Use Of Micronutrient Efficient Genotypes

In general, the traditional method of applying micronutrients to plants subjected to their deficiency provides effective control of their deficiencies but this method involves high-energy inputs. Costs on both labour and fertilizers are high. Continuous fertilizer usage also contributes to resource depletion and, at times, soil pollution. The alternative approach of 'tailoring the plant to fit the soil' (Foy, 1983) circumvents these problems. Growing plant genotypes that can produce reasonably well under conditions of low micronutrient availability, where the other commonly cultivated crop genotypes fail, offers a low input and ecologically safe approach for amelioration of the deficiencies.

Ability to withstand or tolerate the deficiency stress is a function of the nutrient efficiency of the genotype, which is a heritable trait. High nutrient efficiency could be due to high ability for nutrient uptake (uptake efficiency), shoot to root transport (transport efficiency) and/or biochemical utilization (utilization efficiency). Considerable information has been gathered on genotypic differences in micronutrient efficiency of plants and the mechanisms which contribute to these differences (Viets, 1966; Lucas and Kezek, 1972). Different mechanisms contributing to nutrient efficiency of genotypes have been discussed by Sattelmacher et al. (1994) and reviewed by Rengel (1999b). In general, uptake efficiency is a function of root morphology, root exudations and nutrient transport across plasmalemma. Not only nutrient influx but efflux contributes to genotypic differences in nutrient efficiency. Rengel and Graham (1996) showed high efflux of zinc by Zn-inefficient wheat genotypes. Mechanisms contributing to nutrient efficiency may be constitutive or inducible (inductive). The latter are expressed by the efficient genotypes in response to deficiency stress, as is the case with iron deficiency stress. In Fe-efficient genotypes of both strategy I and strategy II plants, the efficiency mechanisms are induced only in response to Fe-deficiency stress (Jolley et al. 1996; Pearson and Rengel, 1997). Likewise, Zn-efficient genotypes of wheat are reported to show increased rates of Zn uptake only when subjected to Zn deficiency stress (Rengel and Wheal, 1997). Better nutrient efficiency could result from more efficient transport of nutrients from root to shoot involving their loading into the xylem sap, and in case of nutrients transported as organic complexes, the availability of the organic ligands. Genotypes also differ in their capacity to make functional use of the nutrients. It has been shown that for about the same concentration of Zn, as in Zn-inefficient genotypes of wheat, the Zn-efficient genotypes show higher activity of carbonic anhydrase (Rengel, 1995a), which may contribute to higher rates of photosynthesis (Fischer et al. 1997), and superoxide dismutase (Cakmak et al. 1997c), which functions as a component of the antioxidative defense system (Cakmak, 2000).

Notwithstanding the mechanisms contributing to higher nutrient efficiency, which may, except for isogenic lines, differ in different genotypes, the use of efficient genotypes provide an opportunity to manage the deficiencies unless they are very severe. Under conditions of severe deficiency, a combination of efficient genotypes and fertilizer amendment, at a much lower rate than needed for an inefficient genotype, may provide the desired yield response. Efficient genotypes are of particular advantage in management of deficiencies on problem soils, where their correction through fertilizer use alone is difficult.

In view of the fact that use of nutrient-efficient (= tolerant) genotypes provides a low-input and environmentally friendly approach for management of deficiencies (Lynch, 1998), increasing thrust is being laid on breeding and identifying the existing genotypes for nutrient efficiency. Nutrient efficiency traits being inheritable and efficiency being the dominant factor, development of nutrient efficient genotypes should be possible through conventional breeding (Cianzio, 1999). So far, major emphasis has, however, been laid on identifying existing crop cultivars for tolerance to micronutrient deficiencies and toxicities through appropriate screening methods.

