Application Of Zn And B On Citrus Biology Essay

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Chapter 2

Citrus is the pioneer fruit crop of the country and its productivity depends on many factors such as climate, site, varieties, root stock, fertilization, irrigation, soil management, pest and disease control etc. Among the all above factors, adequate supply of plant nutrients seems to be a very important factor not only for development of vegetative structures and flowers but also give regular harvest of good quality fruits. Along macronutrients, availability of micronutrients is also needed for higher yield as well as good quality fruits (Ioannis, 2004).

2.1: Zn and B essentiality

Along with macronutrients micronutrients are also essential for the survival of an animal and as well as of a plants. Among various micronutrients Zn and B are needed for healthy growth of humans, animals and plants among the other 16 essential elements existing in nature (Miller et al., 1991; Fageria et al., 2002; Warington, 1923). In Australia, the first recorded increase in plant growth to Zn fertilizer was for citrus (Pitman and Owen, 1936). Zn is involved in the activation of many growth hormones like auxins, NAA, gibberellic acid. It is required for the activity of various enzymes, such as dehydrogenases, aldolases, isomerases, transphosphorylases, RNA and DNA polymerases (Kessler and Monselise, 1959; Kessler, 1961). It is involved in carbohydrate and protein metabolism (Jyung et al., 1975). The B requirement for plant growth was first demonstrated in the early 1920s (Warington, 1923), and since then B has been established as an essential micronutrient for all vascular plants. B is one of seven essential micronutrients required for plant growth and development (Gupta, 1979). However, B is unique as a micronutrient in that the threshold between deficiency and toxicity is narrows (Yau and Ryan, 2008). The role of B in plant nutrition is little understood, which is surprising, since on a molar basis the requirement for B is, at least for dicotyledons, higher than that of any other micronutrient (Nable et al., 1997). B has restricted mobility in some plant species and is freely mobile in others (Brown and Shelp, 1997). It is also involved in photosynthesis (El-Shintinawy, 1999) as well as affects the antioxidant system of plant (Han et al, 2009). B is more active in growing parts of plant and required for the maintenance of structure and functions of membranes especially of plasma membrane, involved in cell elongation (pollen tube) and root growth (Goldbach et al., 2001; Brown et al., 2002).

2.2: Zn and B deficiency

2.2.1: Soil

The deficiency of Zn is an important nutrient problem in the world's soils. Total Zn concentration is in sufficient level in many agricultural areas, but available Zn concentration is in deficient level because of different soil and climatic conditions. Soil pH, lime content, organic matter amount, clay type and the amount of applied P fertilizer affect the available Zn concentration in soil. Zn deficiency rate was determined as a 30% in the world (Sillanpaa, 1982). Deficiency of mineral nutrients and particularly of Zn is observed in many citrus growing region of the country. The calcareous soils of Pakistan are low in plant available Zn and about 70% agriculture soils of the country are Zn deficit (Alloway, 2008). A survey was carried out to evaluate the nutritional status of 288 orchards of the country. Out of that 269 were deficit, 19 low and none in optimum range. It was concluded that soils and orchards of Pakistan are deficient in Zn and its application is required either through soils or through foliage (Ibrahim et al., 2007).

B deficiency is a widespread problem in many agricultural crops, including citrus (Sharrocks, 1997). Whilst of lesser prevalence than B-deficiency, B-toxicity is also an important disorder that can limit plant growth in soils of arid and semi-arid environments (Nable et al., 1997) where citrus trees are cultivated (Papadakis et al., 1997). High soil calcium and dry soils limit the availability of B and induce deficiencies in crops grown under these conditions (Gupta and MacLeod 1981). The B is also readily leached from soils and should be regularly replaced (Gupta, 1983). Low remobilization of B in plants is a problem compounding the management of this nutrient, and strategies should be aimed at applying a number of small amounts of B throughout the growing season (Whily et al., 1996).

