Maize roots under mechanical stress induce increased ethylene concentration in the soil gas phase

Mechanical stress induced by soil compaction affects drastically root system architecture and the morphology of individual root segments. Typical effects of mechanical stress on the root responses include decreased root elongation, increased radial thickening and sloughing of cap cells. Many studies have established a link between the mechanical stress encountered by roots (generated in triaxial cells or by restricting root growth with a barrier) and the gaseous plant hormone ethylene; the data suggests that ethylene acts as an endogenous root growth regulator. To the best of our knowledge however, none of the studies on the ethylene and mechanical stress feedback mechanisms measured ethylene concentrations under realistic soil conditions, i.e., in the soil gas phase and over a long period of time. In this study, we aimed at filling this knowledge gap and set up an experiment which allowed to measure ethylene concentration directly in the soil gas phase with passive diffusive samplers and for maize plants subjected to different levels of soil compaction over a period of 21 days in repacked soil columns in the laboratory. With the help of X-ray computed tomography, we investigated the spatiotemporal patterns of ethylene concentrations in the vicinity of roots, in order to assess which type of roots act as a major source of ethylene in the soil. In accordance with the literature, soil compaction induced a significant increase in ethylene concentration in the soil gas phase, which impacted root growth by reducing significantly the growth of fine roots and increasing the share of thicker roots. A visual inspection of the X-ray CT images at different time points of gas sampling showed that high concentrations of ethylene (i.e., above the third quartile of the distribution) were not strictly ascribable to the abundance or type of roots in the vicinity of a probe. Yet, the highest concentrations of ethylene were recorded on the occasions where roots were present close to a probe. The sampling depths and time of sampling had no or very little effect on the measured ethylene concentrations. Our results suggest that ethylene was diffusing rather homogeneously in the soil columns and that microbial activity was also responsible for a good fraction of the ethylene production. Future experiments are planned to assess the contribution of microbes to the total ethylene production.