Giant reed (Arundo donax L.) grows so tall thanks to its deep roots. However, it’s not just a matter of physical support. The plant can use the deepest roots to draw water when the upper layers of soil dry out. “[D]eep soil water sources constitute an important and more reliable resource for plant survival, growth and development than erratic and scarce rainfall surface water sources,” said Walter Zegada-Lizarazu and Andrea Monti in the Annals of Botany. “These sources become even more important in the context of a climate change scenario that foresees a reduction of precipitation and increasing temperatures which in turn will result in increased risks of more severe and frequent drought events.” But how do the different roots respond?
To find out Zegada-Lizarazu and Monti turned to a rhizotron, a deep compartment with glass at the sides to allow them to see how the roots grew in different circumstances. There was still the problem of how you control different circumstances in the same soil column, but the scientists had a simple solution.
They had two 50cm tall columns of soil. Between them they put a 1cm thick petroleum jelly / paraffin mix, to create a water-impermeable layer, that roots could grow through. The roots could disrupt the layer as they passed through, so the authors also put a small mesh in the layer to help keep it in place.
With the now metre tall soil column divided into two, they could grow A. donax. When it was in both halves, the scientists could both measure and manipulate the water content of the soil of the upper and lower halves of the column, independently.
“The drought stress imposed in upper compartments significantly altered the growth and development patterns of the giant reed plants,” said Zegada-Lizarazu and Monti. “When drought stress was imposed to upper soil layers the main water source for the plant became the deeper compartments, and water uptake from deeper soil layers was on average (across the growing season) 69 % higher than in the control treatment.”
The levels of abscisic acid (ABA) in the plants varied between the experiment and control too, but not always significantly, said the authors. “Levels of foliar ABA (averaged across the sampling dates) were 13 % higher in the DR treatment than in the control, but this difference was not statistically significant… On the other hand, at 137 DAT the ABA in the roots of the upper drought-treated layers was 2.6 times higher than in the corresponding layer under well-watered conditions…”
So what is going on with ABA in the roots? “Changes in root hydraulic conductivity could be related to increased ABA synthesized in roots subjected to stress conditions… which may at the same time induce transcription factors involved in the gene expression of aquaporins…, the water channel proteins that facilitate intense water flow across root tissues…,” This extra ABA has to go somewhere and have a possible target. “[W]e speculate that the deep root system (i.e. basipetal transport through the phloem) is one of the sinks for the ABA synthesized due to drought either in the roots or in the leaves (as suggested by Manzi et al., 2015). This may have led to improved hydraulic conductivity, especially at the apoplastic level, where the effect of ABA is more pronounced.”
Zegada-Lizarazu and Monti conclude that understanding how ABA signals to roots and leaves will be important in growing A. donax. “These plant responses may have significant implications for selection of the areas where giant reed can be cultivated (i.e. arid or semi-arid marginal areas with a shallow water table). They also have implications for the implementation of some agronomic management practices (irrigation, planting density, etc.) aimed to develop more efficient water use strategies, as the main concern in arid and semi-arid Mediterranean climates is production per unit of applied water rather than absolute production, so more efficient and sustainable cropping systems could be developed.”