2.2.2: Plants

Plant root cell membrane permeability increases in Zn deficient soils (Çakmak and Marschner, 1988), which may lead to accumulate B and other nutrient elements in plant roots (Singh et al., 1990). Therefore, excess B uptake by plants may cause B toxicity in Zn deficient soil conditions. The B deficiency causes a plethora of rapid biochemical, physiological, and anatomical aberration, which, along with the lack of relevant information on B chemistry, have made determining the primary function of B in plants one of the most difficult tasks in plant nutrition. While great progress has been made in the last few years, the primary role of B remains undefined (Bolanos et al., 2004). The B deficiency in Avocado reduces pollen viability, pollen tube growth and the potential for ovule fertilization (Robbertse et al., 1990). Whereas, the most pronounced Zn deficiency symptom in plants are stunted growth and little leaf rosette, particularly in fruit trees (Chandler et al., 1931), appears to be related to the physiological function of Zn in auxin production (Spiller and Terry, 1974). Giberellic acid metabolism seems impaired by Zn deficiency in plants (Skoog, 1940; Suge et al., 1986). Since B content in roots remains relatively low compared with that in leaves, even at very high level of B supply, visible symptoms of B-toxicity do not appear to develop in roots (Sharrocks, 1997). Contrary to B-toxicity, mostly the B-deficiency symptoms start in the actively growing parts of plants (Han et al., 2008; Dell and Huang, 1997). Root growth is more sensitive to B-deficiency than shoot growth (Dell and Huang, 1997). While earlier studies have shown that B-deficiency decreases plant photosynthetic capacity (Kastor et al., 1995; Zhao and Oosterhus, 2002).

2.3: Soil vs foliar application

Foliar feeding is the practice of applying liquid fertilizers to plant leaves. The leaves are green factories where the complex chemical processes of photosynthesis produce the compounds needed for plant growth. Therefore, foliar fertilization can be used to correct nutritional deficiencies in plants which are caused by improper supply of nutrients to roots particularly on alkaline and calcareous soils (Silberbush, 2002). Foliar spray of different micro nutrients has been reported to be equally or more effective as soil application (Modaihsh, 1997; Grewal et al., 1997). Postharvest foliar application of B in combination of urea or without urea was efficiently transported from the leaves into storage tissues than soil application in mature 'Delicious' Apple trees (Sanchez and Raghetti, 2005). The uptake of foliar-applied Zn was more rapid than that of soil applied Zn in Mango (Bahadur et al., 1998). Foliar application seems to be an effective method because amount utilized through foliar application by plant is about 85% and early spring foliar sprays of Zn can increase its concentration in the targeted organs (Boaretto et al., 2002). It had been reported that foliar application of micronutrients increases crop production of many fruits. Further, it was concluded that foliar application of micronutrients may be 6 to 20 times more efficient than soil application, depending on soil type (Leiw, 1988). Foliar application of B as post bloom spray was more effective in increasing the B levels in Navel oranges leaves than pre-bloom application (Maurer and Taylor, 1999).

2.4: Application of Zn and B on citrus

2.4.1: Leaf mineral contents

Leaf analysis is generally used to determine the nutrient status of the tree. It is valuable aid in obtaining a better understanding of the fertilizer requirements of citrus tree; however, the data obtained as a result of leaf analysis must be used in conjunction with other facts known about the tree. The use of leaf analysis for evaluating the nutritional status of fruits assumes that yield, fruit quality and plant growth are related to concentration of the essential elements in the leaves at the time of sampling nutrition has a great influence (Bould, 1966; Beyers, 1962).

2.4.2: Macronutrients

N, P and K concentrations of plants increased with higher levels of B and Zn application (Adiloglu and Adiloglu, 2006). B toxicity decreased with the high level N applications (Alpaslan et al., 1996). The effect of B on fruit yield can be attributed to both significant increases in N, P, and K uptake by the trees and to significant increases in fruit set and fruit sizes (Cifu et al., 2008). N, P and K concentrations in shoot of lemon seedlings is affected by B and Zn application all of them increased with optimum application of Zn and B (Rajaie et al., 2009). Leaf mineral contents are generally correlated with vegetative and reproductive growth, fruit yield and also the quality of fruit in citrus. B applied as boric acid in combination with potassium citrate on 'Washington Navel' orange increased N and K content in leaves (Abd-Allah, 2006). Foliar application of Zn alone or with combination of K has been reported to increase the concentration of Zn and K along with N and P concentrations in the leaves of Washington Navel oranges (Omaima and El-Metwally, 2007).

