*Email: yinglong.chen@uwa.edu.au

Understanding root system morphology in bread wheat is critical for identifying root traits to breed cultivars with improved resource uptake and better adaptation to drought stress and other abiotic stresses. Variability in root morphological traits at early vegetative stages was examined among 184 bread wheat genotypes originating from 37 countries grown in a semi-hydroponic phenotyping system, followed by validation study in rhizoboxes, early-season drought, and terminal drought experiments in soil-filled columns.

The phenotyping experiment showed that, at the onset of tillering (Z2.1), phenotypic variation existed for both shoot and root traits. Of the 41 measured traits, 24 root traits and four shoot traits had larger coefficients of variation (CV ≥ 0.25). Strong positive correlations were identified for some key root traits (i.e., root mass, root length) and shoot traits (P ≤ 0.05). Strong relationships between performance traits with functional traits suggest their linkage to plant growth and fitness strategies.

The validation experiment in rhizoboxes (1.0 m deep) showed that genotypes with large root systems had 25% more leaf area and biomass than those with small root systems, which presumably reflects high canopy photosynthesis to supply the demand for carbon assimilates to roots. Phenology, particularly time to anthesis, was associated with root system size. Early-season drought delayed phenology, but there was recovery of root and shoot biomass at anthesis. The restricted water supply at sowing reduced leaf water potential, stomatal conductance, leaf photosynthetic rate, shoot and root biomass.

At maturity, early season drought reduced grain yield more in small-rooted Tincurrin than large-rooted Bahatans-87. Terminal drought reduced stomatal conductance, leaf photosynthesis, and transpiration rates faster in Bahatans-87 than Tincurrin. Terminal drought reduced grain yield in both cultivars, more so in Bahatans-87 than Tincurrin due to a reduction in grain number and grain size in Bahatans-87 and grain size in Tincurrin.

These studies demonstrated phenotypic variability in root system morphology in wheat genotypes at the tillering stage in a semi-hydroponic phenotyping system, which maintained their size ranking at booting when grown in soil.

Phenotypic differences and trait correlations among some interesting root traits may be considered for breeding wheat cultivars with efficient water acquisition and better adaptation to dryland environments.