16 1月 2018

Citrus Nutritional Management – Nutrition Function

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Growing citrus

1.1  Growing conditions

Being a tropical and subtropical crop, citrus can be grown in a belt between 40 °N and 40 °S, except at high elevations. Minimum temperature and its duration time are the limiting growth factors sensitivity depends on variety, rootstock, dormancy of the trees and the absolute minimum temperature and its duration.

Intensive citrus cultivation requires the use of fertilizers, close monitoring and control of pests, diseases and weeds, effective irrigation and control of tree size. The trees begin their productive life on the third year, and peak productivity takes place when the trees are 10-30 years old, average yields under these conditions are 30-60 t/ha.

Extensive citrus cultivation requires with the use of fertilizers, but only moderate monitoring and control of pests, diseases and weeds. They are generally rain-fed only. Their productive life starts on the fourth year, and peak productivity takes place when the trees are 8-15 years old, average yields under these conditions are 15-25 t/ha.

1.2 Soil type

Citrus can be grown on a wide variety of soils, from sand to loam and clay. Both acidic and alkaline soils are acceptable.

1.3 Varieties

The genus Citrus is an evergreen tree belonging to the family Rutaceae. It has about 150 genera and 1500 species, all native to the tropical and subtropical regions of Asia and the Malay Archipelago.

The principal citrus scions are:

Orange (C. sinensis Osbeck)
Mandarin (C. reticulata Blanco)
Lemon (C. limon [L.] Burm.)
Lime (C. aurantifolia [Christ.] Swing.
Grapefruit (C. paradisi Macf.)
Pomelo (C. grandis [L.] Osbeck)
The most commonly used rootstocks are:
Rangpur lime (C. limonia Osbeck)
Rough lemon (C. jambhiri Lush.)
Sour orange (C. aurantium L.)
Cleopatra mandarin (C.reshni Hort.)
Trifoliata (P. trifoliata [L.] Raf.)

1.4 Climate

Both arid and humid climates are acceptable. As citrus trees are sensitive to low temperatures, the limiting parameter for growing citrus is the minimum temperature prevailing in winter time.

1.5 Irrigation

Irrigation is one of the most important factors in producing a good yield of quality citrus. Irrigation scheduling, knowing how much water to put on and when, has a direct impact on tree health as well as fruit yield, size and quality. Without correct irrigation scheduling, orchard is more susceptible to nutrient deficiencies, physiological disorders, pests and disease.

Correct irrigation scheduling requires an understanding of:

How much water can be held in the crop root zone

How much water the crop uses each day

How much water the irrigation system applies.

Shallow root system

Citrus have a shallow root system. It is important to aim irrigation at the effective root zone, minimizing the amount of water leaching past. For citrus, the effective root zone is usually in the top 30 to 40 cm, depending on the soil type.

How much water can the root zone hold?

The amount of water that can be held in the root zone and thus available to the tree varies with the irrigation system, soil type, depth of the effective root zone, and proportion of stone or gravel in the soil.

Examples of water holding capacity

Both trees in this example (Fig. 1) are the same size (9 m2 canopy area) growing in a hedge row in a loam soil. The root-zone depth of both trees is 30 cm. One tree is irrigated with two drippers, the other with a fully overlapping micro sprinkler system. The tree irrigated with two drippers has only 34 liters of readily available water. The tree irrigated with the fully overlapping sprinkler system has a much larger volume of readily available water (189 liters). The more soil is wetted within the root zone, the greater the volume of readily available water.

Figure 1: Example of water holding capacity

Citrus tree water holding

Citrus tree water holding

Scheduling irrigation

To schedule irrigation, the amount of water available in the crop root zone with the tree’s daily water requirement should be compared. If the daily water requirement exceeds the amount of water that can be held in the root zone, there will be a need to irrigate more than once a day. If the soil can hold more than the daily water requirement, there is an option of irrigating when the available water is depleted (this may be every second or third day).

