Phenotypic Plasticity of Photochemical Traits and Antioxidant Responsiveness Confer Photosynthetic Resilience in Peanut (Arachis hypogaea L.) Under Phosphorus Deficiency: The Pivotal Role of Cyclic Electron Flow

Phosphorus (P) deficiency is a major factor governing peanut (Arachis hypogaea L.) productivity, and the physiological mechanisms by which different genotypes (with contrasting photosynthetic capacities) coordinate carbon assimilation and photoprotection remain elusive. This study elucidated the strategic divergence among different peanut genotypes in their foliar photosystems to perform physiological homeostasis under low-phosphorus (LP) conditions. Based on a peanut mini-core collection, six representative accessions with contrasting photosynthetic capacities were selected and categorized into high- and low-photosynthetic functional groups. We integrated leaf gas exchange, chlorophyll fluorescence, the trans-thylakoid proton gradient (ΔpH), and antioxidant enzyme assays to evaluate their adaptive responses to low-P stress relative to the high-P (HP) control. Our results demonstrated that LP stress induced widespread photosynthetic inhibition across all accessions; this suppression was primarily driven by non-stomatal limitations. Under LP stress, high-Pn accessions exhibited superior cyclic electron flow (CEF) plasticity synergized with highly plastic guaiacol peroxidase (POD) activity, suppressing the leaf-level ROS burst and maintaining a substantial ΔpH for ATP synthesis and PSI stability. Conversely, low-Pn accessions suffered from severe oxidative overload and relied heavily on passive thermal dissipation, characterized by elevated non-photochemical quenching (NPQ) values and restricted CEF engagement. Principal component analysis (PCA) confirmed that while baseline biochemical impairments were universal, the capacity to dynamically modulate this ΔpH-dependent regulatory network—which integrates CEF, cytochrome b6f photosynthetic control, and antenna-level NPQ—served as the decisive determinant underlying genotypic variations in photosystem resilience under P deficiency. This study demonstrated that peanut genotypes deploy divergent, ΔpH-centered strategies to balance light energy distribution under P-limited conditions. These findings provide a novel and plausible mechanistic framework for selecting and breeding P-efficient peanut cultivars in poor soils with enhanced photosystem resilience.