers to how fast the process works relative to how fast photons are being absorbed. Other C4 grasses cannot take advantage of sunflecks because they cannot maintain a high state of photosynthetic induction (increasing CO2 uptake temporarily when light is temporarily increased). Still others fail to reduce carboxylase in the shade, and thus leaves the photosynthetic enzymes  (PEP Carboxylase and Rubisco) with nothing to do (Sage and McKown, 2006).

But it turns out that M. vimineum overcomes the limitations of its photosynthetic pathway by taking advantage of sunflecks. A study showed that it is able to rapidly take in CO2 under flashing lights, and then rapidly close stomata in very low light in order to prevent excess water loss (Horton and Neufield, 1998), although the exact mechanisms of this adaptation has not been elucidated.

I know many people hate this species for being invasive, but one cannot help but admire its adaptability, as well as its ability to overcome the seeming limitations of its kind, and perhaps that is why I dreamed of it. I've been watching X Factor UK auditions on YouTube recently  (don't ask!), and I was awed by the passion and abilities of some of the singers, some of whom surprised the judges - just like M. vimineum surprises me because of its tolerance to shade. 

References

Horton JL, Neufeld HS. Photosynthetic responses of Microstegium vimineum (Trin.) A. Camus, a shade-tolerant, C4 grass, to variable light environments. Oecologia. 1998 Mar;114(1):11-19. doi: 10.1007/s004420050414. PMID: 28307549.

Sage RF, McKown AD. Is C4 photosynthesis less phenotypically plastic than C3 photosynthesis? J Exp Bot. 2006;57(2):303-17. doi: 10.1093/jxb/erj040. Epub 2005 Dec 19. PMID: 16364950.