Just as humans have arteries and veins that transport blood from the heart out to tissue and back again, plants have two types of tissues that transport water up to the leaves—and nutrients down to the roots. However, unlike humans, plants do not have a pumping muscle. Other physical principles must therefore come into play in order to produce the same effect. Together with researchers from MIT and Cornell University, Associate Professor Kaare Hartvig Jensen from DTU Physics has described a new model in the journal Nature Plants, explaining where the considerable forces required to transport water in a tree come from.
By means of photosynthesis in the leaves, sunlight, CO2 from the air, and water from the roots combine to produce the sugars the tree needs for nourishment. The waste product is the oxygen we breathe. Following photosynthesis, the substances have to be transported around the vascular system. Small plants have small membrane pumps in their cells which use energy to pump the sugar into the tissue.
“Trees, however, do not, and given their size, you would expect them to need pumps capable of creating extremely high pressure,” says Kaare Hartvig Jensen.
To explain this phenomenon, the researchers have created a chip that imitates the tree’s ‘pump mechanism’, which is a purely physical process—namely diffusion.
“Large structures need a lot of pressure, but trees don’t imitate small plants, nor do they have these membrane pumps. So it’s very strange,” says Kaare Hartvig Jensen, adding that it has not previously been possible to demonstrate the trees’ pump mechanism in the laboratory.However, add diffusion to the mix—i.e. the spread of molecules from high to low concentration between cells through small nanochannels as others have previously suggested—and you have a very important component in the system. And then, suddenly, it all makes sense. By adding sugar to the chip, a smooth transport of liquid over hours or days can be maintained.
Similarly, by means of so-called passive diffusion—where it is solely the difference in the sugar content of the liquid which maintains transport—the trees are able to send water from the root to the crown and back again. Thus the transport of the sugars is effected by the sugars themselves. The researchers also discovered that the pressure in the chip could reach 10 atmospheres (equivalent to 100 metres below sea level), which is more than sufficient—even for large trees.
New knowledge—new technology
So far so good. But if you are an engineer, the work does not stop here—for a chip that can create up to 10 atmospheres and over a period of hours and days can stably transport liquid without pumps and electronic equipment of any kind—is interesting as a technology.
“In principle, you can imagine it running a robot, but not a big machine, of course. But in ‘soft’ robots (made of soft materials) performing fine motor control tasks, it could easily work,” says Kaare Hartvig Jensen and continues:
“Another obvious application is in cell factories, as a plant is really just lots of bioreactors producing glucose and exporting it in a very pure form—and in principle—it’s the same thing we want in cell factories. So you can easily imagine that when a cell factory has produced a chemical substance, you use a chip similar to ours to extract the substance and in this way mimic the strategy used by plants.”