Mitigation by Crop Management

1. Avoidance of Nutrient Stress 

The efficiency of fertilizers might be lower than fifty per cent either due to mismanagement or due to reactions of the fertilizers with soil constituents. only a fraction of the fertilizer applied to the soil is taken up by the crop, the rest is either remains in the soil or lost through leaching, physical wash off, fixation by the soil, or release to the atmosphere through chemical and microbiological processes (Hera, 1996). Fertilizer losses especially of nitrogen could happen during the period of plant growth and lead to low yields than expected. This

low uptake and utilization of nutrients out of the added fertilizers results in lower production and appearance of nutrient stress symptoms. The poor efficiency of added fertilizers is mainly due to imbalance fertilization or improper management practices. Critical information on the fertilization practices like: method of fertilizer placement, time and rate of application and type of fertilizers are essential for successful yields.

There are three general main types of nitrogen fertilizers where the nitrogen in the fertilizer is found in the chemical form of: ammonium, urea and nitrate. Nitrogen in nitrate form fertilizer are soluble and highly mobile in soil. They are prone to various kinds of losses: leaching beyond the root zone in the light textured soils, denitrification in water logged conditions or short time of over irrigation (Bar-Yosef and Kafkafi, 1972). Ammonium type fertilizers usually transform in the soil within a week or two to nitrate form and the further consideration is the same as for nitrate fertilizers.

 Phosphate fertilizers, which are highly reactive, are fixed in soil and become immobile. Potassic fertilizers are less mobile, since they are adsorbed on the clay complex. The entire quantity of phosphate and potassic fertilizers are, therefore, applied in one dose at the time of sowing in annual crops. Whereas, split application of nitrogenous fertilizers increases nitrate reductase activity, uptake of nitrogen (Abrol, 1990), use efficiency (Destain et al., 1997) and grain yield (Sabbe and Batchelor, 1990).

Application of fertilizers in narrow bands beneath and by the side of the crop rows i.e. band placement is preferable when the crop needs initial good start; when the soil fertility is low; when fertilizer materials react with soil constituents leading to fixation (phosphorus) and where volatilization losses are high.

Addition of organic matter to the soil at regular periods, helps to maintain the buffer capacity of the soil to supply the essential elements after its decomposition. Organic matter in soils can be regulated through different agronomic practices such as application of compost, farmyard manure, vermicompost, green manure etc’.

2. Management of Nutrient Stress

When the soil is deficient in a particular nutrient element and the crop grown on such soil show visible deficiency symptoms, there is urgent need to correct it to improve the crop production. Methods of correcting nutrient deficiencies vary according to agro-climatic regions, the socioeconomic situations of the region, the magnitude of disorder, and nutrient element involved. A generalized description of these methods is presented in Table 2. Use of nutrient-efficient cultivars in combination with fertilizers or amendments may be the best solution for correcting nutrient disorders in field crops, but this will vary according to situation. Fertilizer recommendations are usually based on the results of field trials in which crop response to various rates of fertilizer application is determined. Such response curves provide relationships between yield and the amount of fertilizer required for a particular crop grown in a specific agro-climatic region.

In a standing crop, if the nutrient deficiency is confirmed, the foliar application of selected nutrients by means of spray is a quick way to get rid of stress symptoms and avoiding yield loss. Foliar fertilization of macro and micronutrients is best practice whenever nutrient uptake through the roots is restricted due to adverse growing conditions (El-Fouly and El-Sayed, 1997), when topsoil is dry particularly in semiarid regions (Grundon, 1980), under saline soils (El-Fouly and Salama, 1999), and when root activity decreases during reproductive stage (Ikeda et al., 1991). In calcareous soils iron availability is generally very low and chlorosis is quite common, foliar spraying under these conditions is very beneficial (Horesh and Levy, 1981). For example, iron chlorosis was corrected by foliar application of 0.1% aqueous solution of Fe applied at 2 Kg Fe ha^-1, either as iron citrate or as iron sulfate + 400 g ha^-1 citric acid (0.02%), in four equal splits at 30, 45, 60 and 75 days after emergence in groundnut (Singh and Joshi, 1997). Manganese deficiency in such soils is also widespread and two or more sprays may be required within a growing season (Gettier et al., 1985). Not all crops respond equally to foliar nutrition and the nutrient elements differ in their rate of uptake through foliage and the degree of mobility within the plant once absorbed (Table 3).

