Journal of Plant Nutrition Zinc Nutrition in 'Nagpur ...

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Jun 6, 2009 - Srivastava, A. K., R. R. Kohli, H. C. Dass, A. D. Huchche, and L. Ram. 1999. Evaluation of the nutritional status of Nagpur mandarin (Citrus ...
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Zinc Nutrition in ‘Nagpur’ Mandarin on Haplustert a

A. K. Srivastava & Shyam Singh

a

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National Research Centre for Citrus , Nagpur, Maharashtra, India Published online: 06 Jun 2009.

To cite this article: A. K. Srivastava & Shyam Singh (2009) Zinc Nutrition in ‘Nagpur’ Mandarin on Haplustert, Journal of Plant Nutrition, 32:7, 1065-1081, DOI: 10.1080/01904160902943114 To link to this article: http://dx.doi.org/10.1080/01904160902943114

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Journal of Plant Nutrition, 32: 1065–1081, 2009 Copyright © Taylor & Francis Group, LLC ISSN: 0190-4167 print / 1532-4087 online DOI: 10.1080/01904160902943114

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Zinc Nutrition in ‘Nagpur’ Mandarin on Haplustert A. K. Srivastava and Shyam Singh National Research Centre for Citrus, Nagpur, Maharashtra, India

ABSTRACT Zinc (Zn) deficiency is the most prevalent nutritional disorder in citrus orchards world over. The management strategy of Zn deficiency today is still governed by the efficacy of two conventionally used methods of Zn supply to plants via soil or foliar fertilization. A field experiment with 12-yr-old ‘Nagpur’ mandarin (Citrus reticulata Blanco) orchard was, therefore, carried out during 2004–07 comparing soil application versus foliar application of Zn, each at three levels viz., 100, 200, and 300 g tree−1 with constant doses of N (600 g tree−1 ), P (200 g tree−1 ), K (300 g tree−1 ), and Fe(60 g tree−1 ) on Haplustert soil type with reference to response on flowering intensity, fruit set, tree volume, fruit yield, changes in soil fertility/leaf nutrient status, fruit quality, and transformation of native soil Zn fractions. Soil application of Zn at all the three levels, produced significantly higher increase in tree volume over foliar application on equivalent rates viz., T 1 (2.53 m3 ) vs. T 4 (2.06 m3 ) and T 2 (4.30 m3 ) vs. T 5 (2.23 m3 ). The yield-determining parameters like flowering and fruit set intensity (no. m−1 shoot length) were, respectively, much higher with soil applied (135.74 and 21.90) than foliar applied Zn (31.20 and 11.6). These observations set the favorable conditions required for yield response, e.g., all the three treatments involving soil application of Zn, T 1 (32.1 kg tree−1 ), T 2 (52.6 kg tree−1 ), and T 3 (51.8 kg tree−1 ) were correspondingly superior over T 4 (22.5 kg tree−1 ), T 5 (34.3 kg tree−1 ), and T 6 (42.1 kg tree−1 ) as foliar application treatments. All the three major fruit quality parameters (juice, acidity, and TSS) were likewise more influenced by soil application than foliar application of Zn. Improvements in soil Zn fractions (mg kg−1 ) viz., exchangeable Zn (0.25–0.60), complex-Zn (2.71 to 4.86), organically bound Zn (0.86 to 2.0), and Zn-bound to carbonates and acid soluble minerals (2.56–4.96) were observed in response to Zn fertilization with treatments T 1 –T 3. On the other hand, foliar applied Zn treatments (T 4 –T 6 ) produced no such changes in any of the soil Zn fractions. Keywords: ‘Nagpur’ mandarin, Haplustert, Zn fertilization, transformation

Received 10 October 2007; accepted 11 May 2008. Address for correspondence to A. K. Srivastava, National Research Centre for Citrus, P. O. Box 464, Shankarnagar Post Office, Amravati Road, Nagpur – 440 010, Maharashtra, India. E-mail: aksrivas [email protected] 1065

