A recent comment in Nature highlights the shift from the Green revolution to the Root revolution in crop research and culture (Bishopp and Lynch 2015). During Green revolution plant breeding targeted above-ground plant features. Nowadays, improving crop root systems has become a response to face increasing water stress and developing soil infertility. A lot of progress has already been done but more remains to be done, in connection with identification of root traits important for root resource capture. For that, traditional breeding techniques (though a slow process selection due to necessary monitoring of yield or other selected traits for proper selection), as well genetic technologies (including very recent gene editing and regulation using the CRIPS-CAS system) are now available.
Cultivating and harvesting crops is a much faster process that cultivating and harvesting trees; breeding techniques and genetic technologies are much faster applicable.
Traditional forestry research techniques are mostly descriptive and root data are largely missing. Roots represent half of the plant body – and arguably the more interesting half. Once established, consequences of root functioning can be measured in above-ground parts and fine root biomass is strongly related to the above-ground biomass (Finér et al. 2007). A widely observed phenomenon is that for forests, root to shoot ratios increased significantly as shoot biomass decreased (Mokany et al. 2006). Carbon allocation to roots also generally increases with decreasing soil nutrient availability, which is the case in nutrient-poor boreal ecosystems.
Despite its obvious importance for the whole plant, until recently our knowledge of the root apparatus was very limited, mostly due to the inadequacy of the techniques available. This is even more true in tree research due to the large size of tree roots. The lack of appropriated techniques is the major bottleneck in root research and is probably the area in which most efforts have to be concentrated. Techniques to either quantify roots in a species-dependent manner other than using hand-sorting methods, but as well nondestructive methods providing information about the physiological status of roots in situ are needed.
Methods based on biochemical markers approaches have been developed but present the drawback of being dependent on the chemical composition of roots, and thus indirectly on the possible changes in growing conditions. Among the nondestructive techniques for root analysis in soil and in field conditions, infrared spectroscopy, and particularly near infrared reflectance spectroscopy (NIRS) is a very promising method allowing qualitative and quantitative analysis of root mixtures, but the technique is dependent on mycorrhizal colonization, the degree of lignification, and nutrient status of the roots (Rewald and Meinen 2013). The use of electrical properties (impedance, capacitance, resistance) of roots is certainly a powerful tool to develop nondestructive methods providing information about physiological status and growing conditions of roots (Repo et al. 2015). Root growth in situ can also be monitored using minirhizotron. Among the other methods developed for root analysis, DNA-based techniques also allow for quantifying the root biomass proportion of different species without manual sorting (Rewald et al. 2012). Such molecular methods, based on genomic differences between species, circumvent the problem of changing anatomical, morphological and biochemical properties under different environmental growing conditions.
Focusing on tree root research appears to be a key step to improve forest regeneration and tree growth. Prerequisite is to go deeper in understanding tree root development, physiology and metabolism, interactions with soil, plants and micro-organisms (micorrhyza, bacteria, such as plant growth promoting bacteria) and responses to abiotic and biotic stresses. For example, there is a huge gap of knowledge on root functioning during dormant season, though the dormant season lasts several months. But before all, efforts are needed to develop non-destructive qualitative and quantitative analysis techniques. The increasing number of tree root studies published in scientific journals probably suggests that these efforts are under way.
Bishopp, A., and Lynch, J.P. 2015. The hidden half of crop yields. Nature Plants 1: 15117.
Finér, L., Helmisaari, H.S., Lõhmus, K., Majdi, H., Brunner, I., Børja, I., Eldhuset, T., Godbold, D., Grebenc, T., Konôpka, B., Kraigher, H., Möttönen, M.-., Ohashi, M., Oleksyn, J., Ostonen, I., Uri, V., and Vanguelova, E. 2007. Variation in fine root biomass of three European tree species: Beech (Fagus sylvatica L.), Norway spruce (Picea abies L. Karst.), and Scots pine (Pinus sylvestris L.). Plant Biosystems – An International Journal Dealing with all Aspects of Plant Biology 141: 394-405.
Mokany, K., Raison, R.J., and Prokushkin, A.S. 2006. Critical analysis of root : shoot ratios in terrestrial biomes. Global Change Biol. 12: 84-96.
Repo, T., Korhonen, A., Lehto, T., and Silvennoinen, R. 2015. Assessment of frost damage in mycorrhizal and non-mycorrhizal roots of Scots pine seedlings using classification analysis of their electrical impedance spectra. Trees 1-13.
Rewald, B., and Meinen, C. 2013. Plant roots and spectroscopic methods – analysing species, biomass and vitality. Frontiers in Plant Science 4:
Rewald, B., Meinen, C., Trockenbrodt, M., Ephrath, J., and Rachmilevitch, S. 2012. Root taxa identification in plant mixtures – current techniques and future challenges. Plant Soil 359: 165-182.