Rainfall during the irrigation season may reduce the irrigation requirement of trees. Not all rainfall is available to the trees; some is lost to run-off, percolation below the root zone, and interception by leaf litter or mulch.

Over-irrigation, especially surface irrigation, may wet the trunks of the trees and increase the incidence of root rot caused by Phytophthora. Lime-induced chlorosis can be aggravated by over-irrigation, and tends to be reduced by drip irrigation. Irrigation timing is considered crucial for reproductive development, fruit set and fruit enlargement. However, cropping in one season influences both root extension and top growth, often with a carry-over effect on yield in the successive year.

Recommendations

The following recommendations are for healthy and productive orchard. Citrus orchards that above average yield is expected, there is a need to increase the irrigated water quantity by 10% as of the bearing fruit stage (early summer).

Irrigation schedule should start according the soil moisture that can be determined by soil samples with auger or tensiometers (a moisture measurement tool). It directly measures the physical force that the root system must overcome in order to access water held in the soil (also known as matric potential).

Placement of tensiometers in the orchard, according to the irrigation method:

Drip irrigation – 15-20 cm of the second emitter from the tree’s trunk.

Micro-sprinkler – 0.5 m from the mini sprinkler.

Sprinkler – 1 m from the sprinkler.

Depth placement of tensiometers:

Upper – 20-30 cm deep (most of the active root zone) and the reading from this tensiometer with determine the irrigation schedule. Lower – 50-60 cm deep, verifies the irrigated depth.

Irrigation intervals will be determent by the age of the citrus orchard or tree size, irrigation method, soil type and the daily water requirement (Table 1).

Table 1: Irrigating intervals (days):

Irrigation of young orchard (up to 4 years from planting):

The water quantities are per single tree and related to irrigation method to the tree size.

Table 2: Drip irrigated young citrus orchard (up to 4 years from planting)

Irrigation of bearing trees

Due to the variation in water consumption between different growing areas, varieties, expected yield, soil type, drainage problems and so on, it is impossible to provide irrigation schedule that will fit all citrus orchards.

It is important to use additional parameters to determine irrigation schedule:

1.Soil sampling auger

2.Tensiometers

3.Measuring fruit size

How to calculate the water requirement

The daily required amount of water is calculated by multiplying the seasonal irrigation factor, varies from area to area and differs according to varieties, by transpiration factor generated from a Class ‘A’ Pan Evaporation data. To determine the required water quantity, the daily quantity should be multiplied by number of days from the last irrigation (irrigation cycle).

Irrigation factor – is the correlation between the transpiration from the tree and the evaporation from Class ‘A’ Pan (Evapotranspiration) of a citrus orchard during a particular period.

Group A: Varieties no fear of too large fruit size and plots with above average yield.

Group B: Varieties with low yield and fear of oversized fruits.

Group C: Late grapefruit varieties – yield less than 60 ton/ha, Topaz variety yields less than 40 ton/ha.

Group D: As of late spring, irrigation according to fruit size.

Table 3: Irrigation factors according to different citrus groups

Example of calculation:

0.55 (irrigation factor) X 7 mm (daily evaporation data from Class ‘A’ Pan) X 4 days irrigation interval = 15.4 mm, or X 10 = 154 m3 water/ha.

1.6 Planting density and expected yield

Tree spacing is affected by factors such as the species of citrus concerned, the cultivar, type of rootstock, environment, size of the orchard and the management practices the grower will be using. For example, if a grower uses machinery, he must leave enough space between the rows for the machines to pass when the trees are mature. Site quality in terms of soil characteristics and water availability should be considered. Expected lifetime of the orchard is also important and may be influenced by freeze potential, disease incidence, or non-agricultural development potential. Thus, decisions must be based on a number of situations.

Spacing of 6 – 7.5 m between rows and a middle width of 2 to 2.5 m provides adequate access for production and harvesting operations. Within this range, more vigorous trees, such as: grapefruit, lemons, tangelos, and other varieties with more spreading growth habits should be planted at wider spacings than oranges.

