Experimental Investigation of Plant Root Growth Through Granular Substrates

It is well known that granular materials are not homogeneous, i.e. the forces between grains are localized in force chains. However, previous work on plant root growth has neglected this variability in granular soils and reported bulk characteristics or used homogeneous media, such as agar, to grow plant roots. In this paper we report the results of pinto bean root growth through a granular system where photoelastic grains are used to visualize and quantify the local forces in the system. Two issues are addressed: how plant roots respond to different levels of force between grains, and how the growing roots alter the force distribution in a granular system. We find that pinto bean roots are less likely to grow between grains as the force between those grains increases and that roots can exert, on average, 110 mN of force on the granular system. However, both of these observations are time-dependent. Both the inter-grain forces as the roots grow and the forces that the roots impart to the system increase in time without observable concomitant geometric changes in root cross-section.