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INTRODUCTION Citrus is a highly nutrient responsive perennial crop. Sub-optimum production due to nutrient deficiencies is well recognized in citrus orchards (Srivastava and Singh, 2006). Amongst essential nutrients zinc (Zn), after nitrogen (N), is undoubtedly the most widely reported deficient nutrient in citrus orchards world over (Srivastava and Singh, 2005), limiting the citrus yield and quality, on both acidic (Xie et al., 1993; Swietlik, 1996) as well as alkaline soils (Neilsen et al., 1987; Bell et al., 1995) to varying proportions. The process of diagnosis and management of Zn-nutrition is further complicated by the simultaneous occurrence of other nutrient deficiencies like iron (Fe) and manganese (Mn) because of their similar mobility within the plant (Embleton et al., 1988). This is the reason that research carried out in the past suffered from putting forward a plausible explanation about the clear-cut superiority between the two most conventionally used methods of Zn fertilization, via soil application (Embleton et al., 1966; Alva and Tucker, 1992; Swietlik and Zhang, 1994) or foliar fertilization (Garcia-Alverez et al., 1983; Wang, 1999). Studies carried out by Swietlik and Zhang (1994) demonstrated the involvement of two distinctly different mechanisms with regard to fraction of applied Zn entering through foliage or alternatively through the root. Availability of Zn is a function of its partition amongst different forms (Viets, 1962) and their interactions with other nutrients (Fisichella et al., 1994). Chemical transformation of Zn, although extensively studied, has equally inconclusive interpretations due to vulnerability of nutrient to different inactivation reactions, leading to sub-optimum availability at the crop’s more nutrient demanding growth stages. Response to Zn fertilization is often obtained despite comparatively higher supply level of available Zn status in soil. Such responses fail to address soil test-crop response relationship unless different soil Zn fractions are partitioned in relation to fruit yield. Little work on the fate of Zn applied via soil application versus foliar fertilization into various fractions of soil Zn using citrus as a test crop has been done, especially with the mandarin cultivars, ‘Nagpur’ mandarin being one of them. Studies were, therefore, carried out with the objectives to evaluate: 1) comparative response of soil versus foliar application of Zn on growth, yield, quality, and leaf nutrient composition of ‘Nagpur’ mandarin; 2) transformation of applied Zn into different fractions of soil native Zn; and 3) their relationship with fruit yield. MATERIALS AND METHODS Experimental Setup Field experiment was conducted in a 12-yr-old ‘Nagpur’ mandarin (Citrus reticulate Blanco) orchard (varying in initial canopy volume 31.2–36.5 m3 ), predominantly showing multiple nutrient deficiencies in form of N (1.92–2.00%), P (0.08–0.09%), Fe (28.4–34.6 ppm), and Zn(14.8–16.5 ppm) with Zn-deficiency

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symptoms being most conspicuous as small pointed leaves with many interveinal chlorotic specks on both the sides of the mid-rib of leaves covering > 20% canopy area of each test tree. The experiment was initiated during 2004–05 with the six treatments and continuing up to the fruiting season of 2006–07 in a randomized complete block design with five replications on a semectite-rich (swells when wet, shrinks when dry) Typic Haplustert soil type. The initial soil properties at 0–20 cm soil depth were: particle size distribution of clay 438.2 g kg−1 , silt 310.8 g kg−1 , and sand 251.0 g kg−1 , pH 7.8, electrical conductivity (EC) 0.23 dS m−1 , calcium carbonate (CaCO 3 ) 31.4 g kg−1 , and available nutrients (mg kg−1 ) N 101.7, P 10.8, potassium (K) 382.4, Fe 15.3, Mn 10.9, copper (Cu) 2.8, and Zn 0.65. Sub-humid tropical climate of the site is characterized by mean annual rainfall of 800–1200 mm with ustic temperature regime (temperature difference of > 5◦ C between of mean summer of 35–45◦ C and winter temperature of 22–28◦ C). The treatments using zinc sulfate (ZnSO 4 7H 2 O) as Zn source comprised of: T 1 -soil application (100 g ZnSO 4 tree−1 year−1 in two splits, at anthesis and marble fruit size), T 2 -soil application (200 g ZnSO 4 tree−1 year−1 in two splits, at anthesis and marble fruit size), T 3 -soil application (300 g ZnSO 4 tree−1 year−1 in two splits at anthesis and marble fruit size), T 4 -foliar application (100 g ZnSO 4 tree−1 year−1 in two splits, at concentration of 0.50% ZnSO 4 with 10 L solution to each plant, at anthesis, and marble size fruit), T 5 -foliar application (200 g ZnSO 4 tree−1 year−1 in four splits, at concentration of 0.50% ZnSO 4 with 10 L solution to each plant, at anthesis, pea size, marble size, and initiation of fruit enlargement stages), and T 6 -foliar application (300 g ZnSO 4 tree−1 year−1 in six splits, at concentration of 0.50% ZnSO 4 with 10 L solution to each plant at anthesis, pea size, marble fruit size, and three- times during fruit enlargement stage). Different key growth stages in the phenology of trees used were: anthesis, pea size, marble fruit size, and initiation of fruit enlargement stages corresponded to 25–30 days after (DA) floral primordial formation, 15–20 DA anthesis (DAA), 50–60 DAA, and 60–90 DAA, respectively. The common doses of fertilizers carrying 600 g N as urea (three splits), 200g P as single superphosphate (two splits), 300g K as muriate of potash (one split), and 60 g Fe tree−1 as FeSO 4 tree−1 (one split) were supplemented uniformly to all the treatments as basal application. Data on flowering intensity, fruit set, tree spread (E-W × N-S) expressed as tree volume using the formulae 0.5233 HW2 where H and W represent tree height, stem height, and canopy diameter, respectively, (Obreza, 1991) and fruit yield were recorded.