Spacing wider than 7.5 m take longer to fill their allocated space, thus reducing early yield potential.

Spacings between rows as close as 4.5 m can be managed with conventional production equipment with timely row middle hedging, however, fruit handling at these closer row spacings becomes a problem.

Spacing in the row of 3 to 4.5 m is considered suitable for new plantings. Tree vigor, site selection and external fruit quality requirements again are important considerations within this range. With the rapid tree growth occurring in many new plantings, trees at this spacing will grow together to form a continuous hedgerow relatively early in the life of the planting.

Spacings less than 3 m in the row have been tried experimentally and in a few commercial plantings. However, trees planted too closely may compete with each other for space at such an early age (before significant production) that the advantage of the higher density does not justify the additional cost of trees. Spacing trees at regular intervals in the row is preferable to grouping trees. For example, regular 3 m spacing is more desirable than grouping two trees 1.5 m apart and then skipping 4.5 m, even though the tree density per an area is the same.

Tree densities range from 286 trees per hectare for the 4.5 X 6.5 m spacing, to 540 trees per hectare for the 3 x 6 m arrangement. Small acreages of densities of up to 865 trees per hectare might be considered on a trial basis.

Tree vigor is of fundamental importance in determining the tree spacing, density, topping, and hedging in new citrus orchards. Citrus tree are flexible and adapt to arrangement of space allocations. Adaptability is limited, however, and maximum economic returns are generated only when tees perform well within their allocated space.

Plant nutrition

Main functions of plant nutrients

Table 4: Summary of main functions of plant nutrients

Nutrient demand/uptake/removal

Removal of mineral elements in the harvested fruit is one of the major considerations in formulating fertilizer recommendations. The table below shows the quantities of nutrient elements contained in one metric ton of fresh fruit. The large amounts of K reflect the high K content of citrus juice.

Table 5: Nutrients removed from the orchard by the fruit

Table 6: Micronutrients removal from soil, by fruit, of different citrus varieties

2.2.1 Leaf analysis as a guidance tool for the nutrition of citrus

Using leaf analysis as one of the guides in planning citrus fertilizer programs has yielded a considerable cultivation progress. The table below relates to spring flush leaves, 4-6 months old, from non-fruiting terminals. Leaves from fruiting terminals (used in S. Africa and some S. American countries) have lower N, P and K, and higher Ca and Mg contents than leaves of the same age from non-fruiting terminals, a fact which should be borne in mind when interpreting leaf analysis data. It is common to sample spring flush of healthy, undamaged leaves that are 4-6 months old on non-fruiting branches (Fig 2a). Selecting leaves that reflect the average size leaf from the spring flush. Typically, 75 to 100 leaves from a uniform 5-hectare block of citrus are sufficient for testing.

In some countries, it is recommended to sample spring grown leaves from bearing shoots, nearest to a fruit (Fig 2b).

Figure 2a: Sample leaves from the middle of non-fruiting shoots as shown above

Figure 2b: Sampling spring grown leaves nearest to fruit

Table 7: Leaf analysis standards for Citrus (Florida)

2.3  Nutrient deficiency symptoms in citrus

Nutrient deficiency symptoms appear on different plant parts, most frequently on leaves, fruits and roots. The mobility of nutrients is an important property from this point of view. Therefore, nutrients undergoing redistribution process e.g., when plant enters into reproductive- from vegetative phase. Accordingly, various nutrients are classified as very immobile (B and Ca), very mobile (N, P, K, and Mg), immobile (Fe, Cu, Zn, and Mo), and slightly mobile (S). Symptoms are noticed on fruits for immobile nutrients like B and Ca. Development of visible symptoms is accountable to metabolic disorders, which cause changes in micro-morphology of plants before these symptoms are identifiable.

