Root Anatomical Adjustments of Triticale and Soybean to Soil Compaction.

Roots are exposed to a set of abiotic stresses in compacted soils resulting in decreased root growth and crop yields. This includes increased mechanical impedance due to increased soil bulk density and prolonged anaerobic conditions especially after heavy rainfalls due to reduced hydraulic conductivity. While the compaction-induced change in soil physical growth conditions is well studied, relatively little is known about anatomical adjustments of roots to such conditions. The investigation of these phenotypic responses is crucial to identify key traits in crop roots, which facilitate root growth in compacted soils and therefore maintain crop yields.

Triticale and soybean were grown under controlled laboratory conditions for 2 weeks and in the field for 13 weeks, in order to quantify root anatomical adjustments to soil compaction at different plant developmental stages. The experiments included two levels of bulk densities, representing compacted and non-degraded soil. In both environments the same loamy soil was used. Root cross sections from primary, seminal and nodal roots in triticale and from the tap root and adventitious roots in soybean were imaged with bright field microscopy.

Across both environments the proportion of the root cross section occupied by the root cortex increased significantly due to soil compaction for both crops. Increased cortical proportions correlated with increased root cross section areas (R²= 0.44, p-value< 0.01) in triticale, whereas in soybean larger cortical proportions were related to smaller cross sections (R²= 0.42, p-value< 0.01). In triticale the occurrence of root cortex aerenchyma was increased due to soil compaction, which was consistent over both environments and different root classes. Aerenchyma also occurred in young soybean roots exposed to soil but not in mature roots, in which secondary thickening closed aerenchyma. The occurrence of aerenchyma showed that roots were exposed not only to increased penetration resistance but also to anaerobic conditions under soil compaction. This needs to be taken into account, when screening for root phenotypes adapted to compacted soils.