The roots of drought resistance

Future strategies of crop production must increasingly focus on an efficient resource use. Water is yet a limiting factor for plant growth in many regions of the world and global change raises additional concern of more frequent drought situations. The root system is considered a key trait for better drought resistance via efficient water uptake. Root systems are expected to have high natural diversity which is still largely unexploited for crop improvement. Furthermore, dehydration avoidance via efficient root water uptake is considered to be compatible with high yields, which is not the case for many aboveground water stress response mechanisms. Until now there is no comprehensive understanding which root system types and particular root parameters are most suitable under a determined hydrological situation to efficiently supply the plant with water. However, an informed choice of promising plant material would be crucial when targeting the root system for crop improvement, because still there is no screening method to phenotype large unknown populations for root traits. We suggest that adequate plant genetic resources for root mediated superior drought resistance can be readily found when understanding the ecohydrological background that shaped the evolution of root system diversity. This hypothesis builds on the assumption that root system evolution is an optimization process of the plant to the hydrological regime at its site of origin. The main objective of our project therefore is to establish the relation between the diversity of plant root systems and the ecohydrological conditions at their sites of origin. This relation will be studied for a set of durum wheat landraces from origins along an aridity gradient, selected wild relatives (einkorn and emmer) and modern cultivars. We expect that particularly landraces are efficient uptake types ("water spenders"), while wild relatives conserve growth limiting traits of "water savers" that reduce their aboveground losses. Initially the project will characterize the root system architecture of the genotypes with a new type of hyperspectral imaging system for rhizoboxes. Distinct rooting strategies and their relation to environmental characteristic will be established by cluster and principal component analysis. Building on the empirical findings, an innovative coupling of ecohydrological and root architecture modeling will provide a tool to properly analyze the hypothesis of root systems being an optimization to site hydrology. The significance of different root system types to convey improved drought resistance will be validated in a field experiment. We will particularly study if there is a clear distinction among genotypes using a strategy of water spending with efficient root water uptake, and others relying on water saving with a conservative water use. From the outcome of the project we expect to enable breeders to make a targeted choice of genetic resources, e.g. in genebank search, that provide them plant material with adequate root characteristics to improve the water uptake capacity of agricultural crops.

Drought is the main yield limiting factor on about 30 % of arable land worldwide. Therefore the future of agriculture in many parts of the world depends on strategies to effectively use the limiting water resources to ensure stable yield under dry conditions. The project The roots of drought resistance investigated the contribution of plant root diversity within a set of durum wheat landraces, wild relatives and modern cultivars from different dry regions with the aim to find root characteristics with best adaptation to water limited environments. It could be demonstrated that modern cultivars bred in dry environments (Iran) were best adapted to grow under water limitation. They optimally combine an effective root water uptake with high dry matter accumulation. The wheat genetic resources showed two contrasting behaviours in their plant traits: (i) water savers with dense root system and small leaf area with quick closure of stomata minimizing water losses but also growth potential; (ii) water spenders with vigorous vegetative canopies that allow high radiation interception and growth, but require high water supply with the risk to run into dehydration damages during prolonged dry periods.The ratio between root and shoot has changed with domestication of wheat: wild einkorn and emmer genotypes invest very high proportion of dry matter into surface near root axes. The more recent durum wheat on the contrary has evolved towards deeper root allocation with finer root axes, requiring less belowground dry matter investment and still providing better usage of stored soil water.Using simulation modelling we highlighted the dependence of optimum root traits for drought resistance on their interplay with soil hydrology: although deep rooting, as found predominantly in the durum germplasm, is critical in most dry environments, we could show that in agro- ecosystems with shallow and/or light soils with low water storage capacity rainfall capturing via dense surface near root systems can become an advantageous rooting type.Within the project a new methods has been established to concurrently measure root architecture and root water uptake via near infrared imaging, providing a chemical image of the root zone containing information on soil water as well as root properties such as senescence using distinct optical light absorption properties. Taking into account the international trend towards novel non-destructive imaging methods for plant breeding (phenotyping), the method established in this project was a particularly important contribution to advances in structural-functional root phenotyping.