Plants undergoing severe drought stress will eventually experience vascular embolisms – pockets of air in the xylem where the water column breaks and the vessel stops functioning. A plant’s ability to resist embolism is an indicator of its likelihood of surviving under drought conditions. Climate change is expected to result in an increase in aridity globally, making an understanding of different plant species’ related mortality risk critical in predicting outcomes. So far, studies of embolism resistance have mostly focussed on conifers and dicotyledonous angiosperms, with little known about arborescent monocots such as palms, which are important players in carbon and water cycling in tropical forests.
A recent article published in Annals of Botany aims to change this. Lead author Thaise Emilio and colleagues measured embolism resistance in the leaflets and petioles of six palm species from a range of habitats and genera. The sample species represented tropical rainforest, tropical seasonal forest, temperate rainforest, temperate seasonal forest, and desert biomes. Using three different techniques, in vivo X-ray-based microcomputed tomography, the in situ flow centrifuge technique, and the optical vulnerability method, the researchers quantified the pressure required to cause 50% embolism (P50) within either the petiole or the leaflet as the plants progressively dried.
The authors found that P50 varied widely within the palm family – as widely as across all reported angiosperms, in fact. While the first embolisms were seen one to three days after drying began, one species, Trachycarpus fortunei, had such great resistance that P50 was never reached over the course of the experiment. Little difference was found between the petioles and the leaflets, but where differences occurred, the leaflets were unexpectedly less vulnerable than the petioles. Palms’ wider xylem vessels were expected to make them more vulnerable to embolism overall, but this turned out not to be the case. Though the exact mechanisms for palms’ embolism resistance remain unclear, it is thought to be due to there being fewer vessel-to-vessel connections which might allow embolisms to spread, and more parenchymatous tissue, which acts as a water storage space. When drought pressure increases, water may be mobilized from the parenchyma to the xylem to decrease tension to a sustainable level.
“The mechanisms by which palms achieve such a wide range of embolism resistance remain unclear and merit further investigation,” the authors write. “This investigation provides the first step towards understanding hydraulic adaptations in palms by showing how high embolism resistance can be sustained in long-lived arborescent monocots.” They also note that water storage may play a particularly important role in drought tolerance in cases where the drought is a result of increasing seasonality, as opposed to simply less rainfall. “The ability to store water that can sustain transpiration until water again becomes available could ultimately be as important in changing climates as embolism resistance.”