Synthetic iron chelates

The prevention and the cure of Fe chlorosis in fruit trees have been traditionally approached through the application of synthetic Fe chelates (Lucena, 2003). Several Fe chelators are available, those mainly used being EDTA (ethylenediaminetetraacetic acid) and DTPA (diethylenetriaminepenta-acetic acid) characterized by a low stability constant and suitable for foliar applications, and EDDHA [ethylenediaminedi(o-hydroxyphenylacetic) acid], EDDHMA [ethylene-diaminedi(o-hydroxy-p-methylphenylacetic) acid] and EDDHSA [ethylenediamine-di(2-hydroxy-5-sulfophenylacetic) acid], with higher stability constants (Lucena et al., 1996) and suitable for soil supply. Soil application of Fe chelates aims at enhancing Fe availability for uptake at the root level and represents an efficient prevention method, provided no major problems of root absorption, transport and leaf utilization arise. Soil applied chelates are ineffective when applied too early in the spring, when soil temperature is too low or when root uptake is impaired by waterlogging. Field studies on the effectiveness of various soil-applied synthetic Fe-chelates have recently been performed by Alvarez-Fernandez et al. (2003a and 2004). Foliar applications of Fe chelates are used as alternative to soil Fe supply or to integrate the latter to provide more rapid leaf Fe availability during specific phenological stages. Due to the low Fe mobility in the phloem, repeated leaf sprays in chlorotic trees should be made to meet the Fe requirement during rapid shoot development (Rombola et al., 2000).

Iron chelates are usually effective but do not represent a sustainable approach to prevent or cure Fe deficiency (Tagliavini and Rombola, 2001). Soil applied Fe chelates are water soluble and easily leached out of the root zone if excessive irrigation regimes are applied or during the autumn-winter (Rombola et al., 2002b). A likely underestimated problem related to synthetic chelates is the potential of some of them to bind heavy metals (Grcman et al., 2001). Some iron chelates have a scarce degradability in the soil (Nortemann, 1999) and may cause toxic effects on soil microorganisms and mycorrhizae (Grcman et al., 2001). Moreover, it has been reported that plants may take up Fe-chelates directly and EDDHA traces could reach the fruits (Bienfait et al., 2004).

The efficiency of Fe uptake from chelates can be enhanced by improving the technology related to their distribution and choosing the best application timings. Repeated additions of small amounts of Fe chelates through the irrigation water (by drip or micro sprinkler systems) likely maintain optimal Fe availability in the portion of soil where most roots are located and reduce the risk of Fe chelate leaching. Field experiments with clementine (Banuls et al., 2003) indicate that the frequency of Fe chelate supply by fertigation during the vegetative season does not consistently affect yields and fruit quality. In kiwifruit, late summer-early fall applications of Fe chelates were more effective than those carried out before bud burst in preventing the occurrence of early spring Fe chlorosis, possibly through mechanisms of storage and successive spring remobilization (Tagliavini and Rombola, 2001). Attempts have been followed to reduce the risk of leaching of Fe chelates by fixing them to an organic matrix (Yehuda et al., 2003).

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