NITROGEN FIXATION UNDER PHYSIOLOGICAL AND SALT-STRESSING CONDITIONS
3. DIAGNOSIS OF IRON CHLOROSIS IN FRUIT TREES
appear to be contradictory since plants vary in their requirements for nutrients.
Moreover, the methods used in the assessment of nutritional status are sometimes very specific. A brief summary of some of the results obtained is presented next.
In lemon trees grown on calcareous soil, the iron concentration in leaves was related to the concentrations of phosphorus, potassium and manganese in leaves (Fernandez-Lopez et al., 1993).
In a field experiment with different pear rootstocks, Tagliavini et al. (1993) concluded that not only the uptake of iron but also manganese can be impaired by lime in soils, and that elevated copper levels can also induce iron chlorosis.
Romera et al. (1991c) observed the accumulation of manganese in young leaves of tolerant peach rootstocks growing in a nutrient solution without iron, but not in susceptible rootstocks. In field-grown peach trees, iron chlorosis lead to a sharp increase in the concentration of potassium in leaves, and to slight increases in nitrogen, magnesium and manganese, while phosphorus, copper and zinc were relatively unaffected by the chlorosis (Abadía et al., 1985; Belkhodja et al., 1998b;
Köseoglu, 1995a; Köseoglu, 1995b). In nutrient solution, the peach rootstock
‘Montclar’ had only small concentrations of nitrogen, phosphorus, calcium and iron in the new branches grown in the presence of bicarbonate (Shi and Byrne, 1995;
Shi et al., 1993a, b).
The different tolerance of several grafted grapevines became evident when iron uptake was expressed on a fresh weight basis. Total chlorophyll concentration was positively related to iron, calcium and magnesium, and negatively related to potas-sium contents of leaves (Bavaresco et al., 1992). According to Bavaresco (1997) the mineral composition of leaf blades and petioles of chlorotic and green leaves of grapevines were not significant different, but chlorosis seemed to affect the remo-bilisation of nitrogen, phosphorus, calcium and magnesium to the fruits.
scion-root-stock incompatibility. The roots of fruit trees as they grow explore deeper soil layers that can have high levels of calcium carbonate and greater water content, factors that favour iron chlorosis. Also the root length density of trees is usually much less than in annual crops (Goss, 1991).
Since there are several types of iron chlorosis it is important to properly identify the cause in a particular situation. Therefore, both soil and plant analysis might be needed to investigate the origin of the problem.
3.1. Soil analysis
Soil analysis is routinely used as the basis for fertilisation recommendations of annual crops. However, soil tests have limited value when applied to trees because the root system is deep and unevenly distributed, making it difficult to obtain a repre-sentative soil sample.
Two major approaches can be taken to diagnose lime-induced iron chlorosis based on soil analysis (for a review see Hartwig and Loeppert, 1993), i) to analyse for available iron using extractants capable of chelating the metal, and ii) to deter-mine the lime content of the soil. The active lime (Drouineau, 1942), i.e. the fine and reactive fraction of lime, can be used as an indicator of the risk of iron chlorosis, especially when the amount of extractable iron is also known. Rootstocks are ranked according to their tolerance to active lime, but very often susceptible-rootstocks have other characteristics that make them more eligible for commercial operations, such as tolerance to disease.
3.2. Plant analysis
Iron chlorosis can be identified by visual symptoms, a fast and economic method.
Several authors proposed the use of visual scores, from 0 (without symptoms) to 5 (trees with dead branches and white young leaves) (McKenzie et al., 1984; Romera et al., 1991b; Sanz and Montañés, 1997). The degree of chlorosis can now be rapidly quantified by the measurement of chlorophyll content using a SPAD apparatus.
However, by the time symptoms become apparent it is often too late to prevent the negative effects of the disorder on yield and fruit quality.
Tissue analysis offers a number of advances as well as some challenges.
