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

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.