In soybean production, the shade avoidance response can affect yield negatively in both mono-and inter-cropping systems due to increased heterogeneity of the crop and lodging. This is mainly regulated by photoreceptors responding to the ratio between red and far-red light (R:FR) and photosynthetic photon flux density (PPFD). In this study, three soybean cultivars were grown under different R:FR and PPFD in a light emitting diode (LED) climate chamber to disentangle the effect of each on morphology and dry matter. Results showed that plant organs were influenced differently and indicated an interaction with the increase in assimilates at high PPFD. Internode elongation was mainly influenced by low PPFD with an additive effect from low R:FR, whereas petiole elongation responded strongly under low R:FR. Hence, petiole elongation can be seen as the main response to the threat of shade (high PPFD and low R:FR) and both petiole and internode elongation as a response to true shade (low PPFD and low R:FR). Interactions between cultivar and light treatment were found for internode length and diameter and leaf mass ratio, which may be unique properties for specific cropping systems.Agronomy 2019, 9, 428 2 of 15 PPFD by recognizing the change in blue light intensity (400-500 nm) associated with changed PPFD [7]. In Arabidopsis thaliana (L.) Heynh, several studies have shown that phytochrome and cryptochrome interact to fine-tune SAR [5,[8][9][10]. Interacting of cryptochrome with Phytochrome Interacting Factors (PIFs) under low blue light enhanced the response of petioles [8] and hypocotyl [10] to low R:FR. Not only photoreceptors regulate the SAR, as increased leaf thickness under high PPFD has been found, independent of a blue light level [11] and cryptochrome mutations [12].In crop production, the light environment can change with weed pressure or cropping system. Emerging weeds increase R:FR due to the increased reflection of FR while intercropping reduces both PPFD and R:FR perceived by the shorter crop. The SAR of the crop to the given environment can affect yield negatively e.g., SAR to emerging weeds reduced the yield of mono-cropped soybean (Glycine max (L.) Merr.), due to increased crop heterogeneity [4]. The intercropped soybean is often the shorter crop grown together with a taller crop like maize and a strong elongation response of soybean to the given light environments, which can lead to lodging [13]. Earlier studies showed that soybean shaded by maize had a reduced stem diameter [13,14], root length, aboveground biomass, total root biomass, and root-shoot ratio [14,15] and an increased seedling height [13,14,16]. Additionally, the shaded soybean has increased chlorophyll content and reduced the photosynthetic capacity and chlorophyll a/b ratio [16,17]. Plants are considered more shade tolerant if they have a high specific leaf area (SLA), leaf area ratio (LAR), and leaf mass ratio (LMR) [18]. Intercropped soybean leaves had reduced leaf area and mass per area [19] and increased SLA and LAR [16].From field ob...

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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.

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