The nutrient elements which are required comparatively in small quantities are called as micro or minor nutrients or trace elements. Micro-nutrients are essentially as important as macro-nutrients to have better growth, yield and quality in plants. The requirement of micro-nutrients (boron, iron, copper, zinc, manganese, chloride and molybdenum) is only in traces, which is partly met from the soil or through chemical fertilizer or through other sources. The major causes for micro-nutrient deficiencies are intensified agricultural practices, unbalanced fertilizer application including NPK, depletion of nutrients and no replenishment.
Horticultural crops suffer widely by zinc deficiency followed by boron, manganese, copper, iron (mostly induced) and Mo deficiencies. Cl, Cu, Fe and Mn are involved in various processes related to photosynthesis and Zn, Cu, Fe, and Mn are associated with various enzyme systems; Mo is specific for nitrate reductase only. B is the only micro-nutrient not specifically associated with either photosynthesis or enzyme function, but it is associated with the carbohydrate chemistry and reproductive system of the plant. The significance of micro-nutrients in growth as well as physiological functions of horticultural crops fruits are briefed here nutrient wise.
Plants need iron to produce chlorophyll and to activate several enzymes including those involved in the oxidation /reduction processes of photosynthesis and respiration.
Iron concentrations of 50-100 ppm are often quoted as satisfactory leaf analysis values for most crops. But leaf analysis is not a reliable guide as there is poor mobility between tissues. Iron deficiency is rarely caused through insufficient iron in the soil but usually because it is rendered unavailable for the uptake by alkaline soil conditions or an excess of manganese or phosphorous. Iron deficiency is a problem of high pH calcareous soils and is often described as lime induced chlorosis. Custard apples are relatively sensitive suffering from iron deficiency while other crops such as bananas are usually not affected much. This sensitivity appears to be related to crop’s poor ability to absorb or utilize iron. The common deficiency symptoms include development of light green chlorosis of all the tissues between the veins. A distinctive pattern results from the network formed by the midrib and veins, which remain green for example, custard apples. If the chlorosis is severe and persistent, yellowing increases to the point of bleaching and burns can develop within this chlorotic area. Because iron does not move easily within the plant, older leaves can remain green while flushes of new growth are chlorotic. In pine apples, chlorosis is strongest towards the margins of young inner leaves. The fruits are small, reddish in color, hard and prone to cracking. The effects of iron deficiency in different fruit crops are discussed below.
Zinc deficiency is the most widespread and limiting growth and yield in fruit crops. It commonly affects banana, custard apple and mangoes. Problems often appear in spring when crops are growing quickly but have difficulty in absorbing nutrients from cold soil.
Zinc is important for the formation and activity of chlorophyll and in the functioning of several enzymes and the growth hormone, auxin. The severe stunting of leaves and shoots, which is so typical of zinc deficient crops is a consequence of low auxin levels in tissue. Young leaves are usually the most affected and are small, narrow, chlorotic and often rosette due to failure of the shoot to elongate. Bloom spikes are small, deformed and drooping. In young pine apple plants, zinc deficiency is indicated by the young heart leaves bunching together and then tilting horizontally. This condition is commonly called crook neck. Older plants may develop yellow spots and dashes near the margins of older leaves that eventually coalesce into brown blister like blemishes giving the leaf surface n uneven feel. The symptoms and corrective measures for zinc deficiency in different tropical fruit crops are mentioned below.
Boron is much required for cell division and development in the growth regions of the plant near the tips of shoots and roots. It also affects sugar transport and appears to be associated with some of the functions of calcium. Boron affects pollination and the development of viable seeds which in turn affect the normal development of fruit. A shortage of boron also causes cracking and distorted growth in fruit. Boron does not easily move around the plant and therefore the effects of deficiency appears first, and are usually most acute in young tissues, growing points, root tips, young leaves and developing fruit. The fruits of boron deficient papaya are deformed and bumpy due to the irregular fertilization and development of seeds within the fruit. Ripening is uneven and the developing fruit secrete pinkish white to brown latex. Premature shedding of male flowers and impaired pollen tube development can lead to poor fruit set. Growth is ceased at the growing point.
Generally copper deficiency appears to be minimal in tropical fruit crops. Copper is essential for photosynthesis, for the functioning of several enzymes, in seed development and for the production of lignin which gives physical strength to shoots and stems. The deficiency symptoms include restriction of terminal growth, die back of twigs, death of growing points and sometimes resetting, and multiple buds form at the end of twigs. Foliage can be chlorotic in bananas or darker than normal or dull and brownish in color. In pineapple, growth is severely stunted and leaves are narrow, U shaped in section, and curved downward at their tips. Tip necrosis occurs in some young leaves. Since foliar application of copper can result in burning of foliage, soil application of copper sulphate @ 30 kg/ha is recommended to correct the deficiency.
Manganese is necessary for chlorophyll formation for photosynthesis, respiration, nitrate assimilation and for the activity of several enzymes. The concentration of manganese in leaves can range widely from 10-15ppm when deficient and in thousands of ppm when it is toxic. Manganese is only moderately mobile in plant tissues so symptoms appear on younger leaves first, most often in those leaves just reaching their full size. Manganese availability is reduced in high pH calcareous soils but is often very high in the acid soils commonly chosen for tropical fruit production. Over liming of the soils particularly well drained, poor, coastal sandy soils can induce deficiency. Manganese deficiency causes a light green mottle between the main veins. A band of darker green is left bordering the main veins while the interveinal chlorotic areas become pale green or dull yellowish color. Soil application of manganese can be ineffective due to immobilization especially in heavier soils or soils which have been over limed. Two to three sprays of 0.1 % manganese sulphate can be recommended.
Molybdenum functions in enzyme nitrate reductase which is responsible for reduction of nitrate to nitrite during N assimilation in plants. Although molybdenum deficiency is observed in many soils and pasture legumes, vegetables and occasionally cereals, it is very rare in fruit crops. There are few reports that molybdenum deficiency called as yellow spot is observed in citrus. Soil application of molybdenised single superphosphate @ 250-500 kg/ha is the usual means of satisfying the need for 2-5 years.
Although general recommendations are available for most of the nutritional disorders in horticultural crops, application of micro-nutrients based on soil and plant nutrient status, at the critical stages of crop growth is absolutely necessary to achieve higher yield and quality.