2.4.3: Micronutrients

Foliar application of Zn, alone and in combination with Fe and Mn (as ZnSO4, MnSO4, and FeSO4 respectively) on 'Kinnow' mandarin resulted in increased concentration of respective micronutrient without affecting the level of N, P, and K in the leaves. However, the increase in Zn content was more when spraying of Zn was conducted alone rather than in combination with Fe and Mn (Monga and Josan, 2000). In another study, foliar application of Zn along with Mn and B has been found to increase the leaf Zn content in sweet orange up to the optimum range (Rehman and Huq, 2006). An antagonistic relationship between Zn and other cationic micronutrients (Fe, Mn and Cu) appears as a result of competition at the absorption sites of plant root (Loneragan and Webb, 1993). Leaf concentrations of P, K, Mg, Fe, Mn and Cu were within the optimal range for citrus with the exception of Calcium Ca which was low. Although B and particularly Zn treatments modified the concentration of some of these elements in leaves and roots, these changes were too small to explain the observed growth responses in sour oranges (Swietlik, 1995). High B concentration was associated with increased Mn and Zn in lemon leaves ( Leon et al., 1983), while in case of sweet orange leaves the foliar sprays of Mn, Zn and B with urea not affected the B concentration (Tariq et al., 2007). Increase in Fe, Mn and Cu concentration was found in lemon seedlings when Zn and B was applied (Rajaie et al., 2009). Fe concentration of maize plant increased with B application while decreased with Zn application. But these increases and decreases were not found statistically significant. Cu concentration of plant was also affected by B and Zn applications, increased with increasing B and Zn applications, similarly Mn also increases with the B and Zn application (Adiloglu and Adiloglu, 2006).

2.5: Vegetative growth

The Zn being an essential nutrient is found to stimulate the growth of citrus and avocado trees, whereas, B is more important in apple trees but show vital importance in other monocotyledon crops and citrus in some extent. B is required for normal growth and development of all higher plants. The B also plays an important role in starch metabolism in plants. It is well known that Zn acts a co-factor of many enzymes and affects many biological processes such as photosynthesis reactions, nucleic acids metabolism, protein and carbohydrate biosynthesis (Marschner, 1995). Both B-deficiency and excess resulted in a similar decrease in the activities of rubisco & ultimately cause reduction in photosynthates (Han et al., 2008b). Similarly, Zn is essential for chlorophyll formation and function of normal photosynthesis and is also associated with water relations in plants and improves water uptake (Zekri and Obreza, 2003). The application of B had been reported to increase the concentration of B in all vegetative parts of 'Clementine' mandarin plants (Papadakis et al., 2004).

2.5.1: Leaves

Numbers of leaves per shoot were significantly increased by spraying Zn. Almost 41.0% increase in no. of leaves in guava is observed by spraying 0.4 % Zn, similar there was significant effect of Zn spray on leaf size almost 100.0% with 0.4% Zn (Arora and Singh, 1967). The CO2 assimilation and Chlorophyll content decreased to a greater degree in B-deficient leaves than in B-excess ones in Citrus sinensis (L.) Osbeck cv. Xuegan (Han et al., 2008b). The effects of B deficiency on vegetative growth of citrus are well known, and occur when leaf B concentrations are less than 15 ppm. Some of these symptoms include translucent or water-soaked flecks on leaves and deformation of those leaves, yellowing and enlargement of the midrib of older leaves, death and abortion of new shoots, dieback of twigs, and gum formation in the internodes of stem, branches and trunk (Reuther et al., 1968). Kiwifruit plants produced the longest shoots with more leaves and the greatest fresh weight of shoots and leaves when treated with 0.025 m M B combined with the two levels of salinity (Sotiropoulos et al., 2004). Foliar application of B resulted in increased leaf B and in decreased root B in radish while B was found in plant tissue of tomato in declining order according to: mature leaves, young leaves, roots and stems (Ben-Gal, 2007).

