Assessment of root foraging behaviour in Aralia elata subjected to drought stress under different light spectra

Drought may be more frequent in foreseen decades that will threat non-wood forest products (NWFP) in temperate forests. Plants cope with drought by increasing root foraging ability, which may be also impacted by exposure to light spectra (components of wavelengths in three colours of monochronic lights). In this study, three hybrid light spectra were provided by light-emitting diode (LED) in red (wavelength: 13.6% blue, 60.0% green, 26.4% red), green (2.9% blue, 84.6% green, 2.5% red), and blue (5.4% blue, 77.8% green, 16.9% red) lights for culturing potted Aralia elata seedlings in homogeneous (67.5 mg nitrogen [N] to both halves of pot) and heterogeneous (135 mg N to left half of pot) under drought and well-watered conditions. The red light spectrum was the unique illumination environment where height growth was promoted under well-watered condition and root biomass in fertilized patch was enhanced under drought. Compared to blue light spectrum, red light spectrum increased root foraging scale and precision and placement ratio. Red light spectrum also promoted aboveground biomass, but reduced root collar diameter. No interaction was detected between any pairs of factors among drought, light, and soil nutrient pattern. The heterogeneous pattern increased root to shoot biomass ratio with decreased shoot biomass and increased root foraging sensitivity and precision and fine root placement ratio. Drought enlarged the overall foraging scale with no effects on foraging sensitivity or precision. Overall, exposure to red light spectrum had the potential to promote fine root foraging behaviour under drought, but more trails deserve being tested in the future on a wider range of wavelengths.