Unraveling the electrical signature of roots

Detailed knowledge of root system architecture and functioning is essential to understand the feedback loops between plants, soil and climate. In-situ characterization of root systems is challenging due to the inaccessibility of roots and the complexity of root zone processes. Developing a suitable method for non-invasive study of crop root systems is therefore a major challenge in sustainable agriculture. To resolve this challenge, it is essential to understand the root electrical properties at segment scale, its variability in time and space, across species and genotypes, while coupling interpretations at both macroscopic and microscopic levels. In this thesis we investigate the frequency-dependent electrical properties in terms of conduction and polarization of single root segments of Brachypodium (Brachypodium distachyon L.) and Maize (Zea mays L.). Hypotheses were developed for the electrical signature of crop root segments, based on a detailed review of plant root systems, plant electrophysiology and geophysical approaches to root investigation using electrical methods.
A sample holder was designed and coupled in a new measurement set-up for SIP studies on root segments. The set-up was tested on ideal resistors and root segments, and was found to be suitable for assessing the electrical properties of crop root segments of 1-5 cm length and up to 2 mm diameter in a frequency range of 1Hz – 45 kHz. The new measurement set-up was used to perform detailed experiments on primary roots of Maize and Brachypodium plants to test these hypotheses. Firstly, complex electrical responses of the root segments of the target plants in the laboratory were obtained at 10 different ages, and the results were interpreted at both macroscopic and microscopic levels. The results show that the electrical response of crop root segments vary with age and species, and is controlled by the anatomy, cellular fluid composition and cell membrane composition. Our results supports the hypothesis that the mechanism of polarization at low frequency is different from that at high frequency. It is also clear from these results that fine root segments of crops such as Maize and Brachypodium can be differentiated from soils based mainly on their phase response which is much stronger than that of other geological materials.
Experiments were further designed and performed to investigate the suitability of SIP method as a tool for investigating crop root sensitivity to salt stress which is a serious threat to sustainable crop production. Results show that SIP can detect the uptake of saline water by crop roots, resulting in low or high resistivity or phase response depending on the salt concentration, duration of exposure and salinity tolerance of the plant species.