Iron Deficiency

Because of the several factors that render iron unavailable to the plant, iron deficiency forms the most common and widespread micronutrient disorder of plants, particularly on calcareous soils. Long-term, sustainable solution to the problem, through a soil-based approach is not a successful story. The problem finds a more practical, sustainable and cost-effective solution in raising crop cultivars or genotypes, which have high efficiency of uptake, transport and/or utilization of iron. Interest in genotypic approach for overcoming the constraints of lime-induced deficiency of iron dates six decades back, when Weiss (1943) showed iron chlorosis in soybean to be a heritable trait, determined by a single recessive gene pair. It was observed that when grown on calcareous soils, soybean cv. PI 54619-5-1 (PI) developed iron chlorosis but cv Hawkee (HA) showed apparently healthy growth (free from iron chlorosis) (Brown, 1961). The difference in the tolerance of the two cultivars to iron chlorosis was found to be related to the difference in the ability of their roots to absorb and translocate iron (Brown, 1963). Higher efficiency of iron uptake by HA roots was related to higher capacity of its roots to reduce Fe1+ to Fe2+, in which form alone soybeans absorb iron. Over the years, it has been established that, under conditions of iron deficiency, transport of iron across the plasmalemma involves the activity of plasmalemma bound Fe(III) chelate reductase (Bienfait, 1985), and that it is the main factor contributing to iron efficiency of strategy I plants. When subjected to iron deficiency, roots of some dicotyledonons plants secrete riboflavin, which leads to enhanced uptake of iron (Welkie and Miller, 1993). The Fe-efficient genotypes have been shown to possess greater capacity for riboflavin secretion in response to Fe-deficiency than the Fe-inefficient genotypes (Jollev et al. 1991; Welkie, 1996).

The Fe-efficiency of strategy II genotypes is essentially a function of the capacity of their roots to produce and release phytosiderophores in response of iron deficiency stress (Romheld and Marschner, 1990; Jolley and Brown, 1991a) and the uptake kinetics of the Fe-phytosiderophores (Von Wiren et al. 1995). The phytosiderophores released by plant roots in response to iron deficiency from a complex with Fe3+ and contribute to increased uptake of Fe3+ in the chelated form. The capacity to produce and release the phytosiderophores is genetically determined. The Fe-efficient genotypes of the strategy II plants, that show tolerance to iron chlorosis on calcareous soils, have been found to be efficient in release of phytosiderophores such as hydroxymugineic acid (HMG). The efficiency of cereal crop species growing on calcareous soils is related to the capacity of their roots to release phytosiderophores. It has been suggested that release of phytosiderophores by roofs can form a criterion for evaluation of Fe-efficiency of the graminaceous plants (Hansen et al., 1995, 1996). Such an attempt should, however, take into account the fact that release of phytosiderophores by roots shows diurnal variations (Rómheld and Marschner, 1981a; Zhang et al. 1991b; Walter et al, 1995; Cakmak et al. 1998).

A prerequisite to management of iron-deficiency using the iron-efficient genotypes is the identification of such genotypes through suitable screening procedures. Identification of bicarbonate as the soil factor inducing chlorosis in susceptible genotypes of soybean grown on calcareous soils (Coulombe et al. 1984a) led to development of a screening method in which bicarbonate was included in the nutrient solution to simulate soil calcareousness (Graham et al. 1992). The method developed for screening soybean genotypes for iron effectivity included bicarbonate and low (1.5 to 2pm) concentration of iron supplied as Fe-EDDHA (Coulombe et al. 1984b). Subsequently, Fe-EDDHA was replaced with Fe-DTPA as the source of iron (Coulombe and Chaney solution) because DTPA has higher iron buffering capacity than EDDHA (Chaney et al. 1989). The method was further improved for screening a wide range of dicots (strategy I plants) for Fe-chlorosis (Chaney et al. 1992a,b). While use of 25 ¡im Fe-DTPA was suggested for general screening, 15 }im Fe-DTPA was recommended for screening highly chlorosis resistant cultivars. Methods have also been developed for screening of plant genotypes for iron chlorosis by raising seedlings on a calcareous soil under greenhouse conditions. Ocumpaugh et al. (1992) found the method suitable for screening oats for iron deficiency. A positive correlation between root Fe3+-reducing capacity and Fe-efficiency (Camp et al. 1987; Jolley and Brown, 1987; Tipton and Thawson, 1985) formed the basis for development of large scale short-term methods for screening of plant genotypes for iron efficiency (Jolley et al. 1992; Stevens et al. 1993). The Fe3+ reduction capacity of roots of soybean genotypes was found to be highly correlated to their field susceptibility rating for iron chlorosis (Jolley et al. 1992). Lin et al. (1998) made a comparative study of linkage map and quantitative traits loci (QTL) in F lines from two soybean populationsPride B 216 X A 15 and Anoka X A7 grown in nutrient solution and on calcareous soils under field conditions. Identical traits were expressed under both the conditions, which showed that both the systems identified similar genetic mechanisms of iron uptake and/or utilization.