The way in which the symptoms develop and manifest on younger or older leaves, or the fruits, gives a reliable indication about the cause of nutritional disorders. Both deficiency and excess of nutrients can lead to reduction in crop yield, coupled with inferior fruit quality. Mild visible leaf symptoms, for some of the essential element deficiencies, can be tolerated without a reduction in yield in some citrus varieties, but not in others.

Various forms of deficiency symptoms are usually summarized as:

  1. Stunted or reduced growth of entire plant with plant remaining either green or lacking in normal green luster or the younger leaves being light colored compared to older ones
  2. Older leaves showing purple color, which is more intense on the lower side
  3. Chlorosis of leaves either interveinal or the whole leaf itself, with symptoms either on the younger and/or older leaves or both
  4. Necrosis on the margins, or interveinal areas of leaf, or the whole leaf on young or older leaves
  5. Stunted growth of terminals in the form of rosetting, frenching, or smalling of leaves coupled with reduced terminal growth, or subsequent death of terminal portion of plants

2.3.1 Nitrogen (N)

Function: Nitrogen is one of the primary nutrients absorbed by citrus roots, preferably in form of nitrate (NO3 -) anion. It is a constituent of amino acids, amides, proteins, nucleic acids, nucleotides and coenzymes, hexosamines, etc. This nutrient is equally essential for good cell division, growth and respiration.

Deficiency Symptoms: Deficiency is expressed by light green to yellow foliage over the entire tree in the absence of any distinctive leaf patterns. With mild deficiency, foliage will be light green progressing to yellow as conditions intensify (Fig. 3a, 3b). New growth usually emerges pale green in color, but darkens as foliage expands and hardens. With yellow vein chlorosis, the midribs and lateral veins turn yellow, while the rest of the leaf remains with normal green color (Fig. 4). This chlorosis is frequently attributed to girdling of individual branches or the tree trunk. It may also occur with the onset of cooler weather in the fall and winter, due to reduced nitrogen uptake by the plant from the soil. Nitrogen deficiency is also associated with senescing foliage, which can develop a yellow-bronze appearance prior to leaf abscission (Fig. 5). Nitrogen deficiency will limit tree growth and fruit production, while high nitrogen applications produce excessive vegetative growth at the expense of fruit production, reducing fruit quality and threatening groundwater, particularly on vulnerable soil types.

Figure 3a: Nitrogen deficiency (Dark green leaf is normal; the other two leaves are deficient.)

Figure 3b: Nitrogen deficiency (Aging, senescing leaves.)

Figure 4: Pale new growth under N deficiency

Figure 5: Nitrogen deficient lemon foliage. Note that deficiency appears on the older leaves.

2.3.2 Phosphorus (P)

Function: Phosphorus is one of the three primary nutrients, and is absorbed by citrus roots in the form of orthophosphate (H2PO4-) or HPO42-. It is a component of sugar phosphates, nucleic acids, nucleotides, coenzymes, phospholipids, phytic acid, etc. It plays a key role in the reactions involving ATP. The element is necessary for many life processes such as photosynthesis, synthesis and breakdown of carbohydrates, and the transfer of energy within the plant. It helps plants store and use energy from photosynthesis to form seeds, develop roots, speed-up the maturity, and resist stresses.

Deficiency Symptoms: Phosphorus has the tendency to move from older to younger tissues; therefore, symptoms appear first on older leaves, which lose their deep green color. Leaves turn small and narrow with purplish or bronze lusterless discoloration. Some leaves may later develop necrotic areas, and young leaves show reduced growth rate. Fruits are rather coarse with thick rinds (Fig. 6) and have lower juice content, which is higher in acid. Although rarely observed, foliage may exhibit a bronze appearance. Phosphorus deficiency is unlikely to occur in orchards that have received regular P applications in the past.

