Like all living creatures, a plant needs water. Water is sipped in by the roots, together with nutrients ; it ascends in the stem through conduits and is finally evaporated out at the surface of the leaves.
In fact, evaporation is the motor of the whole circuit. During a period of drought, if the water lost by evaporation up there in the leaves is not compensated by the volume sucked in at the root level, the sap column may be under excessive tension… and break !
Breaking here corresponds to the appearance of a cavitation bubble. You cannot pull on vapour : the ascent of water is stopped immediately in affected conduits. If bubbles continue to form this causes the plant’s death by dehydration. In order to prevent such embolisms, we have to know more about how cavitation happens in plants. But stems are thick and not made of glass, hence they are really hard to scrutinize.
Fortunately, it’s easier with leaves, where the light can go through. Plus the leaves present a large variety of network architectures depending on the species.
Under sever drought, the leaf shrinks a little, and suddenly bubbles appear in the veins, starting embolisms. With the raw pictures, it’s really difficult to observe the appearance of these bubbles. Luckily, this is better seen by image analysis, looking at colour changes.
And it’s always the same, no matter the leaf network. Bubbles start at its base and spread into smaller and smaller veins. In more complex networks, the leaf’s water supply is not disconnected at the first bubble : the main vein gradually fades away delaying the network failure.
This work was published in PNAS