Plasticity of Root and Shoot Phenotypes in Response to Soil Compaction in Sorghum (Sorghum bicolor L. Moench)

Highly compacted soils limit crop production and affect millions of hectares of agricultural land globally. One of the leading causes is heavy traffic of farm equipment. Hard soils impact root system growth and function, including nutrient and water uptake. Crop genotypes, however, vary in response to soil compaction and root system architecture (RSA) shows genetic “plasticity” to this soil condition. We are testing if RSA plasticity with soil strength depends on plant size. Destructive and non-destructive shoot phenotyping was conducted on 30 genotypes of sorghum in complementary experiments using two soil penetration resistances (0.5 and 1.5-4.0 MPa). Based on a screening experiment, shoot biomass was lower in compacted soil than in the loose condition. Degree of plasticity, calculated as [(shoot mass _loose) – (shoot mass _compact)], depended strongly on genetic background (i.e. large genotype-by-treatment interaction), and increased slightly with shoot size. To study both belowground and aboveground plasticity, six genotypes varying in shoot plasticity were selected for in-depth root and shoot phenotyping. Degrees of below- and aboveground plasticity differed among genotypes, but in general shoot and root biomass decreased under compaction. Non-invasive leaf area phenotyping showed that the effect of soil strength appeared between the first and second week of growth, manifested earlier in those genotypes with final higher plant biomass. Notably plasticity in root length was expressed mainly in fine root length (diameters< 0.2 mm). Plants in compacted soil produced 10-15% less fine root length than those in looser soil. Although high correlations were found among traits under both looser and compacted soils, below- and aboveground plasticity were not always correlated under compaction. This suggests decoupled shoot and root responses to soil compaction, and variation in growth compensation in plant tissues, including RSA-plasticity. These are subject to further studies.