Methods of root investigation in open fields: what we can learn to improve crop management

Although knowledge of root morphology and architecture is essential in order to improve crop productivity, it is greatly hampered by the poor accessibility of plant roots and the lack of suitable methods of investigation. The first international handbook on root methods goes back to the late 1970s (Bhom, 1979) and mainly proposed destructive procedures. In the last few decades, significant progress has been made with the setting up and improvement of conservative methods. Among these, the use of minirhizotrons – transparent tubes inserted into the soil which allow micro-video cameras to observe individual roots or specific soil areas periodically – has received great attention in open field studies, especially those on root turnover. Although of interest, the use of non-destructive root electrical capacitance or NMR imaging is restricted to single plants or pots, although 3D soil resistivity imaging has recently been found to correlate well with root distributions at field scale. New methods are being developed, but destructive soil coring and auger sampling still remain as reference methods for measuring root length, diameter and deepening. At the University of Padova (Italy), the roots of several crop species have been studied since the early 1990s by means of soil coring, minirhizotrons, rhyzoboxes and electrical capacitance, and we have actively contributed to improve root image analysis (Vamerali et al., 2003). Prominent results include the feasibility of reducing ploughing depth in maize without modifying root profiles; the importance of fast root deepening in sugar beet through correct nitrogen dosage, in order to exploit deep water reserves (water-table); and more suitable choices of maize hybrids in semi-arid environments, guided by the frequency distribution of roots in 2-D maps arranged by means of geostatistic procedures.