e-mail: mariano.cossani@sa.gov.au

Genetic yield gain of wheat in Australia since the 1960s is around 0.6% yr-1. Selection pressure for yield and agronomic adaptation has changed crop traits related to the efficiency in the use of water, radiation, and nitrogen.

In this work, we report changes in root biomass and in its vertical distribution associated with selection for yield and agronomic adaptation. We sampled roots at anthesis using DNA as a proxy for biomass in a field experiment combining 4 wheat cultivars released during the last 55 years and 4 environmental conditions resulting from the combination of water and N availability. Yield ranged between 3.1 and 4.9 t ha-1. Root biomass at the deeper soil layers (60-100 cm) related negatively to yield (R2=0.31, P<0.03) and year of release (R2=0.5, P<0.003).

Root biomass at deeper layers related negatively with water use efficiency (R2=0.47, P<0.004), nitrogen use efficiency (R2=0.28, P<0.04), and soil moisture at flowering (R2=0.27, P<0.04). The proportion of roots allocated to deeper layers in relation to topsoil (10-30 cm) declined with year of release (R2=0.63, P<0.0003).

Our findings in the field support previous work in rhizo-boxes and indicate that modern wheat cultivars adapted to winter rainfall environments produce higher yields with less root biomass in deeper soil layers.

This partially accounts for changes in water and nitrogen use efficiency with implications for breeding and crop management.