Root Research

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In relation to the linkage between physiological and ecosystem functions of fine roots in tree species, this article is reviewed from our work about forming and maintenance processes of fine root architecture in Hinoki cypress (Chamaecyparis obtusa Sieb. et Zucc.). Apical roots are higher in physiological activities than the basal roots, because of not only aging, but also ontogenetic difference, ‘heterorhizy’. These physiological functions affect the demography of individual roots within a fine root architecture, i.e., mortality rate is higher in apical roots than in a basal roots. Moreover, apical roots tend to die before secondary growth, whereas basal roots tend to die after secondary growth. These life cycle processes relate with decomposition rates. Thus, the heterorhizic fine root architecture is important not only for physiological functions but also for ecosystem functions. Further studies in plasticity of fine root architecture among inter- or intra-species to environmental factors will be needed to reveal the linkage between physiological and ecosystem functions of fine roots.

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It is unusual that roots of terrestrial plants are in bare, but they normaly form symbiotic associations with mycorrhizal fungi. The most common association is vesicular-arbuscular (VA) mycorrhiza, and it is well known that forming VA mycorrhiza can improve phosphorus (P) acquisition by the host plants. However, as for nitrogen (N) nutrition, it is still debatable whether the mycorrhizal fungi are involved or not, althhough the availability of N rather than P limits plant growth in many ecosystems. Recently, we have found that N delivery via the mycorrhizal hyphae depends on the form of N supplied; while the fungus fed with ammonium can rapidly deliver N to the host plants, the fungus fed with nitrate scarcely delivers N despite of that nitrate is metabolized by the fungus. On the basis of this finding, I discuss the involvement of the mycorrhizal symbiosis in plant N nutrition with critical review for the previous studies.

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