3.2.1. Leaf analysis
Chemical plant analysis, in particular leaf analysis, is still the most common method used for diagnostic purposes in trees, and is based on the relationship between growth rate of plants and nutrient content (Moreno et al., 1998; Sanz and Montañés, 1995a, b). Leaf analysis integrates all the factors that might influence nutrient availability in the soil and plant uptake, and pinpoints the nutritional balance of the plant at the time of sampling. However, the use of leaf analysis presents limitations when applied to lime-induced chlorosis, since in many field-grown plants there is no correlation between leaf iron concentration and the degree of chlorosis expressed as chlorophyll content (Abadía, 1992; Hamzé and Nimah, 1982; Mengel et al., 1994;
Pestana et al., 2001b). Moreover, iron concentration in chlorotic leaves, expressed on a dry weight basis, is frequently even greater than in green leaves (Abadía, 1992; Aktas and Van Egmond, 1979; Bavaresco et al., 1993a; Bavaresco et al., 1999;
Deckock et al., 1979; Fernandez-Lopez et al., 1993; Mengel, 1995; Morales et al., 1998c; Rashid et al., 1990; Terry and Low, 1982). This was called the ‘chlorosis paradox’ by Römheld (2000) and results from the inactivation of iron in leaves or from an inhibition of leaf growth due to iron chlorosis (Morales et al., 1998c).
Morales et al. (2000c) observed a greater iron concentration in the petioles and veins of chlorotic leaves of peach trees than in the lamina, where active iron is located.
In apple leaves under iron deficiency, Vedina and Toma (2000) observed a decrease in organic iron content, indicating low mobility of iron compounds.
Bavaresco et al. (1999) proposed the expression of iron concentration per leaf (µg Fe leaf–1) rather than on a dry matter basis, as it allowed the separation of dark green from chlorotic leaves.
Another limitation of leaf analysis is the fact that the sampling date recommended for fruit trees is late in the growing season, generally very close to harvest. At this point it is no longer possible to correct nutritional disorders in time to avoid decreases in yield (Sanz and Montañés, 1995b). In fact, according to Igartua et al.
(2000), at the recommended date for foliar analysis of peach, 120 days after full bloom, most of the varieties grown in the Mediterranean area are already har-vested or are very close to harvest. This also happens with pear (Sanz and Montañés, 1995b). It is therefore important to develop a useful method to diagnose iron defi-ciency in fruit trees before yield is affected.
The standard method used to interpret the results of leaf analysis is to compare nutrient concentrations to reference values for a particular crop and sampling method.
At most, this procedure can identify a single deficiency at a time, but does not evaluate the nutrient balance. Due to the complexity of the nutritional imbalances resulting from iron chlorosis, several authors have proposed the use of indexes to interpret plant analysis data. These include i) nutrient ratios, ii) Diagnosis and Recommendation Integrated System (DRIS), and iii) Deviation from Optimum Percentage (DOP) (Beverly et al., 1984; Guzmán and Romero, 1988; Guzmán et al., 1991; Köseoglu, 1995b; López-Cantarero et al., 1992; Montañés and Heras, 1991;
Valenzuela et al., 1992).
The use of nutrient ratios to interpret foliar analysis was proposed for apple (Tagliavini et al., 1992), peach (Abadía et al., 1985; Alcántara and Romera, 1990;
Köseoglu, 1995a), quince (Tagliavini et al., 1995b), pear (Tagliavini et al., 1993), citrus (Fernández, 1995; Hellín et al., 1984; Wallace, 1990), grape (Bavaresco, 1997), and berries (Bavaresco, 1997). The nutritional relationships identified were the ratios P:Fe (Köseoglu, 1995a; Mengel et al., 1984; Wei et al., 1995), K:Ca (Abadía et al., 1989b; Abadía et al., 1985; Garcia et al., 1999; Mengel et al., 1984; Montañés et al., 1990a), Fe:Mn (Lucena et al., 1990; Monge et al., 1993) and Zn:Fe (Nenova and Stoyanov, 1999). These ratios express nutritional imbalances that appear when iron immobilization in the plant takes place. However, no absolute values could be established for any of the ratios to enable the diagnosis of iron chlorosis under field conditions (Chaney, 1984).
The analysis of an ‘active’ pool of iron in leaves (usually identified with Fe