Root biochemical and photosynthetic responses of Lupinus albus under different nitrogen forms and phosphorus sources addition

Nitrogen (N) and Phosphorus (P) are vital nutrients for plant growth and photosynthesis, and they influence rhizosphere processes. In this study, we evaluated the effects of N forms (NO3-vs. NH4+) and soluble P sources (Orthophosphate versus Polyphosphate) on plant biomass, photosynthetic performance, and root traits. Nitrate combined with polyphosphate (Poly-P) increased shoot biomass by 84% and root biomass by 32% compared to the control. Also, shoot P uptake increased by 19-fold, shoot N uptake by 48%, and chlorophyll content by 20% compared to the control. These improvements were associated with a high photosynthetic efficiency (as reflected in PIabs, φPo, ψEo), along with an enhanced root length, surface area, and volume. Conversely, NH4+ addition reduced root biomass and all root traits compared to NO3 regardless of P sources, except average root diameter as an eventual strategy for NH4+ toxicity mitigation. Moreover, photosynthesis activity was impaired, showing lower fluorescence and a destabilization of the oxygen-evolving complex with a higher heat dissipation energy. Moreover, the role of P supply as a key element for energy metabolism was restricted. Under P deficiency, white lupin secreted large amounts of organic acids (OAs), which were negatively correlated with shoot P uptake (r = -0.88 for NO3, r = -0.99 for NH4) and acid phosphatase activity (r = -0.98 for both). Overall, our findings indicate that N form is the key driver of white lupin's growth and metabolism, while P supports mainly biomass allocation and rhizosphere/root activity. However, the interaction N form P source is crucial in maintaining a more favorable rhizosphere environment and therefore an improved nutrient uptake efficiency and consequently photosynthetic efficiency.