Soil surface pressure reduces post-emergent shoot growth in wheat

Aims: An emerging shoot experiences mechanical impedance (MI) prior to initiating photosynthesis, when it needs to break through soil that has a surface crust. This is the one of the first physical stresses that the shoot experiences. Surprisingly, few measurements have been made to understand the impact of this stress upon post-emergent shoot growth. Methods: A system employed wax layers of different strengths to investigate shoot responses to MI of the soil surface. Experiments tested the responses of plants to MI using wax layers with different strengths, and tested different seed sizes, nitrogen and phosphorus nutrition and different wheat genotypes. Detailed leaf and root morphological responses and photosynthetic gas exchange and fluorescence were measured. Results: MI produced permanent impairment to limit plant size, leaf growth rate and leaf photosynthetic function. Large seed sizes and N and P fertilization were able to overcome MI, especially for moderate levels of impedance. There was strong genotypic variation in the response to MI among 14 diverse wheat cultivars, and breeding for varieties suitable to no-tillage cropping systems appears to have facilitated selection in the ability to overcome MI of the soil surface. Conclusions: This study has highlighted the importance of MI stress of the soil surface in limiting shoot growth and has broad implications for plant genotype selection and agricultural systems management, particularly with regard to nutrition and tillage systems.Yi Zhou, David R. Coventry, Matthew D. Dento