Cucumber is one of the most widespread horticultural crops in countries with a Mediterranean climate (Mediterranean basin, western North America, Chile, South Africa, and South Australia). About 90% of cucumber cultivation in the Mediterranean Basin takes place in greenhouses under conditions of intensive production, using alkaline soils with high calcium carbonate content [Ministry of Agriculture, Fisheries and Food (MAPA), 2004]. Iron deficiency (Fe chlorosis) is one of the most serious problems for horticultural crops, including cucumber, and fruit trees cultivated in alkaline and calcareous soils (Álvarez-Fernández et al., 2006; Lucena et al., 2007; Tagliavini and Rombolà, 2001). Two main strategies can be used to correct Fe deficiency in commercial crops. The most effective one is to provide readily available Fe for plants through the use of iron compounds capable of maintaining a sufficient level of soluble Fe in soil solution (Abadía et al., 2004; Lucena, 2006). Among these compounds, certain classes of Fe chelates have proven to be highly efficient (Lucena, 2006). However, the high cost of Fe chelates and similar compounds makes this strategy suitable only for high value crops, primarily fruit trees (Abadía et al., 2004).
Alternatively, a second strategy is based on growing plant cultivars resistant to Fe chlorosis. This strategy may be important in the case of horticultural and field crops (Hansen et al., 2006; Jolley et al., 2004).
Although the Fe-efficiency capabilities of certain cucumber cultivars have been characterized for research purposes, some authors highlighted the need for developing specific studies to better understand the real Fe efficiency of cucumber cultivars commonly used in Mediterranean climate countries (Cadahía, 1998). It is clear that such a study would be of great interest for seed producers and farmers.
In general, plant Fe efficiency has been related to the plant's capacity to develop specific physiological and morphological responses at the root level under Fe-deficient conditions (Briat, 2008; Römheld and Marschner, 1986; Schmidt, 2006). The so-called “Strategy I” plants (nongraminaceous monocots and dicots) activate physiological and biochemical responses that include the development of subapical swelling with abundant root hairs, transfer cells, the increase of Fe3+ enzymatic reduction at the root surface, the acidification of the rhizosphere, the increase in Fe2+ transporters, and the release of organic molecules with reducing capacity (Bienfait, 1988; Briat, 2008; Curie and Briat, 2003; Hell and Stephan, 2003; Jin et al., 2007; Römheld and Marschner, 1986; Schmidt, 2006). In recent years, major advances regarding the control at the molecular level of some of these responses have been made (Barton and Abadía, 2006; Briat, 2008; Curie and Briat, 2003). The genes encoding some of the enzymes and Fe transporters involved in root Fe uptake have also been characterized in certain plant species (Briat, 2008; Curie and Briat, 2003; Fox and Guerinot, 1998; Robinson et al., 1999; Vert et al., 2002; Waters et al., 2002). The main response of the Strategy II plants (graminaceous monocots) under Fe deficiency is characterized by the increase in the production and release to the rhizosphere of a specific family of organic molecules named phytosiderophores that have the capacity to form soluble Fe complexes, thus increasing Fe bioavailability (Briat, 2008; Kobayashi et al., 2006; Mori, 1999). In fact, some studies indicate that the concentration of Fe-phytosiderophore transporters in Strategy II plants also increases under Fe deficiency (Briat, 2008; Kobayashi et al., 2006).
However, a number of studies have shown that the capacity of certain plant cultivars to better grow under conditions of Fe deficiency was not clearly related to any of the Fe-deficiency root responses (De la Guardia and Alcántara, 2002). This fact suggested that other mechanisms could also be involved in the expression of Fe-efficiency capacities in these plants. These mechanisms may probably be related to the special ability of these plants to optimize the metabolic use of the Fe present in the shoot (Briat et al., 2007). This optimization of the Fe metabolic use in the shoot would be reflected in the shoot growth rate and in the amount of Fe extracted from the shoot under Fe-limiting conditions.
In this context, the aim of our work was to characterize the Fe-efficiency capacities of four commercial cucumber cultivars commonly used in the Mediterranean area (Serena, Trópico, Ashley, and Anico). We have considered the relationships of two complementary strategies. On the one hand, we have studied the capacity of these cucumber cultivars to develop the main Fe-stress root responses (reductase activity, rhizosphere acidification, and root morphological changes). On the other hand, we have evaluated the growth and nutrient extraction of the different cucumber cultivars under conditions of Fe deficiency (plant growth, nutrient extraction, leaf chlorophyll concentration, and efficiency of photosystem II).
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