Figure 6. Phosphorus deficiency symptoms on fruits

2.3.3 Potassium (K)

Function: Potassium (K) is required as a cofactor for 40 or more enzymes. It is required for many other physiological functions, such as formation of sugars and starch, synthesis of proteins, normal cell division and growth, neutralization of organic acids, regulating carbon dioxide supply by controlling stomatal opening and improving efficiency of sugar use, overcoming environmental stress such as frost by decreasing the osmotic potential of cell sap.

Due to higher ratio of unsaturated/saturated fatty acid, a role in imparting drought tolerance, regulation of internal water balance and turgidity, regulating Na influx and/or efflux at the plasmalema of root cells, chloride exclusion behavior through selectivity of fibrous roots for K over Na, and imparting salt tolerance to cells to enable holding the K in the vacuole against leakage, when Na incurs in external medium.

Table 8: Positive effect of K on yield, fruit size and quality in sweet oranges

Deficiency Symptoms: Early K-deficiency symptoms are commonly in the form of stunted growth, sparse somewhat bronzed foliage, and a lusterless appearance of leaves. Under acute K-deficiency, the leaves wrinkle and twist, and only weak new lateral shoots emerge, because of lack of mechanical strength. Other symptoms are: reduced fruit size (Fig. 7a, 7b) with very thin peels of smooth texture, premature shedding of fruits (Fig. 11), leaf scorching, appearance of resinous yellow spots, and shoots turning S-shaped.

Figure 7a: K affects the size of orange fruit

Figure 7b: Potassium deficiency symptoms on fruits (Left one is normal and smallest fruit is the most K-deficient).

Figure 8: K affecting the rind – have smoother, thinner rinds, susceptible to diseases Plugging – removal of the peel in the stem-end area of the fruit; increases incidence of postharvest decay.

Figure 9: Creasing – rind disorders, albedo breakdown, causing narrow sunken furrows on the rind surface as the peel disintegrates easily.

Figure 10: Splitting – vertical split at the styler or blossom-end and opening longitudinally towards the stem end.

Figure 11: Fruit drop – early fruit drop may occur when K is deficient.

Fading of chlorophyll, appearing as blotches, takes place in distal half of leaf. These blotches appear pale yellow at first, but later deepen to bronze, as they spread and coalesce, with leaf tips turning brown, showing abnormally variegated chlorosis in the form of amarelinho. Appearance of yellow to yellow-bronze chlorotic patterns on older leaves, along with corky veins, is also seen (Figs. 12a-c).A screw-type of curling towards the lower leaf surface, particularly on lemon is more common. Many a times, fruits attain more growth in the longitudinal direction leaving restricted fruit growth equatorially; eventually fruits turn elliptical in shape.

Figures 12a-c: Chlorosis of older leaves. K-deficient leaves turn golden yellow, and bend downward at the tip and margins

Potassium deficiency is likely to occur on calcareous soils due to elemental antagonism, and where large crops of fruit are produced with high nitrogen rates. A rarely observed bronzing of foliage may sometimes be observed, particularly on lemons. Potassium deficiency is associated with accumulation of cadaverine, acid invertase, lysine, histidine, arginine carboxylase, and reduced activity of carbamyl putrescine amine hydrolase, and pyruvate kinase. These are considered useful biochemical markers for establishing K-deficiency in citrus.

Table 9: Response of citrus yield and quality parameters to K fertilization as indicated by leaf K *

Thicker arrow = more pronounced effect

2.3.4 Calcium (Ca)

Function: Calcium is one of the secondary nutrients, absorbed by plant roots as Ca2+. Calcium is a constituent of the middle lamella of cell walls as Ca-pectate. It is required as a cofactor by some enzymes involved in the hydrolysis of ATP and phospholipids. It is an important element for root development and functioning; a constituent of cell walls; and is required for chromosome flexibility and cell division. Calcium deficiency appears to be a special case in which leaf chlorosis indeed reflects a broader interference involving alterations in nitrogen metabolism. The reduced activity of pyruvate kinase is considered an indicator of Ca-deficiency.

