Giant reed genotypes from temperate and arid environments show different response mechanisms to drought.

Studies at the root level and how the root-shoot interactions may influence the whole crop performance of giant reed (Arundo donax L.) under limited water conditions are largely missing. In the present study, we illustrate the effects of water stress on some phenotypic traits at the root-shoot levels of two giant reed genotypes (from Morocco and Northern Italy) that were reported to have different adaptive hydraulic stem conductivities despite the limited genetic variability of the species. The trial was carried out in 1 m rhizotrons (1 × 1 × 1 m) for two consecutive growing seasons. As expected, both genotypes showed an effective behavior to contrast water shortage; however, the Moroccan genotype showed a higher leaf water potential, a lower root length density (RLD) and thinner roots in the upper soil layer (0-20 cm), and similar to control RLD values at deep soil layers (40-60 cm). On the other hand the Italian genotype showed the opposite pattern; that is no drought (DR) effects in RLD and root diameter at upper soil layers and reduced RLD in deep layers, thus revealing different DR adaptation characteristics between two genotypes. This DR adaptation variability might bring new insights on DR tolerance of giant reed identifying potential traits aimed to improve the integral plant functioning, to a more efficient use of water resources, and to a more effective crop allocation to targeted stressful conditions under a climate change scenario that foresees the increase of DR periods.