Sampling and Analysis Six-to-seven month-old-leaves at 2nd, 3rd, or 4th leaf positions from nonfruiting terminals covering 10% of the trees between the heights of 1.5–1.8 m from the ground were sampled. Likewise, soil samples (10 samples per

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treatment) were collected from perimeter of trees, the zone having maximum feeder roots concentration at soil depth of 0–20 cm (Srivastava et al., 1999). The time of leaf and soil sampling was same. The initial leaf and soil samples were collected before treatment and thereafter, in every growing season, sampling was done only once. Leaf samples were thoroughly washed (Chapman, 1964) and ground using a Wiley grinding machine to obtain homogeneous samples. Tri-acid [perchloric acid (HClO 4 ): nitric acid (HNO 3 ): sulfuric acid (H 2 SO 4 ) in 2:5:1] extracts of leaf samples (Chapman and Pratt, 1961) were subjected to analysis of P by vanado-molybdo-phosphoric yellow color (0.002 N ammonium molybdate + 0.01 N ammonium metavanadate) method, K by flame photometrically, and micronutrients (Fe, Mn, Cu, and Zn) by atomic absorption spectrophotometer (Model GBC-908, GBC, Dandenong, Victoria, Australia). Nitrogen was determined in prepared leaf samples without digesting them, using autonitrogen analyzer (Model Perkin Elmer-2410 Series, Perkin Elmer, Waltham, MA, USA). Collected soil samples were likewise air dried, ground, and passed through 2-mm sieve, and analyzed for available N using alkaline potassium permanganate (KmnO 4 ) steam distillation (Subbiah and Asiza, 1956), Olsen-P using sodium bicarbonate extraction by shaking 2.5 g soil in 50 mL of 0.5 M sodium bicarbonate (NaHCO 3 ) for 30 min. Potassium was extracted in 1 N neutral ammonium acetate (NH 4 Oac) in 1:2 soil:extractant ratio after shaking for 30 min. (Jackson, 1973), and micronutrients (Fe, Mn, Cu, and Zn) in 0.05 M (pH 7.3) diethylenetriaminepentaacetic acid (DTPA)- calcium chloride (CaCl 2 ) after shaking 20 g soil and 50 mL extractant together for 2 hrs (Lindsay and Norvell, 1978). Different fractions of Zn (Figure 1) viz., water soluble Zn (WS-Zn), exchangeable Zn (Exch.–Zn), complex Zn (Comp.-Zn), organically bound Zn (Org.-Zn), Zn bound to carbonates and amorphous oxides (Carbonate-Zn), Zn bound to crystalline oxides (Cryst.-Zn), and residual-Zn (Resi.-Zn) were determined following the extraction procedures as suggested by Edward Raja and Iyengar (1986). Fruits were randomly sampled from pre-treated tagged terminals. Different fruit quality parameters viz., juice content, total soluble solids (TSS), and acidity were determined (Ranganna, 1986). Fruits were cut in half and juices from both halves were manually pressed through 0.8 mm mesh screen. Three drops of juice were placed on a hand-held temperature-compensated refractometer (Elma Hanbai, Nagasaki, Japan) to determine soluble solids content. Titratable acidity was determined from 10 mL of juice by adding 5 drops of 1% phenolphthalein solution in 50% isopropyl alcohol, and stirring well while adding 0.1 N NaOH until the color changed from yellow-orange to pink (pH endpoint = 8.2). For juice content analysis, each fruit was cut in half and squeezed using an electric hand juicer. Expressed juice from all five fruits from each quadrant of each tree was combined and calculated using the formula: total fruit weight–pulp weight/total fruit weight. Data generated were statistically

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Figure 1. Path diagram and correlation coefficient of soil Zn fractions influencing the fruit yield.