Deficiency Symptoms: Deficiency of Ca is mainly characterized by fading of chlorophyll along the leaf margins and between the main veins, especially during winter months. Small necrotic (dead) spots develop in the faded areas. Small thickened leaves cause loss of vigor and thinning of foliage under severe conditions. Twig die-back and multiple bud growth develop from new leaves with undersized and misshapen fruits having shriveled juice vesicles, commonly referred as bronzing or copper leaf.Chlorosis along leaf margins, leaves misshapen, often abscise rapidly.Roots underdeveloped, may decay .Tree shows dieback and stunted Breakdown’ Fruits show creasing and cracks form underneath the rind, with splitting and separation of the rind (Fig. 13).

Figure 13: Creasing resulted by calcium (Ca) deficiency

2.3.5 Magnesium (Mg)

Function: Magnesium is another secondary macronutrient absorbed as Mg+2. Magnesium is a central constituent of the chlorophyll molecule. A large number of enzymes involved in phosphate transfer require it nonspecifically. It is involved in photosynthesis, carbohydrate metabolism, synthesis of nucleic acids, related to movement of carbohydrates from leaves to upper parts, and stimulates P uptake and transport, in addition to being an activator of several enzymes.

Deficiency Symptoms: The deficiency of Mg is commonly referred to as bronzing or copper leaf symptoms. Symptoms of Mg-deficiency occur on mature leaves following the removal of Mg to satisfy fruit requirement. Disconnected yellow areas or irregular yellow blotches start near the base along the mid-rib of mature leaves that are close to fruits. These blotches gradually enlarge and coalesce later to form a large area of yellow tissue on each side of the mid-rib. This yellow area gradually gains in size, until only the tip and the base of the leaves are green, showing an inverted V-shaped area pointed on the mid-rib. With acute deficiency, leaves may become entirely yellow-bronze and eventually drop (Fig. 15 a-c). Seedy fruits are more severely affected than cultivars producing seedless fruits. Within the tree itself, heavily fruited limbs develop extreme Mg-deficiency symptoms, and may even become completely defoliated, while limbs with little or no fruits may not show deficiency symptoms. An increase in concentration of alkaline invertase is considered biochemical markers of K-deficiency in citrus.

With acute deficiency, leaves may become entirely yellow-bronze and eventually drop (Fig. 13). Magnesium deficiency occurring in calcareous soil may have to be corrected with foliar applications. Spraying Magnisal®, magnesium nitrate, is an effective treatment to correct Mg2+ deficiency.

Figure 14: Severe (left) and moderate (right) Mg deficiency

Figure 15 a-c: Magnesium (Mg) deficiency symptoms on citrus leaves

2.3.6 Iron (Fe)

Function: Iron, a micronutrient, is a constituent of cytochromes, non-haeme iron proteins, involved in photosynthesis, and N2 fixation and respiratory linked dehydrogenises. Iron is also involved in the reduction in nitrates and sulfates, the reduction in peroxidase and adolase. The increase in carbonic anhydrase activity is considered effective marker of Fe -deficiency.

Deficiency Symptoms: Lime induced Fe-deficiency is the most common form of deficiency. Interveinal white chlorosis due to Fe-deficiency appears first on young leaves. In mild cases, leaf veins are slightly darker green than interveinal areas with symptoms appearing first on new foliage (Fig. 16). In severe cases, interveinal areas become increasingly yellow with entire area eventually becoming ivory in color with emerging foliage, which is smaller. In some cases, leaves remain completely bleached, and margins and tips are scorched. In acute cases, the leaves are reduced in size, turn fragile and very thin, then shed early (Fig 17).Trees dieback severely from the periphery, especially at the top, and some trees have dead tops with the lower limbs showing almost normal foliage. Trees may become partially defoliated with eventual twig and canopy dieback. Iron deficiency is usually an indication of calcareous soil condition and is more likely to be expressed on high pH-sensitive rootstocks like Swingle citrumelo. An early expression of flooding damage to roots and of copper toxicity may be iron deficiency symptoms.

