Plant roots and spectroscopic methods - analyzing species, biomass and vitality

Abstract

In order to understand plant functioning, plant community composition, and terrestrial biogeochemistry, it is decisive to study standing root biomass, (fine) root dynamics, and interactions belowground. While most plant taxa can be identified by visual criteria aboveground, roots show less distinctive features. Furthermore, root systems of neighboring plants are rarely spatially segregated; thus, most soil horizons and samples hold roots of more than one species necessitating root sorting according to taxa. In the last decades, various approaches, ranging from anatomical and morphological analyses to differences in chemical composition and DNA sequencing were applied to discern species' identity and biomass belowground. Among those methods, a variety of spectroscopic methods was used to detect differences in the chemical composition of roots. In this review, spectroscopic methods used to study root systems of herbaceous and woody species in excised samples or in situ will be discussed. In detail, techniques will be reviewed according to their usability to discern root taxa, to determine root vitality, and to quantify root biomass non-destructively or in soil cores holding mixtures of plant roots. In addition, spectroscopic methods which may be able to play an increasing role in future studies on root biomass and related traits are highlighted.

Keywords: IR spectrometry; electrochemical impedance spectroscopy; fine root; root biomass; root taxa; root vitality.

FIGURE 1

FT-MIR–ATR spectra recorded from dried, ground roots of Triticum aestivum (TA), Apera spica-venti (AS), Brassica napus (BN), and Sisymbrium officinale (SO) grown in a greenhouse experiment. Spectra are means of four replications, vector-normalized and offset-corrected (C. Meinen, unpublished data).

FIGURE 2

Cluster analysis of FT-MIR–ATR spectra (Figure 1) recorded from dried, ground roots of Triticum aestivum (TA), Apera spica-venti (AS), Brassica napus (BN), and Sisymbrium officinale (SO) grown in a greenhouse experiment. Data were pre-processed by first derivative and vector normalization. Spectral distances were calculated by Euclidean distance and Ward’s algorithm in the frequency range of 374–3999 cm^-1 (mean, n = 4; C. Meinen, unpublished data).

FIGURE 3

Classification of Populus spp. rhizosphere components using different combinations of visible (VIS) and near infrared (NIR) wave bands to produce spectral reflectance images at days 20, 55, 70, and 170 after planting (DAP). Wavelengths of the three VIS bands (upper row) and the VIS–NIR bands (lower row) are 650, 550, and 480 nm, and 886, 679, and 522 nm, respectively. A color photo at DAP 70 and the false color classification scheme are given. Source images were taken under wet soil conditions (see Nakaji et al., 2008 for details). Images courtesy of T. Nakaji, K. Noguchi and H. Oguma, Japan.

FIGURE 4

Fresh weight of hydroponically grown Salix spp. roots and the reciprocal sum of resistances R₁ and R₂ during growth (mean, n = 3; Repo et al., 2005, modified). Electrochemical impedance spectroscopy (EIS) was used to determine R₁ and R₂ (see Repo et al., 2005 for details).