When plants experience drought-induced water stress, the ability to resist embolism – the formation of air bubbles in the xylem tubes that prevent their function – is essential to their survival. Though embolism resistance has been well-studied in trees, less is known about how herbaceous plants cope under water stress. The model plant Arabidopsis thaliana is a useful species in which to test this feature because it is able to produce a small amount of wood in both its hypocotyl and in the base of the inflorescence stem. Wood is thought to play a role in determining embolism resistance.
In a new study published in Annals of Botany, lead author Ajaree Thonglim and colleagues studied four A. thaliana accessions with differences in growth form, woody tissue production, and drought response in order to understand the traits underlying embolism resistance. The researchers made detailed anatomical observations using light and transmission electron microscopy, as well as establishing vulnerability curves for each accession.
The woodier a stem was, the greater its embolism resistance, though this trait wasn’t found to be directly responsible for that resistance. Rather, woodiness is strongly linked to other characters such as intervessel pit membrane thickness and vessel wall thickness, which themselves increased resistance.
The authors found that of the various anatomical traits measured, intervessel pit membrane thickness best explained embolism resistance differences among accessions. In functional terms, this can be explained by the air-seeding hypothesis, which predicts that gas bubbles will pass into adjacent, non-embolised xylem vessels by way of pores in pit membranes, quickly expanding the embolism and hastening hydraulic failure and plant death.
Given that the majority of all food crops are herbaceous, and that climate change is increasing the frequency and severity of both drought and heat waves, it is important that we gain a better understanding of resistance in these plants.
“Adding more accessions and performing complementary measurements related to drought tolerance in stems, leaves and roots will undoubtedly shed more light into the complex mechanism that this short-lived, herbaceous model species has developed in order to cope with periods of water shortage,” write the authors.