Figure 16: Iron (Fe ) deficiency symptoms on young citrus leaves

Figure 17: Development of iron (Fe) deficiency in lemon. Left- healthy leaf; right- severe deficiency

Figure 18: Iron (Fe) deficiency symptoms on lemon foliage. Note that leaves are young, but are generally full size.

2.3.7 Manganese (Mn)

Function: Manganese is one of the redox micronutrients, absorbed by the plant roots in the form of Mn+2. It is required for the activity of some dehydrogenases, decarboxylases, kinases, oxidases, peroxidases, and non-specifically by other divalent, cation-activated enzymes, and is required for photosynthetic evolution of O2, besides involvement in production of amino acid and proteins.

Manganese has equally strong role in photosynthesis, chlorophyll formation and nitrate reduction, and is indispensable for the synthesis of ascorbic acid, emerging from secondary effects of the fertilizer. A metalloenzyme peroxidase concentration is considered the marker of Mn deficiency. Accumulation of xylose and increased activity of peroxidase are considered useful biochemical markers of Mn deficiency in citrus in addition to reduced activity of phenylanine lyase, tyrosine ammonia lyase, and polyphenol oxidase.

Deficiency Symptoms: Deficiency appears as dark green bands along the midrib and main veins surrounded by light green interveinal areas (Fig. 19) giving a mottled appearance (Fig. 20). In more severe cases, the color of leaves becomes dull green or yellowish green along the mid-rib, and main lateral veins turn pale and dull for the interveinal areas. Chlorosis appears first on younger leaves, then spread gradually to older leaves (Fig. 21. Stems remain yellowish green, often hard and woody. Young leaves commonly show a fine network of green veins on a lighter green background, but the pattern is not distinct as in Zn and Fe deficiencies, because the leaves remain green. Both manganese and zinc deficiencies may occur on calcareous soil and may be more severe on trees with highly pH-sensitive rootstocks. Incipient manganese symptoms may sometimes disappear as the season progresses, so leaves should be observed several times before remedial action is taken. Soil and foliar applications may be effective in correction of manganese deficiency.

Figure 19: Manganese (Mn) deficiency

Figure 20: Mn deficient leaves show mottled appearance

Figure 21: Manganese deficiency symptoms in lemon, showing normal leaf (left) and typical interveinal chlorosis (right).

2.3.8 Zinc (Zn)

Function: Zinc deficiency is a common problem world over. It is an essential constituent of many enzymes such as alcohol dehydrogenase, glutamic dehydrogenase, lactic dehydrogenase, carbonic anhydrase, regulating equilibrium between carbon dioxide; alkaline phosphatase, carboxypeptidase, and other enzymes such as dehydropeptidase and glycylglycine dipeptidase for protein metabolism. It also regulates water relations, improves cell membrane integrity, and stabilizes sulflahydryl groups in membrane proteins involved in ion transport.

Deficiency Symptoms: Little leaf, rossetting, mottle leaf, frenching etc. The young leaves from vegetative shoots are more affected than reproductive shoots. The symptoms of Zn-deficiency are also characterized by irregular green bands along the mid-rib and main vein on a background of light yellow to almost white (Fig 22). Relative amounts of green and yellow tissue vary from a condition of mild Zn-deficiency showing in small yellow splotches between the larger lateral veins, to a condition in which only a basal portion of the mid-rib is green and the remainder of the leaves remain yellow to white. In severe Zn-deficiency, the leaves turn abnormally narrow and pointed with the tendency to stand upright coupled with extremely reduced size (Fig 23). As the deficiency progresses, the leaves are affected over the entire periphery of the tree, and the twigs become very thin and later dieback rapidly. A profuse development of water sprouts takes place. Some of the metallo-enzyme viz., carbonic anhydrase, nitrate reductase, and indole-acetic acid are suggested as biochemical markers of Zn-deficiency.

