What determines the maximum height of bamboos?

Bamboos are by far the tallest grasses, with the species Dendrocalamus giganteus capable of growing up to 50 m tall. By comparison, the other grass subfamilies can only boast of species like Arundo donax, which tops out at around 6 m tall.

So how do bamboos accomplish this feat?

Researchers have found that the maximum height of tall plants is limited by the ability of the plant to pull water up from the ground to supply the highest leaves. The water column is drawn through xylem vessels, which are the specialized tubes that transport water in plants. When the cost of supplying water to the leaves is higher than the photosynthetic benefits given by those top leaves, then the plant ceases to grow taller. 

Arundo donax in New Mexico

But that's not the entire story.

In addition to gravity, tall plants also have to deal with cavitation, which involves air bubbles forming in the xylem vessels and preventing the water from being drawn up. The formation of such embolisms is a daily occurrence, and can have fatal consequences when the problem is not fixed. In dicot trees which can grow larger in diameter, one solution is for the tree to simply produce more xylem as old ones become non-functional.

But bamboos cannot produce new xylem to replace older ones that have been filled with air bubbles, so researchers tried to determine how bamboos solve this problem of cavitation, which would be a serious limitation on how tall they can grow.

What they found out was something surprising.

They discovered that bamboos repair any embolisms that occur during the day by closing their stomata at night then using  pressure generated from their roots to dissolve the air bubbles back into solution! The root pressure is generated by actively pumping ions and causing water to flow into the roots as it follows an osmotic gradient.

Even more surprising, they tested 59 species of bamboos and measured the heights of the grasses as a function of their basal root pressures, and they found out that  there was a strong correlation between the two values. The higher the root pressure generated, the taller the bamboo.

So it seems that in bamboos at least, the maximum height of each species is directly related to the ability of that species to generate higher root pressure! The greater the ability of the species to generate high root pressure, the higher its potential maximum height.

Reference

Cao KF, Yang SJ, Zhang YJ, Brodribb TJ. The maximum height of grasses is determined by roots. Ecol Lett. 2012 Jul;15(7):666-72. doi: 10.1111/j.1461-0248.2012.01783.x. Epub 2012 Apr 11. PMID: 22489611.