Ray-tracing analysis on the far-red induced light-capturing ability of kale

Abstract

Far-red (FR, 700–800 nm) light has recently been used to regulate phytochrome-mediated morphological responses in vertical farms. Long-term morphological changes in different FR intensities are well known, but changes in photosynthetic capacity have not been sufficiently investigated. The objectives of this study were to quantitatively analyze the growth, photosynthetic responses, and light interception of kale (Brassica oleracea L. var. acephala) depending on the FR fraction. The plants were cultivated at a photosynthetic photon flux density (PPFD) of 240 μmol m−2 s−1 under red and white LEDs with FR LED (peak at 730 nm) for 16 h. The FR fractions determined as FR/R+FR were FR0.07 (control), FR0.28, FR0.60, and FR0.72. To evaluate the morphological and physiological responses in each FR fraction, the growth, chlorophyll content, chlorophyll fluorescence, P700 absorbance change, and photosynthetic rate were measured. Light interception was quantitatively calculated using 3D-scanned plant models with physiological changes and ray-tracing simulations. The dry weight of shoot and root, total petiole length, and leaf area increased with increasing FR fractions. As a result of ray-tracing analysis, the light interception and canopy photosynthetic rate increased with high FR fraction treatment. As the FR fraction increased, the effective quantum yield of photosystem II [Y(II)], the parameters of the JIP test that is related to quantum yield and efficiency for electron transport in photosystem II and photosystem performance indices decreased. Despite the decreased electron transport efficiency, the canopy photosynthetic rate increased in relation to the FR fraction. In conclusion, the increase in the FR fraction increased the yield of kale because of the synergistic improvement in morphological change and canopy photosynthesis.