Figure 22: Zinc (Zn) deficiency

Figure 23: Leaves showing symptoms of severe Zn deficiency

2.3.9 Sulfur (S)

Function: Sulfur is essential for protein formation, as a constituent of the three amino-acids cystine, cysteine and methionine. Sulfur is required for the formation of chlorophyll and for the activity of ATP – sulfurylase. These essential functions permit the production of healthy and productive plants, which are capable of giving high yields as well as superior quality.

Deficiency Symptoms: When plants do not get enough S, they may show visual symptoms of S deficiency. The classical symptom is a yellowing of younger leaves while old leaves remain green. Plants deficient in S often mature late. Symptoms of S deficiency occur particularly in plants well supplied with nitrogen but are sometimes confused with that of nitrogen. In this instance, leaf analysis can be invaluable. Symptoms: Yellowing of new growth smaller leaves and abscise prematurely. Dieback of new shoots. Fruit undersized and misshapen (Fig. 24)

Figure 24: Sulfur (S) deficiency

2.3.10 Copper (Cu)

Function: Copper plays an active role in enzymes performing key functions like respiration and photosynthesis and Cu-proteins have been implicated in lignification, anaerobic metabolism, cellular defense mechanism, and hormonal metabolism. Known forms of Cu in the plants comprise cytochrome oxidase, diamine oxidase, ascorbate oxidate, phenolase, leccase, plastocyanin, protein having ribulose biphosphate carboxylase activity, ribulose biophosphate oxygenase activity, superoxide dismutase, plantacyanin, and quinol oxidase. Copper proteins exhibit electron transfer and oxidase activity. Ascorbic acid oxidase is widely distributed and responsible for catalysing oxidation of ascorbic acid by oxygen. Copper is also a constituent of cytochrome oxidase and haeme in equal proportions. It also acts as a terminal electron acceptor of the mitochondrial oxidative pathway.

Deficiency Symptoms: copper deficiency is commonly known as exanthema. Wilting of terminal shoots, frequently followed by death of leaves is usually seen in Cu-deficiency.

Mild copper deficiency is usually associated with large, dark green leaves on long soft angular shoots. Young shoots may develop into branches, which appear curved, or “S-shaped,” referred to as “ammoniation” usually resulting from excessive nitrogen fertilization (Fig. 25). Twigs can develop blister-like pockets of clear gum at nodes (Fig. 26). As twigs mature, reddish brown eruptions may occur in the outer portion of the wood. Severely affected twigs commonly die back from the tip with new growth appearing as multiple buds or “witches broom”. Necrotic-corky areas on the fruit surface may sometimes occur in extreme situations (Fig. 27) . Copper deficiency is more likely to occur in new plantings on previously uncropped soils, which are usually deficient or totally lacking in copper.

Figure 27Copper (Cu) deficiency – necrotic-corky areas on the fruit surface

2.3.11 Boron (B)

Fruit symptoms most indicative of boron deficiency include darkish-colored spots in the white albedo of fruit and sometimes in the central core (Fig.28). Fruit may be somewhat misshaped with a lumpy surface. Young leaves are dull brown/green, water soaked areas may develop. Leaves are thick, curled with pronounced veins on upper surface. Bark of twigs may split, fruits are small (Figs. 29 a and b).Unlike other micronutrient deficiencies, boron can affect fruit quality and therefore should be avoided. On the other hand, even slight excess of boron can cause toxicity (see page 30), so maintenance or correctional applications should be done very carefully, involving either soil or foliar applications, but not both.

Figure 28: Boron deficiency symptoms in red grapefruit

Figure 29 a-b: Boron deficiency symptoms on leaves

2.3.12 Molybdenum (Mo)

Deficiency of molybdenum in citrus is rare. It can occur under acidic soil conditions. The most characteristic field symptoms are large yellow spots on the leaves that appear first as less defined water-soaked areas in spring (Fig.30), later developing into distinct larger interveinal yellow spots.

Figure 30: Molybdenum deficiency symptoms

 

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