Abstract
This work quantifies Fe uptake in young citrus trees, its partitioning among plant compartments, and the contribution of the Fe absorbed from fertilizer to the development of new tissues. A soil pot experiment was conducted using 4-year-old clementine trees (Citrus clementina Hort ex Tan), and a dose of 240 mg Fe was applied by labeled fertilizer (92% atom 57Fe excess). Plants were uprooted at five different phenologic states: end of flowering (May 15), end of fruit setting and fruit drop (July 1), two fruit growing moments (August 1 and October 15), and at complete fruit maturity (December 10). The Fe accumulated in the root system exceeded 90% of the total Fe content in the plant. All organs progressively enriched with 57Fe (8.5–15.5% and 7.4–9.9% for young and old organs, respectively). Reproductive ones reached the highest increase (111% between May and October). 57Fe enrichment from woody organs reflects an increasing gradient to sink organs. The root system accumulated 80% of the Fe absorbed from the fertilizer, but the young organs accumulated relatively more Fe uptake during flowering and fruit setting (15.6% and 13.8%, respectively) than old organs (around 9.8%). Although iron derived from fertilizer (Fedff) preferably supplied young organs (16.7–31.0%) against old ones (2.5–14.9%), it only represented between 13.8% and 21.4% of its content. The use efficiency of the applied Fe (FeUE) barely exceeded 15%. The lowest FeUE were found in young and old organs of the aerial part (1.1–1.8% and 0.7–1.2%, respectively). Since the pattern of the seasonal absorption of Fe is similar to the monthly distribution curve of the supplied Fe, it is recommended that the application of Fe chelates in calcareous soils should be performed in a similar way to that proposed in this curve.
Highlights
Most soils contain enough iron (Fe) to satisfy the needs of crops, in the presence of oxygen, an alkaline medium, and an excess of soil moisture, this ion precipitates as the insoluble hydroxide, which is a scarcely soluble Fe form that, causes serious problems of iron chlorosis for plants
Since the pattern of the seasonal absorption of Fe is similar to the monthly distribution curve of the supplied Fe, it is recommended that the application of Fe chelates in calcareous soils should be performed in a similar way to that proposed in this curve
The response may vary from one species to another, iron chlorosis reduces the longevity of the plantation, as it affects the development of new shoots due to the low translocation of the element from the adult leaves to the new tissues
Summary
Most soils contain enough iron (Fe) to satisfy the needs of crops, in the presence of oxygen, an alkaline medium, and an excess of soil moisture, this ion precipitates as the insoluble hydroxide, which is a scarcely soluble Fe form that, causes serious problems of iron chlorosis for plants. In alkaline soils such as those in the Mediterranean basin, high levels of bicarbonate ions are the main cause of iron chlorosis in citrus plantations. The deficiency of Fe impairs the photosynthetic capacity of the tree decreasing photoassimilated compounds such as sugar levels and starch. This ion is part of a good number of enzyme systems important for the metabolism of the plants, such as the protein ferredoxin that acts as a final acceptor of electrons and whose high redox potential allows to reduce substances as NADP+, nitrate, oxygen, and sulfate [2,3]. The flavedo of oranges and mandarins characteristically acquire a yellowish color, without reaching the orange-reddish tonalities of the normal fruits, and it diminishes the content of soluble solids in the juice [5]
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