Manganese Deficiency

The general, mechanisms contributing to nutrient uptake, transport and utilization fail to provide a satisfactory basis for manganese efficiency of genotypes (Rengel, 1999). It is largely so because manganese efficiency is expressed in the soil system but not in solution cultures (Huang et al. 1994) that provide a suitable technique for investigating the physiological basis of genotypic differences in nutrient efficiency. According to Rengel (1999), the main factor that contributes to manganese efficiency of a genotype is the capacity of plant roots to excrete substances that facilitate larger mobilization of manganese from the rhizosphere. The root secretions possibly include reductants and organic ligands capable of chelating manganese and microbial stimulants. Secretions that favour the growth of the Mn-reducing organisms or inhibit the Mn-oxidizing microorganisms are likely to contribute to manganese efficiency (Timomin, 1965). Differences in the pattern of internal compartmentalization and remobilization of manganese are other likely factors contributing to genotypic differences in manganese efficiency (Huang et al. 1993).

Differences in tolerance of crop genotypes to manganese deficiency have been reported (Graham, 1988; Bansal et al. 1991). Based on field screening on low manganese soils, Bansal et al. (1991) have identified some manganese deficiency tolerant genotypes of wheat. Ingeneral, information on genotypic differences in crop tolerance to manganese deficiency is limited possibly because manganese deficiency does not pose a serious problem to crops on a large scale.

Copper Deficiency

Graham et al. (1987a) described copper efficiency 'as the ability of a genotype to yield well on a copper deficient soil, and also, taking account of different yield potentials, as relative yield of paired-Cu and +Cu plants i.e. (-Cu/+Cu) x 100'. Differences in Cu-efficiency of genotypes have been made out best in case of cereal crops, which are sensitive to copper deficiency (Agarwala et al. 1971; Graham et al. 1987a). Rye has high copper efficiency, which is attributed to location of gene(s) controlling copper efficiency on the long arm of its 5 R chromosome (5 RL) segment (Graham et al. 1987a). The efficiency of this segment has been successfully transferred to the rye-wheat hybrid triticale (Graham and Pearce, 1979) and, through back-crossing and selfing, into wheat (Graham et al. 1987a). The wheat-rice translocation lines show marked increase in copper efficiency. Their grain yield on copper deficient soils is reported to be more than double that of the parental lines.

Zinc Deficiency

With the possible exception of iron, more studies have been carried out to study the causes for genotypic differences in zinc efficiency than any other micronutrient. The subject has been recently reviewed by Rengel (1999). There are indications that enhanced uptake of zinc by the Zn-efficient genotypes is an inducible response evoked in response to zinc deficiency (Rengel and Wheal, 1997). The Zn-efficient genotypes also show increased apoplast to symplast transport of zinc under conditions of Zn-deficiency (Rengel and Graham, 1996; Rengel and Howkesford, 1997; Rengel et al.