analyzed through critical difference (CD), correlation (r) studies, and partitioning of direct or indirect effect through path coefficient analysis (Gomez and Gomez, 1984). RESULTS AND DISCUSSION Vegetative Growth Different growth parameters viz., flowering intensity, fruit set, tree volume and fruit yield were significantly (P < 0.05) influenced by Zn treatments involving both soil as well as foliar application. Flowering and Fruit Set Flowering and fruit set are considered as two most important indices of improved response on fruit yield. Both of these parameters were significantly influenced by Zn-fertilization (Table 1). The treatment T 2 on mean basis registered the highest flowering (flowers m−1 shoot length) intensity (162.3), which was significantly (P < 0.05) superior over other soil application treatment, T 1

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Table 1 Response of differential doses of zinc on flowering intensity, fruit set, tree volume, and yield of ‘Nagpur’ mandarin (pooled data of three growing seasons)

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Treatments T1 T2 T3 T4 T5 T6 CD (P = 0.05)

Flowering intensity (No. m−1 shoot)

Fruit set (No. m−1 shoot)

Increase in tree volume (m3 ) over initial

Fruit yield (kg tree−1 )

84.3e (58.5–10.2.3) 162.3a (148.2–186.7) 160.4ab (152.3–178.0) 73.0f (46.9–91.6) 93.0d (82.5–101.9) 115.0c (98.8–142.2) 9.4

13.2cd (4.7–20.3) 25.4ab (17.2–30.8) 27.3a (18.1–31.2) 7.3ef (3.2–10.6) 10.8c (4.3–12.3) 16.8c (10.2–29.0) 4.8

2.3 (0.8–3.8)∗ 4.3 (1.8–6.0) 3.5 (1.6–4.7) 2.1 (0.5–2.8) 2.2 (0.6–3.1) 2.5 (0.9–3.8) 0.83

32.1de (24.1–40.6) 52.6a (46.3–60.4) 51.8ab (49.6–56.1) 22.5f (18.3–26.1) 34.3d (28.1–40.2) 42.1c (38.9–47.0) 6.4

Numbers in parenthesis indicate the range of values obtained over three growing seasons. Means (10 samples per mean) within column followed by same letter do not differ significantly (P < 0.05) on the basis of Duncan’s multiple range test. T 1 = soil application of ZnSO 4 at 100 g/tree, T 2 = soil application of ZnSO 4 at 200 g/tree, T 3 = soil application of ZnSO 4 at 300 g/tree, T 4 = foliar application of ZnSO 4 at 100 g/tree, T 5 = foliar application of ZnSO 4 at 200 g/tree, and T 6 = foliar application of ZnSO 4 at 300 g/tree.

(84.3) and all the three foliar treatments viz., T 4 (73.0), T 5 (93.0), and T 6 (115.0). The fruit set (no. m−1 shoot length) expressed the similar response, being highest with T 2 (25.4), and superior over either T 1 (18.2), T 4 (7.3) or T 6 (16.8). Difference in nutrient status of trees was observed to be one of the causal factors to differential response on flowering as described through nutrient diversion theory (Sachs, 1977). Studies relating nutritional status to flowering intensity (no. m−1 shoot length) in ‘Nagpur’ mandarin (Srivastava et al., 2000) showed comparatively, a much higher flowering intensity (170.2) at 2.48% N and 28.4 ppm Zn than flowering intensity of 92.0 at 1.80% N and 18.2 ppm Zn. Tree Volume and Fruit Yield An improvement in tree volume provides the fruit bearing area, a pre-requisite to increased response on fruit yield. The increase in the tree volume (over initial observations) on mean basis was maximum with treatment T 2 (4.3 m3 )

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compared to other treatments involving soil application of ZnSO 4 viz., T 1 (2.3 m3 ), T 3 (3.5 m3 ), foliar application of ZnSO 4 viz., T 5 (2.2 m3 ) and T 6 (2.5 m3 ). These observations suggested the non-linear response of Zn application to canopy increase up to its optimum level only. All the three treatments (T 4 , T 5 , and T 6 ) involving foliar application of zinc produced no significant response on tree volume (Table 1). Significant response of Zn-fertilization on fruit yield was observed when soil application versus foliar application of zinc was compared on equivalent rates, the former producing much higher magnitude of response than latter treatments (Table 1). In earlier studies, foliar sprays with ZnSO 4 failed to increase the yield of ‘Valencia’ and ‘Washington’ Navel oranges with moderate Zn-deficiency (Embleton et al., 1988) underscoring the hypothesis that the severity of Zn-deficiency symptom is the most important criterion determining the trees’ response to the corrective Zn-treatments. The treatment T 2 on mean basis recorded maximum fruit yield (52.6 kg tree−1 ). The effectiveness of various treatments on equivalent rates, demonstrated that treatment T 1 (32.1 kg tree−1 ) was superior over T 4 (22.5 kg tree−1 ), T 3 (51.8 kg tree−1 ) over T 5 (34.3 kg tree−1 ), and T 2 (52.6 kg tree−1 ) over T 6 (42.1 kg tree−1 ) showing clear cut superiority of all the treatments of soil application over the foliar application. Considering the vital role of Zn in indole-acetic acid (IAA) oxidase system, Znfertilization improved the root growth enabling better extraction of nutrients and water available within the rhizosphere to impart favorable response on growth and yield (Bausher, 1982). Swietlik (1996) demonstrated increase in fruit yield of ‘Rio Red’ grapefruit with application of Zn during March-June in spring season due to increase fruit set rather than fruit size.