2.5.2: Spur length

Elongation of terminals shoots was observed by spraying 0.4% Zn, maximum increase observed is 50.2% as compared to control in guava (Arora and Singh, 1967). The combined application of 1% urea, 50 ppm GA3, and 0.2% ZnSO4 gave the highest number of branches per plant and root length in Citrus volkameriana as well as the tallest plants and the highest number of leaves per plant, stem diameter, root diameter, number of secondary roots per plant, dry matter content of stems and roots and total dry matter contents in Citrus reticulata (Khasi mandarin) and Citrus volkameriana seedlings (Singh and Sheo, 2001). The B application had been found to increase the number of current shoots per tree and decreased their length in 'Dabrowicka' prune trees (Wojcik et al., 1999). Lengths of spring shoots and summer shoots were lowest in the control, and increased with increasing B rates up to 40 g tree−1 of borax when applied as ground application and with the foliar application rates up to 2.0, 4.0, and 8.0 g L−1 of borax in Red Bayberry (Cifu et al., 2007)

2.5.3: Trunk width and tree size

There was a significant B and Zn interaction on plant growth and tissue nutrient concentration which were rate dependent. In general, the effect was antagonistic in nature on nutrient concentration and synergistic on plant growth. Foliar application of Mg, Cu, Zn, and B in the form of 0.2% MgSO4, 0.4% CuSO4, 0.5% ZnSO4 and 0.1% H3BO3 resulted in increase in height (75.25 cm) and spread (70 cm) in Mandarin orange cv. Darjeeling plants (Haque et al., 2000). On the contrary, there was no significant effect of N, Zn and their combinations on trunk girth, tree height and tree spread in sweet oranges (Sahota and Arora, 1981). Both low and excess B reduced root volume and plant height in the two cultivars (Newhall and Skagg's Bonanza) of 'Navel' orange (Sheng et al., 2009). Hosseini et al., (2007) reported that high levels of B decreased plant height and dry matter production of corn (Zea mays L.).

2.6: Reproductive growth

2.6.1: Fruit set and fruit drop

The supply of B needed for reproductive growth in many crops is more than that needed for vegetative growth (Mengel and Kirkby, 1982; Marschner, 1986; Hanson, 1991), and the same may be true in citrus. Increase in leaf Zn, Mn and B concentrations induced more flowering and minimized fruit let drop in 'Valencia' orange trees (Gracia et al., 1984). The reduction in June drop and preharvest fruit drop percentages is proportional to increase in the levels of N, P, K and concentrations of Zn, Fe, Mn. The beneficial effect of the balanced amount of macro and micro nutrients on reducing fruit drop might be attributed to the effect of K as well as the effect of Zn in increasing the biosynthesis of the natural hormone IAA that is responsible for reducing fruit dropping (Nijjar, 1985). It had been found that application of Zn (0.5%) significantly increased final yield through increasing fruit set, efficiency of fruiting and decreasing June drop and preharvest drop as compared to control in 'Balady' Mandarin trees (EL-Baz, 2003). Foliar application of B increases yield and improve fruit quality by increasing the N, P, and K uptake by the trees and preventing excessive fruit drop (Cifu et al., 2007). Improvement of fruit quality to the greater elongation of the pollen tube in Mg, Zn and B treatment favoring fertilization and improving fruit setting in 'Jincheng' orange (Qin, 1996). Urea, H3BO3 and ZnSO4 with 0.5% concentration alone and in combination, were applied to olive tree as foliar spray. It was found that urea spray of 0.5% concentration significantly increased initial fruit set and spraying with B and Zn caused significant increase of final fruit set (Talaie and Taheri, 2001). Two foliar sprays of B as Solubar (20.8%) one in early spring and other after fruit harvest improved fruit set and final fruit number and kernel production in Almond than a single B spray in early spring and control (Rufut and Arbones, 2006). An increase in fruit set has been reported by the application of B on 'Red blush' Grapefruit (Maurer and Davies, 1993). Boric acid with calcium chelate (Ca-chelate) gave the highest number of fruit set per branch in first season and with di-potassium hydrogen phosphate (KH2PO4) in the second season when boric acid was applied alone or in combination with KH2PO4 or ca-chelate on 'Washington Navel' orange ( Abd-Allah, 2006).

