Plant water loss: it’s complicated

Plant water loss mechanisms have been led by science’s long-standing assumption that stomata, the tiny orifices on leaf surfaces, are where CO2 enters the plant, and where water exits it.

On warm days, it was thought, the stomata’s opening to allow the plant to acquire CO2 also allowed water from the humid area inside the leaf to diffuse into drier air.

This water loss is costly, and one of the reasons it takes about 300 grams of water to grow one gram of plant dry matter.

But 14 years of intermittent experiments by Dr Suan Chin Wong, a Visiting Fellow at Australian National University’s Farquhar Laboratory, and colleagues have revealed an equation more complex than just “CO2 in -> H2O out”.

It has long been assumed that relative humidity inside the leaf is always 100% because there is no method directly measuring the relative humidity of the air inside leaves.  But in the late 2000s, Wong did a series of experiments in which he found that the relative humidity inside cotton and sunflower leaves could be as low as 80%. 

In 2018, a collaboration with colleagues with access to new equipment fully corroborated his earlier findings, and sparked a more in-depth investigation into the cause.

“It was thought that a plant taking in carbon dioxide couldn’t avoid losing water,” Wong says. “But we’re saying that it’s possible to take in CO2 and not lose water – at least, much less water than we thought possible.”

Plant water loss: first described in 1889, might just be about to change.

In the subsequent search for other mechanisms, the researchers identified a probable answer in aquaporins. Discovered in 1992 by American researcher Peter Agre – who won a 2003 Nobel Prize for his work – aquaporins are a protein structure that regulate the movement of water molecules through cell membranes. In humans, aquaporins regulate the water content of blood cells, are vital to kidney function, and to the flow of body fluids in general.

And right now, as reported in a recent Nature Plants paper, that’s where this long, patient scientific process currently rests – with an important new understanding, and many questions. 

The understanding that plant water loss and CO2 acquisition are on different control mechanisms overturns assumptions that have been in place since stomata were first described in 1889.

”I’ve been working on stomata for over 40 years,” Wong says ruefully, “and I only realised this in the last few months.”

Next, the questions. Are aquaporins really influential in controlling plant water loss? How? And if that process is understood, what does that mean for future plant breeding? Particularly in a world where the temperature is climbing.

“First we have to really pinpoint the mechanism that’s a work in regulating water loss,” says Wong. “And then we work out what it all means.”