 Mitigation by Plant Resistance

Nutrient concentration and uptake by different plant genotypes are the most important criteria for identifying the existing genetic specificity of plant nutrition (Saric, 1987). The tolerance in a given plant species or genotype to nutrient stress is closely related to its nutrient use efficiency. Genotypic differences in nutrient use efficiency are linked with root nutrient acquisition capacity, or with utilization by the plant, or both. Various plant species have developed morphological and/or physiological mechanisms to improve acquisition and use efficiency of mineral nutrients when grown on poor, infertile soils (Romeheld, 1998). By such mechanisms plants achieve nutrient deficiency stress tolerance. However, sometimes the ability to adapt to stress is limited, depending on the intensity and duration of stress. For example, some nutrient stress factors, when operating for a long period, can result in inhibited growth and root physiological processes responsible for nutrient uptake with the consequence of an impaired capability for adaptation and stress tolerance.

For a given genotype, nutrient use efficiency is reflected by the ability to produce a high yield in a soil that is limiting in one or more mineral nutrients for a standard genotype (Graham, 1984). The efficiency should be compared under stress and adequate nutrient supply to verify plant species or genotypic differences in nutrient utilization under sub-optimal and optimal conditions. The differential response of genotypes to nutrient stress is related to uptake, transport and utilization pattern of nutrients within plants (Fig.2). Isolation of intra-species differences in capacity for growth under specific nutritional conditions, particularly under nutritional stress is a critical aspect in nutritional plant genetics. A wide range of morphological, anatomical and physiological plant traits can be responsible for variations in response to nutrient stress within a plant species. Such traits can function either within or outside the plant to affect nutrient availability, absorption, translocation or utilization. Some physiological and morphological plant features controlling plant resistance to nutritional stress are given in Table 4.

The acquisition of nutrients by plant roots plays the most important role in tolerance to nutrient stress. Genotypes can differ widely in both the affinity of uptake and the threshold concentration, for example, the uptake of phosphorus in corn inbred lines (Schenk and Barber, 1979). The differences in their ability to grow in soils of low phosphorus status might be attributed to several factors including differential influx rates, root length/shoot weight ratios (Fohse et al., 1988) or root system morphology and root hair density (Randall, 1995). Among the more important factors is the role of root exudates in making available soil P ‘fixed’ in forms such as iron or aluminum phosphate. White lupin and pigeon pea are well adapted to acidic P-deficient soils (Hocking et al., 1997). White lupin forms develop proteoid roots (bottlebrush-like clusters of rootlets covered with a dense mat of root hairs) which are considered to be a response to low P availability (Marschner, 1986). Proteoid roots of white lupin secrete citric acid which solubilizes ‘fixed’ P (Gardner et al., 1983; Gerke et al.,1994), thus increasing P uptake by the plant. For pigeon pea, secretion of piscidic, malonic and oxalic acids appears to be the mechanism by which this species is able to release P from iron and aluminum phosphates (Otani et al., 1996). Screening of tolerant Medicago sativa and Lablab purpureus germplasm under phosphorus stress condition was carried out by Mugwira and Haque (1993a and b) based on their root and shoot growth.

Genetic differences in K uptake and utilization efficiency in field crop cultivars have also been observed (Glass et al., 1981). Growth response of 20 wheat genotypes were compared by Gill et al. (1997) under deficient (0.6 mM) and adequate (3.0 mM) K levels in solution cultures. Substantial and significant differences due to K-stress were obvious among genotypes for shoot dry weight, relative reduction in shoot dry weight, root dry weight, and root/shoot ratio. Rengel (1997) screened wheat genotypes tolerant to Zn and Mn stress. Wheat genotypes tolerant to Zn deficiency released greater amounts of phytosiderophore, 2-deoxymugineic acid, than the sensitive genotypes. In addition, Zn deficiency increased the numbers of fluorescent pseudomonas in rhizosphere of all wheat genotypes tested, but the effect was particularly obvious for genotypes tolerant to Zn deficiency. Under Mn stress, wheat genotypes tolerant of Mn deficiency had a greater ratio of Mn-reducers to Mn-oxidizers in the rhizosphere compared to sensitive genotypes. Mn-efficient durum wheat genotypes had greater Mn uptake from Mn deficient soil, produced higher grain yield, relative grain yield and above ground biomass and generally maintained higher seed Mn concentration (Khabaz-Saberi et al., 1997).

