Researchers at the Hawkesbury Institute for the Environment at the University of Western Sidney have recently discovered that trees do more than just move water up and down, water can be moved laterally and even be stored within tree tissues. While traditionally we assumed that trees maintained two distinct systems, xylem and phloem, for the transport of nutrients and water, respectively, this recent study indicates that trees are actually able to store water in phloem for use during drier periods. This seems to help answer why more trees survive extended droughts.
Water moves up trees due to negative pressure maintained in minuscule, xylem vessels, however, sometimes when it is very dry cavitation occurs. Cavitation is the presence of tiny water bubbles which cause the pressure in the vessel to equalize; this can lead to dehydration and possible death for some trees. Theoretically, more trees should die due to cavitation, this sparked interested with the Australian research team. They began to think that trees must be storing water elsewhere in order to survive that kind of stress.
Using dyes and microscopy the team experimented on Sydney blue gum trees (Eucalyptus saligna) and found that water is readily and quickly moved from xylem and phloem and back again. Hawkesbury scientists were able to view their dyes moving into phloem from xylem at night when water was more readily available. Water appears to remain within the phloem until it is needed, then it will be transported back into they xylem via horizontal parenchyma or “ray cells”. These results were published this month in the journal “Plant Physiology”.
The research team summarized their findings; “What this means is that this research sheds new light on how trees actually do shift water into the phloem when they least need it (at night or during very humid or rainy weather) and then make use of it when they do need it in the xylem to maintain a continuous flow all the way up to the top of the tree.”
Sebastian Pfautsch, Justine Renard, Mark G. Tjoelker and Anya Salih; “Phloem as Capacitor: Radial Transfer of Water into Xylem of Tree Stems Occurs via Symplastic Transport in Ray Parenchyma” Plant Physiology March 2015 vol. 167 no. 3 963-971