Since root length density declines with soil depth, plant water uptake results in a soil profile with a drier, strong surface layer and a moister, comparatively weaker deeper layer. Differences in root distribution between wetter and drier soil layers can affect plant growth (Martin-Vertedor and Dodd 2011). To simulate vertical differences in soil strength in the laboratory, a wax layer screening system was developed. Considerable genetic variability in the ability of rice roots to penetrate a strong wax layer was correlated with field performance as the soil dried (Clark et al. 2002). In rice it has been demonstrated that penetration of roots into strong layer is under genetic control (Clark et al. 2008; Norton et al 2008).Differences in the angle at which seminal roots are produced in different wheat varieties affects the depth of root penetration of comparatively weak soil (Oyanagi et al. 1991). However, actual rooting depth in the field will depends on the ability of the roots to penetrate stronger soil below the plough layer, which was assessed using a wax layer system. Considerable genetic variability in the ability of wheat roots to penetrate a strong wax layer was demonstrated, and the dependence of this on the number of roots, penetration angle and root stiffness was explored. An important issue for laboratory screens of root penetration is how to interpret this data in terms of field performance.  The same genotypes were grown in the field (Woburn, UK) under both irrigated and rainfed conditions in 2009. The results of the wax layer screening experiment were compared with the vertical gradients in soil strength and soil moisture in the field experiment. Concurrent determination of soil moisture and soil strength profiles (using a soil moisture profile probe and a penetrometer respectively) can discriminate genotypic differences in root water uptake of wheat.

Acknowledgements

We thank DEFRA (WU0121) for supporting this work. Rothamsted Research is an Institute of the BBSRC.

Clark L.J., Cope R.E., Whalley, W.R., Barraclough, P.B. and Wade, L.J. (2002) Root penetration of strong soil in rainfed lowland rice: comparison of laboratory screens with field performance. Field Crops Research. 76:189-198.

Clark L.J., Price A.H., Steele K.A. and Whalley, W.R. (2008) Evidence from near-isogenic lines that root penetration increases with root diameter and bending stiffness in rice. Functional Plant Biology. 35: 1163-1171.

Norton G.J., Aitkenhead M.J., Khowaja F.S., Whalley W.R. and Price, A.H. (2008) A bioinformatic and transcriptomic approach to identifying positional candidate genes without fine mapping; an example for rice root growth QTLs. Genomics. 92: 344-352.

Martin-Vertedor AI., Dodd IC (2011) Root-to-shoot signalling when soil moisture is heterogeneous: increasing the proportion of root biomass in drying soil inhibits leaf growth and increases leaf ABA concentration. Plant Cell and Environment. in press

Oynangi A., Sato A. and Wada M. (1991) Varietal differences in geotropic response of primary seminal root in Japanese wheat. Japanese Journal of Crop Science 60: 312 319.