Active iron (Fe2⁺) as a reliable indicator of chlorosis in citrus genotypes cultivated in calcareous soils under subtropical conditions in the Jordan Valley

Abstract

Abstract Iron (Fe) deficiency–induced chlorosis is a major constraint to citrus production in calcareous soils, and its severity is often exacerbated under cold stress conditions. This study aimed to evaluate the diagnostic reliability of leaf active iron (Fe 2 ⁺) relative to total Fe and to assess the influence of specific rootstock–variety combinations on micronutrient accumulation under field conditions. The study was conducted in commercial citrus orchards in the Jordan Valley using four citrus varieties (Pineapple, Hamlin, Shamouti, and Ambersweet Tangor) grafted onto three rootstocks (Sour Orange, Cleopatra, and Volkameriana). Trees were visually classified as green (G), moderately chlorotic (MC), or chlorotic (C). Leaf active Fe, total Fe, manganese (Mn), zinc (Zn), and copper (Cu) were determined and analyzed using three-way ANOVA, correlation analysis, and receiver operating characteristic (ROC) curves. Active Fe declined markedly with increasing chlorosis severity; green leaves contained approximately 71% more active Fe than chlorotic leaves, while moderately chlorotic leaves had about 32% higher values. Volkameriana rootstock increased active Fe by approximately 13% compared with Cleopatra and enhanced Mn accumulation by up to 80% relative to Sour Orange. Active Fe showed a strong correlation with chlorosis index (Spearman r = 0.784, p < 0.0001) and excellent diagnostic accuracy (AUC = 0.903), whereas total Fe showed poor performance. The study concludes that leaf active Fe is a sensitive and reliable indicator for diagnosing citrus chlorosis in calcareous soils, providing a practical and sustainable tool for early detection of iron deficiency, optimizing micronutrient management, reducing unnecessary fertilizer inputs, and improving long-term citrus productivity under environmentally challenging conditions. Graphical Abstract