2.6.2: Fruit yield

Foliar application of Zn has been reported to increase fruit yield by increase in fruit weight and size. The increase in fruit weight and diameter with Zn sprays might be due to important component for fruit growth and development which had been influenced via tryptophan by Zn spray in sweet orange (Sahota and Arora, 1981). The Zn foliar spray applied before anthesis may be most beneficial in terms of fruit yield in citrus and grapes (Swietlik, 2002). It is known that under conditions of B deficiency, pollen germination and growth of pollen tube are reduced which consequently lowers crop yields and deteriorates fruit quality (Guttridge and Turnbull, 1975). B application increases fruit set and yield in several fruit and nut trees, including almond, Italian prune, olive, and sour cherry (Slavko et al., 2001; Hanson, 1991; Chaplin et al., 1997: Shrestha et al., 1987). Foliar application of Zn, Mn and B alone or in various combinations to sweet orange trees was found to increase significantly the Zn level and fruit yield as compared to unsprayed trees (Perveen and Rehman, 2002). The application of Zn shortened the maturity period of guava, and increased the yield to great extent; the radical improvement in yield of grape, pomelo, lemon, mandarin and in fig had been recorded (Arora and Singh, 1967). Eleven-year-old plants of 'Kinnow' on 'Troyer citrange' (TC) and 'Karna Khatta' (KK) rootstocks planted at 1.8 m x 1.8 m and 2.4 m x 2.4 m, respectively were sprayed with micronutrients (Zn, Fe and B). The fruit yield of 'Kinnow' mandarin was highest under Zn + B treatment. Irrespective of micronutrient applications, fruit yield were higher on 'Kinnow' budded on KK than on TC rootstock. Plants on KK rootstock with Zn + B treatment recorded the highest yield, while those on TC had general slightly lower yield in both years (Mishra et al., 2003). Application of H3BO3 with Ca-chelate gave maximum results in number of fruits per branch and fruit weight per tree on 'Washington Navel' orange (Abd-Allah, 2006), while in another experiment on sweet orange B treatment alone did not significantly increased the yield, but in combination with Zn and Mn the fruit yield considerably increased (Tariq et al., 2007).

2.7: Fruit quality

2.7.1: Physical quality

Unlike other micronutrient deficiencies, B deficiency can impact fruit quality and should therefore not be allowed to occur (Stephen and Tucker, 2000). B stress is responsible for considerable loss of productivity and poor fruit quality (Han et al., 2008a).

2.7.1.1: Fruit weight

The weight and peel thickness of the fruit has been found to increase with the application of Zn alone or in with combination with GA3 in 'Washington Navel' oranges trees (Eman et al., 2007). Mean fruit weight was not affected by the application of B sprays in apple (Wojcik, 2002). Applying the Zn and K significantly increased fruit weight by (3.27 and 5.58%) as compared with control. The increase in fruit weight per tree could be rendered by spraying 'Washington Navel' orange trees with Zn + K which may enhance the activity of photosynthetic and protein synthesis in leaves, which in turn encourages plant growth. Also, the favorable effect of Zn + K on these characters may be attributed to their effect on growth parameters, which in turn improve fruit weight (Omaima and El-Metwally, 2007).

2.7.1.2: Fruit size

Fruit length was significantly increased by application of foliar spray of Zn, K and Zn + K treatments as compared to control in 'Washington Navel' oranges (Omaima and El-Metwally, 2007). Fruit diameter in Sweet orange was increased with the foliar application of Zn, B and Mn (Tariq et al., 2007). In two field studies with 'Hass' growing in boron-deficient soils in Australia, Smith et al., (1995) reported an 11-15% increase in fruit size (219-246 g and 222-261 g respectively), when trees were treated with soil applications of borax. There was no effect from boron applications on fruit numbers or yield.