The Fe efficient groundnut cultivar ‘ICGV-86031’ released more hydrogen ions and reductants from its roots under iron deficiency stress treatment, maintained higher leaf Fe and had less chlorosis than the Fe-inefficient cultivar ‘TCGS-37’ (Reddy et al., 1997). There are also large differences in tolerance to Fe deficiency among field grown wheat genotypes (Hanson et al., 1996).

Genetic control of Cu efficiency has been located on the long arm of the rye chromosome 5 and has been applied to wheat breeding for CU efficiency (Podlesak et al., 1990).

Similar, increase in Cu efficiency was observed by Graham (1984) in spring form of rye (Secale cereale L.) and genetically manipulated wheat. Wheat genotypes differ in tolerance to Zn deficiency (Graham et al., 1992; Cakmak et al., 1996a), with durum wheat being generally less tolerant than bread wheat (Rengel and Graham, 1995; Cakmak et al., 1996). Tolerance of bread wheat genotypes to Zn and Fe deficiency was expressed in shoot dry weight and relative shoot growth as compared with to sensitive genotypes (Rengel and Romheld, 2000).

Cereal species, as well as genotypes of a given cereal greatly differ in adaptation to Zn-deficient conditions. The susceptibility of cereals to Zn deficiency was found to decline in the order: durum wheat > oat > bread wheat > barley > triticale > rye. Among the cereals, rye showed an exceptionally high Zn efficiency. Biomass production and grain yield of rye was not affected under severe Zn-deficient conditions (Cakmak et al., 1997b). Several mechanisms constitute the physiological bases of Zn efficiency. These are enhancement of root growth (Dong et al., 1995), root uptake and root-to-shoot translocation of Zn (Cakmak et al., 1996a; Rengel and Graham, 1996), release of Zn-mobilizing phytosiderophores from roots (Cakmak et al., 1996b; Erenoglu et al., 1996) and internal utilization of Zn (Cakmak et al., 1996b; Rengel, 1995). Use of disomic wheat-rye addition lines (Triticum aestivum L., cv. Holdfast-Secale cereale L., cv. King-II) increases Zn efficiency (Table 5). The addition of rye chromosomes, particularly 1R, 2R and 7R, into Holdfast reduced the severity of deficiency symptoms (Cakmak et al., 1997a). Transgenic plants have improved nutrient efficiency and resistant to nutrient stress. This was found for copper in wheat (Graham et al., 1987); boron and phosphorus in tobacco (Brown et al., 1999; Lopez-Bucio et al., 2000).

The molecular approach to breeding of mineral deficiency resistance and mineral efficiency is now receiving increasing attention. QTLs (quantitative trait loci) controlling Phosphorus efficiency were identified in rice and maize. Transgenic tobaccos overexpressing ferritin in the plastids or in the cytoplasm resulted in higher leaf concentration of iron, manganese and zinc (see #4791 in this site reference database). Tobacco transformation over expressing the antioxidant superoxide dismutase had greater manganese deficiency resistance (#4512).

Related Publications

• Principles of Plant Nutrition, by K. Mengel and E. A. Kirkby. (International Potash Institute, Berne, Switzerland).
• Diagnosis of Mineral Disorders in Plants. Vol. 1, Principles, by C. Bould, E. H. Hewitt, and P. Needham. (Chemical Publishing, New York, 1984).
• Diagnosis of Mineral Disorders in Plants. Vol. 2, Vegetables, by A. Scarfe and Mary Turner. (Chemical Publishing, New York, 1984).
• Plant Analysis Handbook, by J. Benton Jones Jr., B. Wolf, and H. A. Mills. (Micro-Macro Publishing, Athens, GA, 1991).
• Nutritional Disorders of Plants, Ed. by, Werner Bergmann. (Gustav Fischer Verlag, Gena, Germany, 1992).
• Nutrient Deficiencies and Toxicities in Crop Plants, Ed. by, William F. Bennett. (The American Phytopathological Society, St. Paul Minnesota, 1993).
• Mineral Nutrition of Higher Plants2^nd Ed. by, H. Marschner (Academic Press, London, 1997).
• Growth and nutrition of field crops2^nd Ed. by, N. K. Fageria; V. C. Baligar, and C. A. Jones. (Marcel Dekker, Inc. New York, 1997).

Reviews

• Raun W.R. Johnson G.V.  1999. Improving nitrogen use efficiency for cereal production  Agron. J. 91:357-363.
• Liptay A. Arevalo A.E.  2000. Plant mineral accumulation, use and transport during the life cycle of plants: A review Can. J. Plant Sci. 80:29-38.

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