Background <p>The interaction between phosphorus (P) and magnesium (Mg) significantly influences citrus productivity, especially under Mg deficiency. This study investigated how P availability affects growth, physiological performance, and nutrient partitioning in young navel orange plants under different Mg supplies.</p> Methods <p>A factorial experiment was conducted using sand-cultured navel orange plants subjected to two Mg levels (0 mM as deficient and 1 mM as sufficient) and three P levels (0.2, 1, and 5 mM, supplied as KH₂PO₄/NaH₂PO₄, representing low, moderate, and high supply, respectively). Growth parameters, leaf chlorophyll content, photosynthetic performance, oxidative stress markers, and mineral concentrations in old and new leaves were measured.</p> Results <p>Mg deficiency induced chlorosis and corky veins in old leaves, symptoms markedly accelerated by high P supply. Under Mg deficiency, High P reduced the old leaf biomass but promoted new shoot growth. These responses correlated with accelerated chlorophyll degradation, inhibited photosynthesis, enhanced oxidative damage, and altered nutrient levels (decreased nitrogen but increased potassium and P) in the old leaves, alongside a higher Mg concentration in the new leaves. Adequate Mg nutrition completely mitigated the negative effects of high P to the old leaves.</p> Conclusions <p>High P supply under Mg deficiency triggers a tissue-specific physiological trade-off, promoting old leaf senescence while stimulating new shoot growth via nutrient reallocation. Avoiding high-P inputs in Mg-limited orchards is crucial to prevent aggravating Mg deficiency and ensure sustainable citrus production.</p>