2.7.1.3: Seed

Foliar application of Mg + Cu + Zn + B in the form of 0.2% MgSO4, 0.4% CuSO4, 0.5% ZnSO4 and 0.1% H3BO3 had been reported to increase seeds per fruit in 'Mandarin' cv. Darjeeling plants (Haque et al., 2000). Foliar spray of B (0.05% of Na2B4O5.4H2O) accelerated the sugar and dry matter accumulation and seed maturation when four micro nutrients (B, Mn, Zn and Mo) applied as foliar spray on Elymus sibirus (Xiao et al., 2005).

2.7.1.4: Peel

Average fruit weight, fruit size and peel thickness were improved by application of K, P and B in 'Hamlin' orange (Abd El-Migeed et al., 2000). B appears to accumulate in citrus peel to a much greater extent than in the leaves, ranging in lemon from 1600 to 3500 ug g-1 (Sinclair, 1984). Trees fertilized with 1250 g N + 700 g P2O5 + 800 g K2O / tree plus Zn, Fe and Mn at 0.1% had a slight effect on peel thickness in 'Valencia' orange trees (Abu-Zinada, 1994). The best results with regard to peel thickness was obtained in 'Valencia' oranges grown on 'Troyer Citrange' and fertilized with 1250g N + 700g P2O5+ 800g K2O per tree accompanied with spraying the trees with 0.1 % solution of Zn, Fe and Mn in chelated form (Elham et al., 2006).

2.7.1.5: Juice

The maximum juice was found in fruits from the trees sprayed with 1.0 and 0.8% urea and zinc sulfate respectively in 'Kinnow' Mandarin (Malik et al., 2000), while in another experiment the application of B alone significantly increased the % age juice compared with control treatments on sweet orange (Tariq et al., 2007). H3BO3 + K2HPO4 treatment recorded the highest juice weight followed by Ca-chelate + H3BO3 when H3BO3 applied in combination with Ca-chelate and K2HPO4 on 'Washington Navel' orange (Abd-Allah, 2006). The Zn along with B when applied in combination gave highest juice % age in 'Kinnow' budded on 'Karna Khatta' rootstock (Mishra et al., 2003). Micronutrients (Mn, Zn and Mg) each at 0.5% as foliar spray were applied on fifteen-year-old trees 'Khasi' mandarin. Foliar sprays significantly increased the values of juice % age compared with the untreated control (Babu and Yadav, 2005). According to Dong et al., (2009) high dietary fibre diets are associated with the prevention, reduction and treatment of some diseases, such as diverticular and coronary heart diseases.  And the pre-harvest foliar application of Ca, B is useful for improving the tissue structure of segment membrane, reducing transcript levels and activities of polygalacturonase, pectinesterase and β-galactosidase and regulating the content of dietary fiber on 'Cara Cara navel' orange.

2.7.2: Chemical quality

2.7.2.1: SSC (Brixo)

Highest percent of total acidity and TSS% were recorded with foliar spray of Zn + K, K and Zn, consecutively in average of the two seasons in 'Washington Navel' oranges (Omaima and El-Metwally, 2007). By application of K, P and B in 'Hamlin' orange the total soluble solids were improved (Abd El-Migeed et al., 2000). In citrus, B deficiency leads to low sugar content, granulation and excessive fruit abortion (Reuther et al., 1968). The B fertilization, regardless of application mode, increased soluble solids concentration in 'Bluecrop' highbush Blueberry compared to those of the control (Wojcik, 2005). The Zn, alone and in combination with Fe and Mn (as ZnSO4, MgSO4, and FeSO4, respectively) as foliar spray on 'Kinnow' mandarin was tested and found that Zn significantly increased TSS as compared to trees where Zn was not included in the spray. The highest TSS was obtained from the tree sprayed with Zn (0.3%) alone (Monga and Josan, 2000). Foliar spray of urea and ZnSO4 on 'Kinnow' gave maximum TSS when trees sprayed with 1.0 and 0.8% urea and ZnSO4, respectively (Malik et al., 2000).