1998). Genotypic differences have also been shown to be influenced by rates of zinc efflux. Higher efflux of zinc by Zn-inefficient genotypes could result in decrease in zinc concentration (Rengel and Graham, 1996). Some genotypes may show higher Zn-efficiency because of a higher capacity for biochemical utilization of zinc. Even with little change in uptake or transport of zinc under zinc deficiency conditions, the Zn-efficient genotypes may show higher activities of zinc enzymes such as carbonic anhydrase (Rengel 1995a, Hacisalihoglu et al. 2003) and Cu/Zn superoxide dismutase (Yu et al. 1999; Hacisalihoglu et al. 2003). Hacisalihoglu et al. (2003) have shown that Zn-efficient and Zn-inefficient genotypes of wheat, which show no differences in uptake and transport of zinc and do not differ in water soluble zinc content or sub-cellular compartmentalization of zinc, show marked differences in the expression of zinc enzymes. When subjected to zinc-deficiency stress, the Zn-efficient genotype Kargiz, showed higher activities of Cu/Zn SOD and carbonic anhydrase than the Zn-inefficient genotype BDME. Northern analysis showed that the activities of the two enzymes in the Zn-efficient genotype were upregulated in response to Zn-deficiency stress. Thus, Zn efficiency of Kargiz is a function of its higher ability for biochemical utilization of zinc under conditions of its limited availability. Zinc-efficient genotypes of wheat are also reported to have high tolerance to crown rot decrease caused by Fusarium graminiarium (Grewal et al. 1996).

While little advance has been made in breeding zinc-efficient genotypes (Cianzio, 1999), a large number of crop species have been investigated for genotypic differences in zinc efficiency (Table 11.7). In India, where zinc deficiency forms a major constraint in crop production, promising varieties of Indian crop plants have been evaluated for tolerance to zinc deficiency (Takkar, 1993). Agarwala et al. (1978) examined 35 rice varieties for tolerance to zinc deficiency. Rice variety Sabarmati, showed high tolerance to zinc deficiency (Fig. 11.1). Screening of wheat varieties for tolerance to zinc deficiency showed var. WL 212 to be tolerant to zinc deficiency (Fig. 11.2) (Agarwala and Sharma 1979).

Use of zinc-efficient genotypes offers a low-input approach for overcoming zinc deficiency and minimizing fertilizer use for optimum yields. This approach is especially effective in management of zinc deficiency when it is induced due to adverse soil chemical conditions, such as calcareousness (Sakal et al. 1988; Cakmak et al. 1997a) and sodicity (Qadar, 2002). Recently, Singh et al. (2005) have discussed the factors contributing to Zn-efficiency of cereals and suggested that this could be improved through manipulation of mechanisms involved in synthesis and release of Zn-phytosiderophores and improving root architecture. The latter has been shown to play an important role in determining the Zn-efficiency of wheat genotypes (Dong et al. 1995).

Fig. 11.1. Zn-deficient and Zn-sufficient plants of rice vars. Jagannath, Calrose and Sabarmati (arranged left to right). Grown with Zn-deficient nutrition, var. Jagannath shows severe restricition of growth and Zn-deficiency symptoms, whereas var. Sabarmati shows normal growth, free from Zn deficiency symptoms. Var. Calrose figures intermediate in terms of tolerance to Zn deficiency (Source: Agarwala and Sharma, 1979).

Fig. 11.1. Zn-deficient and Zn-sufficient plants of rice vars. Jagannath, Calrose and Sabarmati (arranged left to right). Grown with Zn-deficient nutrition, var. Jagannath shows severe restricition of growth and Zn-deficiency symptoms, whereas var. Sabarmati shows normal growth, free from Zn deficiency symptoms. Var. Calrose figures intermediate in terms of tolerance to Zn deficiency (Source: Agarwala and Sharma, 1979).

Table 11.7. Some crops investigated for genotypic differences in tolerance to zinc deficiency

Crop

Wheat (Triticum aestivum L.)

