![]() ![]() A low R:FR ratio leads to a low PSS, and results in a series of shade avoidance syndrome (SAS) responses. The ratio of the active Pfr to the total P (Pfr+Pr) is defined as the phytochrome photostationary state (PSS) ( Sager et al., 1988). Phytochromes have active (Pfr) and inactive forms (Pr) ( Chen and Chory, 2011). These plants can detect such differences by the ratio of red to far-red light (R:FR), which is detected by a family of plant photoreceptors, the phytochromes ( Casal, 2000). Plants that normally grow in unshaded or lightly shaded habitats can distinguish differences in the proximity of other plants through alterations in the spectral intensity of light ( Casal, 2013). We conclude that growing tomato plants under artificial light without FR during the light period causes a range of inverse shade avoidance responses, which result in reduced plant source strength and reduced fruit production, which cannot be compensated by a simple EOD-FR treatment. Additionally, the treatments without FR and the EOD-FR showed strongly reduced fruit production due to reduced fruit growth associated with reduced source strength and delayed flowering. Increased petiole angle and internode length had a negative influence on total light absorption. Simulations with a 3D-model for light absorption revealed that the increase in dry mass was mainly related to an increase in light absorption due to a higher total leaf area. ![]() EOD-FR-treated plants responded almost similarly to plants grown without FR, except for plant height, which was increased. Leaf area and plant dry mass were lower in the treatments completely lacking FR than treatments with FR. Increasing PSS above solar PSS resulted in increased plant height. In an additional experiment, the same treatments were applied to plants receiving supplementary low-intensity solar light. Photosynthetic Active Radiation (PAR 150 μmol m -2 s -1) was supplied using red and blue (95/5%) LEDs. Tomato plants were grown at four PSS levels (0.70, 0.73, 0.80, and 0.88), or with a 15-min end-of-day far-red (EOD-FR) application (PSS 0.10). ![]() Additionally, we investigated whether a short period of FR at the end of the day (EOD-FR) could counteract any potentially negative effects caused by a lack of FR during the day. The aim of this study was to investigate how PSS values higher than solar radiation influence the growth and development of tomato plants. In contrast to sunlight, artificial light sources such as LEDs often lack FR, resulting in a PSS value higher than solar light (∼0.70). It has often been suggested that this architectural response is advantageous for plant growth due to its positive effect on light interception. A low PSS induces an array of shade avoidance responses that influence plant architecture and development. Shading by sunlit leaves causes a low red (R) to far-red (FR) ratio that results in a low phytochrome stationary state (PSS).
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