Increasing co-limitation of water and nitrogen drives genetic yield gain in Australian wheat

The interactions between nitrogen (N) and water are manifold, and the concept of co-limitation provides a quantitative framework for integration. This paper tested the hypothesis that selection for yield increased water-N co-limitation in wheat adapted to winter rainfall environments of Australia. To test this hypothesis, we measured evapotranspiration, N uptake and yield in a historic collection of varieties released between 1958 and 2007 (exp. 1) and between 1969 and 2015 (exp. 2). Crops were grown under 5 (exp. 1) or 4 (exp. 2) environmental conditions resulting from the combination of sites, seasons, and supply of N and water. Genetic gain of yield, i.e. the slope of the regression between yield and year of release was 14-24 kg ha(-1) y(-1) or 0.36-0.52% y(-1). This yield gain was associated with a linear increase in water-N co-limitation, in turn associated with an increase in N uptake per mm of evapotranspiration at a rate of 0.0019 kg N mm(-1) y(-1). Our findings highlight the critical role of nitrogen for the adaptation of wheat to low rainfall environments, and the scope for further yield improvement based on traits that integrate resources.