Barley (Hordeum vulgare L.) Rice (Oryza sativa L.)

Triticale (Secale cereale) Chickpea (Cicer arietimum L.) Soybean (Glycine max L.) Navy bean (Phaseolus vulgaris) Oilseed rape (Brassica napus) Potato (Solanum tuberosum L.)

Reference

Agarwala et al. (1971), Sharma et al. (1971, 1976), Graham et al. (1992), Graham and Rengel (1993), Takkar (1993), Rengel and Graham (1995c), Cakmak et al. (1996c)

Frono et al. (1975), Agarwala et al. (1978), Sakal et al. (1988), Qadar (2002)

Ramani and Kannan (1985)

Saxena and Chandel (1992) Jolley and Brown (1991b)

Sharma and Grewal (1990)

Fig. 11.2. Wheat vars. PV18 (left three pots) and WL 212 (right three pots) raised at 0.0001 )M, O.OOtyM and 1.0 ^M Zn (arranged left to right). At low (0.0001 and 0.001 ;iM) Zn supply, var. PV18 shows severe limitation in growth and visible symptoms of Zn deficiency, whereas var. WL 212 shows moderately good growth, and only mild symptoms

Fig. 11.2. Wheat vars. PV18 (left three pots) and WL 212 (right three pots) raised at 0.0001 )M, O.OOtyM and 1.0 ^M Zn (arranged left to right). At low (0.0001 and 0.001 ;iM) Zn supply, var. PV18 shows severe limitation in growth and visible symptoms of Zn deficiency, whereas var. WL 212 shows moderately good growth, and only mild symptoms

Information on genotypic differences in molybdenum efficiency is limited. In an early study, Young and Takahashi (1953) reported differences in performance of alfalfa cultivars on molybdenum-deficient soils of Hawai and suggested avoiding cultivation of the susceptible cultivars as forage. Based on growth depression and severity of visible symptoms induced in response to molybdenum deprivation, Agarwala and Sharma (1979) reported differences in tolerance of several wheat, rice and maize varieties to molybdenum deficiency under sand culture conditions. Wheat var. Sonalika, was reported tolerant to molybdenum deficiency (Fig. 11.3). Franco and Munns (1981) reported differences in molybdenum accumulation in common bean (Phaseolus vulgaris L.) genotypes in acid soils, which are low in available molybdenum. In beans, genotypic differences in molybdenum accumulation corresponded with the differences in their N2 fixation ability (Brodrick and Giller, 1991).

Boron Deficiency

Several crop genotypes are known to show genotypic differences in boron efficiency (Nable et al. 1990; Bellaloui and Brown, 1998; Rerkasem and Jamjod, 1997, 2004, references therein). Based on differences in seed yield

Fig. 11.3. Mo-sufficient and Mo-deficient plants of wheat vars. Sonalika, and WG 377 (arranged in that order from left to right). Grown with Mo-deficient nutrition, var. Sonalika shows apparently normal growth, whereas var. WG 377 exhibits growth depression and

on low boron soils of Chiang Mai (Thailand), Rerkasem and associates (see Rerkasem and Jamjod 1997). have reported genotypic differences in boron efficiency in wheat, barley, green gram, black gram and soybean. Wheat cultivars SW 41 and Fang 60 showed marked difference in boron efficiency. The B-inefficient wheat cultivar SW 41 showed even higher sensitivity to boron deficiency than the dicotyledonous cultivars. Wheat has otherwise been rated as highly tolerant to boron deficiency (Martens and Westermann, 1991). Rerkasem and Jamjod (2004) have suggested that advanced lines of wheat genotypes, found to be B-efficient on the basis of performance (yield) on low boron soils, should be used for development of B-efficient varieties. This has advantage over the use of the international wheat germplasm available with the International Center for Maize and Wheat Research (CIMMYT) in breeding for tolerance to B deficiency as the latter is reported to be largely B-inefficient (Rerkasem and Jamjod, 2004).

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