2.7.2.2: Titrable acidity

The TA was significantly affected with foliar application of Zn, K and Zn + K treatments. The highest value of TA was recorded with foliar spray of Zn + K, K and Zn, consecutively in average of the two seasons. In contrast, the lowest values of TA were recorded when 'Washington Navel' orange trees were not fertilized with Zn or K (Omaima and El-Metwally, 2007). Foliar spray of micronutrients (Mn, Zn and Mg each at 0.5%) on Fifteen-year-old 'Khasi' mandarin (Citrus reticulata Blanco.) reduces the TA which was higher in the control treatments (Babu and Yadav, 2005). Foliar application of Zn, alone and in combination with Fe and Mn (as ZnSO4, FeSO4, and MnSO4, respectively) on 'Kinnow' mandarin resulted in decrease acidity in all treatments compared to the control (Monga and Josan, 2000). Acidity is reduced in guava when sprayed with 0.4% of Zn (Arora and Singh, 1967).

2.7.2.3: SSC: TA ratio

A significant increase in TSS/Acid ratio in 'Washington Navel' orange has been reported by application of Ca-chelate + Boric acid treatment (Abd-Allah, 2006). Foliar application of B reported to increased fruit soluble solids contents and found to decreased firmness and titrable acidity of fruit at harvest in 'Dabrowicka' prune trees (Wojcik, 1999). Young trees of 'Washington Navel' orange, 'Valencia' orange and 'Balady' mandarin, budded on sour orange rootstock were sprayed with ZnSO4 (0.0, 0.20, 0.40, 0.60, 0.80 and 1.0%). Best results with regard to TSS/ TA ratio was obtained with the application of 0.4% Zn spray as compared to control (Dawood et al., 2000).

2.7.2.4: pH

Juice pH is also very important in fruit quality, while foliar application of Zn do not affect the pH of fruit juice in sweet oranges (Tariq et al., 2007).

2.7.2.5: Ascorbic acid

Foliar application of Zn has been reported to increase the ascorbic acid contents in the juice of different citrus varieties (Dawood et al., 2001). A significant increase in vitamin C content of sweet orange was observed when the Zn alone, Zn + Mn or Zn + B applied through foliar application (Tariq et al., 2007). The Zn significantly increased ascorbic acid in fruit juice than the control. High ascorbic acid was obtained by spraying Zn and GA3 sprays (Eman et al., 2007). This means that the presence of GA3 irrespective to its concentration with Zn sprays has a positive effect on ascorbic acid in fruit juice. These results are in line with the findings obtained by El-Menshawi et al. (1997) who concluded that Zn sprays increased ascorbic acid in 'Balady' mandarin trees.

2.7.2.5: Sugars

Sugars are the major constituents of the TSS which determine the quality (Ahmad, 1988) and maturity of the fruits (Raza, 1997). Sugars present in citrus fruit are glucose, fructose and sucrose. There quality and proportion changes with the passage of time. Two types of sugars are present in citrus fruits viz., reducing and non-reducing, making the total sugar contents of the fruit. Non-reducing sugars contains sucrose, while reducing sugars are glucose and fructose (Arthey, 1975).

Application of Mg, Cu, Zn, Fe and B in combination did not show any effect on total and reducing sugars in 'Kinnow' mandarin (Ram and Bose, 2000).Trials were carried out with 13 years old trees of Mango cv. Langra. The B or Zn, each at 0.2-0.08% was applied during full bloom. At higher concentration (0.6-0.08%) B + Zn significantly increased fruit concentration of total sugars (Rath et al., 1980). Foliar application of 100 ppm NAA in combination with 0.3% H3BO3 was found to have an increase of 1.8% in TSS content of strawberry than other treatments (Yan et al., 1998).

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