Leaf development and energy yield of hydroponic sweetpotato seedlings using single-node cutting as influenced by light intensity and LED spectrum

Vine cuttings with six to eight unfolded leaves are utilized as is conventional in sweetpotato (Ipomoea batatas (L.) Lam.) seedling production. However, most vine cuttings wilt after transplanting into the field. Moreover, few researchers have examined the influence of photon flux density (PFD) provided by white or white plus red light-emitting diodes (LEDs) on sweetpotato plantlets. In this study, hydroponic sweetpotato (cv. Beniharuka) seedlings using single-node cutting were grown under 20 combinations of five levels of PFDs of 150, 200, 250, 300, and 350 μmol m⁻² d⁻¹ and four light qualities: white LEDs with a red light to blue light ratio (R:B ratio) of 0.9, white plus red LEDs with R:B ratios of 1.2 and 2.2, respectively, and fluorescent lamp with an R:B ratio of 1.8 as control, for 20 days under a controlled enviroment. Results showed that the number of newly developed leaves on hydroponic sweetpotato seedlings increased with time in a quadratic function, regardless of light quality. Fluorescent lamps led to greater numbers of new leaves on hydroponic sweetpotato seedlings compared with those grown under LEDs. Plant height, leaf area, and fresh and dry weights increased initially and then decreased with increasing daily light integral (DLI) in quadratic funcitons with a highest value under a PFD of 250 μmol m⁻² d⁻¹. However, no significant differences were observed in fresh and dry weights of hydroponic sweetpotato seedlings grown under PFDs of 200 and 250 μmol m⁻² s⁻¹. The quantum yield of photosystem II (ФPSII) decreased linearly as DLI increased from 8.6–20.2 mol m⁻² d⁻¹. Power consumptions based on fresh and dry weights were lowest in sweetpotato seedlings grown under a PFD of 200 μmol m⁻² s⁻¹ provided by white LEDs with an R:B ratio of 0.9. White LEDs also showed higher light energy use efficiency than white plus red LEDs. In summary, it is recommended that a PFD of 200 μmol m⁻² s⁻¹ with DLI at 11.5 mol m⁻² d⁻¹ provided by white LEDs with an R:B ratio of 0.9 is suitable for hydroponic sweetpotato (cv. Beniharuka) seedling production under a controlled environment.