| Summary: | Roots are essential for nutrient uptake and anchorage for the plant, however there is published evidence to suggest that the physical structure of soil has a strong influence on their abilities to grow and develop healthily. Observing roots in 3-dimensions, in situ and non-destructively is important for understanding the complex nature of the physical root/soil relationship, however roots are notoriously difficult to observe due to the opaque nature of soil. This problem can be partially negated by using techniques such as X-ray micro-computed tomography, but is an expensive and time-consuming technique. Furthermore, soil is a growth medium prone to spatial and temporal variation in terms of water, nutrient availability, and microbial populations, making it difficult to observe the effects of soil physical structure alone. The development of transparent soil (TS) by Downie et al. (2012) has brought about a new era in the study of root/soil interactions. TS is a growth medium with the transparency of agar and some of the physical heterogeneity of soil. TS has particles and pores, so roots can explore it in much the same way as they would soil, however the water and nutrient levels can be better controlled and microbial influences are less of an issue, due to the semi-sterile conditions that transparent soil cores are kept under. Downie et al (2012) used TS to study root growth of small Arabidopsis thaliana roots and also imaged Psuedomonas fluorescens colonising lettuce seedling roots. This project scaled the TS system up in order to image larger root systems of Hordeum vulgare (barley) seedlings under different physical conditions. Comparisons of barley roots growing in soil and TS were made, and it was found that roots grew longer in natural soil than in TS. The TS was then sieved into different particle size ranges and it was found that barley roots grew more successfully in the smaller particles (850-1250 μm) than the larger particles (>1676 μm). Vertically stratified split pots, containing large particles down one side and small particles down the other were also used and non-destructively imaged at 24-hour intervals. It was found that the presence of the large particles had an inhibitory effect on root growth across the entire root system, including the roots that were growing in the smaller particles. Finally a device was designed which allowed the application of compression to the TS system. It was found that root growth decreased proportionally with the level of pressure that was applied to the TS cores. Manipulation of TS structure and the development of techniques to quantitatively record root growth and physical soil conditions from 3-D images has enabled us to measure root growth in barley roots under different physical conditions. The results showed that root growth is heavily influenced by particle size, pore structure and soil strength. Root/soil contact was consistently observed as an important soil property for root growth across experiments.
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