• Terahertz spectra of standard saccharides and mung bean seedlings were measured. • Spectral features of day 1 seedlings were consistent with those of standard starch. • Standard starch and day 1 seedlings were hydrolyzed by α-amylase in vitro. • Absorption peak at 9.0 THz observed in seedlings was attributed to starch. • Starch quantification can be successfully achieved using the peak at 9.0 THz. To investigate the potential of terahertz spectroscopy to monitor and quantify starch in plants, terahertz spectra (3.0–13.5 THz) of mung bean plants 1–7 days after germination were examined and compared to those of starch and its constituent saccharides (standard reagents). Day 1 seedlings showed similar spectral features with standard starch, and absorption peaks gradually disappeared in the subsequent 6 day growth period. To interpret this result and identify useful peaks for starch quantification, standard starch and day 1 seedlings were hydrolyzed by α-amylase in vitro. Since both standard starch and seedlings showed that absorption peak at 9.0 THz disappeared after amylase hydrolysis, this peak is sensitive to changes in starch. Additionally, intensity of this peak was correlated with starch content as quantified by chemical analysis (r = 0.98). Our results indicate terahertz spectra of seedlings can provide an identifiable peak that is attributed to starch and not affected by the constituent saccharides. [ABSTRACT FROM AUTHOR]