Have you ever wondered how plants become damaged and die during drought? As a post-doctoral researcher investigating plant physiological responses to water deficit, I spend most of my time thinking about this question. A very smart young lady once told me that it’s obvious…they run out of water. In one sense, she is absolutely correct: when the leaf mesophyll cells run out of water (desiccate), they become unable to maintain normal physiological function, which ultimately leads to death. But this explanation is not entirely satisfactory: It does not tell us about the precise processes that lead to desiccation, whether there is a particular threshold of desiccation that plants fail to recover from, and how different species might vary in their capacity to withstand desiccation. I have been investigating these questions in Californian oaks, and recently published my findings in the scientific journal Plant Physiology.
To find a satisfactory answer to the question, we need to revise some fundamentals about how plants function. Not having a heart, or a mechanism to pump water to the tops of their canopies, trees rely on gradients in water concentrations. Water naturally and passively moves from places of high concentration to areas of low concentration. As water is drawn out of the pores on leaves (stomata), it is replaced by water from the rest of the plant. Plants replace this water lost to the dry atmosphere by taking up moisture from the soil. Within the plant water is transported in xylem: from the roots, all the way to the leaf mesophyll cells. Think of this process as similar to an elastic band being stretched from the tops of the canopies. As the environment dries, plants find it harder and harder to extract moisture from the soil. The elastic band stretches…
We currently think that as the tension in the water column increases, air is eventually drawn into the xylem vessels and the elastic band snaps. This process creates an air pocket that blocks the transport of water, know as embolism. Xylem embolism represents a significant, drought-induced damaging process in land plants. As plants embolise they lose the capacity to hydrate the downstream cells in the leaves. So this partly answers our question: the point at which plants experience embolism is the point at which they begin to suffer (mostly irreversible) damage.
However, substantial debate surrounds the capacity of species with long xylem vessels to resist embolism. Some studies have shown that species are highly vulnerable to embolism, and regularly experience this during dry periods, while others suggest that plants are actually less vulnerable to embolism. Our paper investigated whether recent methodological developments could help resolve this controversy within oaks, a charismatic, ecologically important, temperate angiosperm genus. We were hoping to shed further light on the importance of xylem vulnerability to embolism as an indicator of drought tolerance in long-vesselled trees. To do so we used a non-invasive optical technique to visualise embolism in leaves and stems of eight oak species from the Mediterranean-type climate region of California, USA. Basically we took a lot of photos of trees as they dried down and monitored when they embolised.
We show that the point at which air enters into leaves ranges from −1.70 MPa to −3.74 MPa, and from −1.17 MPa to −4.91 MPa in stems (pure water is 0 MPa and plants normally operate above -1.5 MPa). Consequently, our results show that long-vesselled North American oak species are more resistant to embolism than previously thought, and support the hypothesis that avoiding stem embolism is a critical component of drought tolerance in woody trees. So now we can say exactly when particular oak species start to fail during drought.
This information is useful, as it allows us to predict how oaks might respond to future drought events. It tells exactly when they will start to lose function, and become damaged. In fact, our study shows that accurately quantifying xylem vulnerability to embolism is essential for understanding species distributions along aridity gradients and predicting plant mortality during drought. All we need to do is monitor their water status to know whether they will recover or not. Ultimately, this is why physiological studies, such as ours, are important: They allow us to be precise in understanding how plants respond to climate change.