Understanding the interactions between biomass, grain yield and grain protein content in low and high protein wheat cultivars

Abstract

Grain protein content (GPC) is a key quality attribute and an important marketing trait in wheat. However, a negative relationship between grain yield and GPC has limited selection for increased GPC, since grain yield is the primary driver of breeding programs. GPC is strongly influenced by nitrogen (N) fertilizer application, but the N-use efficiency (NUE) of high and low GPC genotypes appears to be genetically determined. The aim of this PhD thesis was to investigate the grain yield-GPC relationship under controlled and field conditions, and to suggest selection targets and traits for improving NUE in wheat. Firstly, the N responsiveness of six wheat genotypes that varied in GPC were examined under controlled condition. This experiment was designed around non-destructive estimation of biomass using a high-throughput image-based phenotyping system. In parallel, field trials were conducted to allow the comparison of results obtained from the controlled condition study using the six selected genotypes. Estimating the rate of biomass accumulation in breeding plots in the field is difficult. Therefore, the growth rate of biomass related traits such as height and ground cover were assessed in these trials. To examine the grain yield- GPC relationship under multi-environmental conditions, the grain yield and GPC data of over 200 wheat genotypes obtained from the Australian National Variety Trials (NVT) across the Australian wheat-belt were analysed. Results of the controlled environment experiment showed that high GPC genotypes appeared to demand more N to grow their biomass. In both controlled and field environments, high GPC genotypes slowed down the rate of biomass growth under low N supply. Under low yielding conditions, high GPC genotypes seemed able to manage grain N reserves by compromising biomass production. These results indicated the importance of biomass growth analysis to show the differences in the N responsiveness of high and low GPC genotypes. Differences between high and low GPC genotypes in responding to low N could be due to their history of selection. N effect is strongly associated with the amount of available water in the soil. Controlled and multi-environmental studies showed that the slope of the relationship between grain yield and GPC is steeper in low compared to high yielding environments. Therefore, high GPC genotypes bred under stress conditions sacrifice yield in favour of GPC, possibly to enhance the survival chance by producing fewer grains with sufficient nutrient levels. Conversely, low GPC genotypes bred in high yielding environment are less conservative compared to high GPC genotypes in using N for yield production. The outcomes of this PhD project highlight the importance of considering environmental factors for improving NUE in breeding programs. It recommends that wheat breeders focus on selecting in low yielding environments for high yield and high GPC genotypes.