**Tina Hitz, Simone Grae**ff**-Hönninger and Sebastian Munz \***

Institute of Crop Science, Cropping Systems and Modelling, University of Hohenheim, 70599 Stuttgart, Germany; tina.hitz@uni-hohenheim.de (T.H.); graeff@uni-hohenheim.de (S.G.-H.)

**\*** Correspondence: s.munz@uni-hohenheim.de; Tel.: +49-711-22359

Received: 29 October 2020; Accepted: 9 December 2020; Published: 11 December 2020

**Abstract:** Low photosynthetic photon flux density (PPFD) under shade is associated with low blue photon flux density (BPFD), which independent from PPFD can induce shade responses, e.g., elongation growth. In this study, the response of soybean to six levels of BPFD under constant PPFD from LED lighting was investigated with regard to morphology, biomass and photosynthesis to increase the knowledge for optimizing the intensity of BPFD for a speed breeding system. The results showed that low BPFD increased plant height, leaf area and biomass and decreased leaf mass ratio. Photosynthetic rate and internode diameter were not influenced. A functional structural plant model of soybean was calibrated with the experimental data. A response function for internode length to the perceived BPFD by the internodes was derived from simulations and integrated into the model. With the aim to optimize lighting for a speed breeding system, simulations with alternative lighting scenarios indicated that decreasing BPFD during the growth period and using different chamber material with a higher reflectance could reduce energy consumption by 7% compared to the experimental setup, while inducing short soybean plants.

**Keywords:** photomorphogenesis; blue photon flux density; functional structural plant modelling; indoor farming; LED lighting
