Photosynthetic apparatus plays a central role in photosensitive physiological acclimations affecting spinach (Spinacia oleracea L.) growth in response to blue and red photon flux ratios

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Relative abundance of blue (B) and red (R) photons in the incident light plays a major role in regulating the morphology and physiology of plants. Many studies have been carried out to determine the R:B photon flux ratio (PFR) required for eliciting desired traits in various plant species. However, studies investigating the impact of R:B PFR on the underlying metabolic events and their mutual interactions are limited. In the present study, spinach (Spinacia oleracea L. cv. All Green) plants were raised in a plant growth chamber under R:B PFRs of 100R:0B, 75R:25B, 50R:50B, 25R:75B and 0R:100B provided by customized light-emitting diode (LED) panels. Data related to plant growth, status of photosynthetic apparatus, nitrogen metabolism, metabolite profile, oxidative stress and antioxidant defense response were collected. A putative mechanistic concept highlighting the interplay between photosynthesis, C- and N-assimilation and oxidative stress response in regulating plant growth under changing R:B PFR has been presented. Steady growth under B and R combined treatments was associated with high chlorophyll content, PSII quantum yield (Fv/Fm, Fv/Fo, ΦPSII) and vitality index (RFd) along with proper C- and N-assimilation. Reduction in non-photochemical quenching (NPQ) and oxidative stress was also observed under 25R:75B to 75R:25B light treatments. Subdued photosynthetic activity under deficiency of B waveband resulted in arrested growth due to suboptimal C- and N-assimilation. Monochromatic B lighting caused damage to the photosynthetic apparatus due to increased photooxidation. Presence of B photons was favorable for eliciting enzymatic and non-enzymatic antioxidant defense responses. However, the antioxidant machinery was unable to ameliorate the excessive oxidative damage under 0R:100B treatment resulting in suboptimal plant growth. Integrity and proper functionality of the photosynthetic apparatus under B and R combined light quality treatments was found to play a pivotal role in coordinating the non-photosynthetic light-sensitive metabolic processes in response to changing R:B PFR for ensuring proper plant growth.