Changes in Soil Fertility and Leaf Nutrient Status Significant changes (P < 0.05) in soil fertility and leaf nutrient composition were observed in response to zinc application (Table 2). Among the different nutrients (N, P, K, Fe, Mn, Cu, and Zn), only N, P, and Zn were influenced both in soil as well as in leaves. Soil Fertility Changes Changes in available pool of nutrients are considered supplementary to inflict changes in leaf nutrient composition in perennial crop like citrus (Srivastava and Singh, 2006). The treatment T 2 registered the highest increase in available N (Table 2) from initial value of 98.2 mg kg−1 to and 120.1 mg kg−1 in 2006–07 (123.1 mg kg−1 ). These changes in available N were significantly superior to the other treatments viz., T 1 (101.8–118.2 mg kg−1 , mean 112.2 mg kg−1 ), T 3 (103.9—137.1 mg kg−1 , mean 127.4 mg kg−1 ), T 4 (100.4–101.0 mg kg−1 , mean 118.2 mg kg−1 ), T 5 (100.1–105.9 mg kg−1 , mean 112.8 mg kg−1 ), and

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101.8a 98.2a 103.9a 100.4a 100.1a 106.1a ns

T1 T2 T3 T4 T5 T6 CD (P = 0.05)

112.2ef 123.1b 127.4a 113.2d 112.8de 117.7c 3.1

Final 10.4 24.9 23.5 12.8 12.7 11.6 —

Net increase 10.9a 10.8a 11.0a 10.2a 11.2a 11.0a ns

Initial 12.3 16.1 15.8 11.8 12.2 13.1 1.3

Final

P

1.4 5.3 4.8 1.6 1.0 2.1 —

Net increase 0.60a 0.68a 0.64a 0.68a 0.70a 0.64a ns

Initial

0.86bc 1.06a 1.06a 0.82cd 0.86bc 0.88b 0.06

Final

Zn

0.26 0.38 0.40 0.14 0.16 0.24 —

Net increase

Final values are derived on the basis of average of three seasons. Means (10 samples per mean) within column followed by same letter do not differ significantly (P < 0.05) on the basis of Duncan’s multiple range test. T 1 = soil application of ZnSO 4 at 100 g/tree, T 2 = soil application of ZnSO 4 at 200 g/tree, T 3 = soil application of ZnSO 4 at 300 g/tree, T 4 = foliar application of ZnSO 4 at 100 g/tree, T 5 = foliar application of ZnSO 4 at 200 g/tree, and T 6 = foliar application of ZnSO 4 at 300 g/tree.

Initial

Treatments

N

Table 2 Available pool of nutrients N, P, and Zn (mg kg−1 ) in response to different Zn treatments (pooled data of three growing seasons)

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T 6 (106.1–112.2 mg kg−1 , mean 117.7 mg kg−1 ), irrespective of method of application, soil or foliar. On average all the treatments involving soil application of Zn (T 1 –T 3 ) registered significantly higher available P (12.3–16.1 mg kg−1 , mean 14.7 mg kg−1 ) than available P (11.8–13.1 mg kg−1 , mean 12.3 mg kg−1 ) with foliar applied Zn treatments (T 4 -T 6 ). The DTPA extractable Zn showed significant changes in response of Zn fertilization. The treatment T 2 (0.68–1.16 mg kg−1 , mean 1.06 mg kg−1 ) was most effective amongst soil applied Zn treatments. While none of foliar applied Zn produced significant changes in available Zn in soil. However, available Zn on equivalent rates, T 2 was superior over T 5 and T 3 over T 6 , but not amongst T 4 , T 5 , and T 6 or between T 2 and T 3, supporting the supremacy of soil applied Zn over foliar applied Zn (Table 2). These observations showed that Zn-fertilization has some triggering effect in enhancing the availability of both native and applied Zn (distinction of the two however not made in the study) through transformation of solid phase to solution phase complexes that later accelerated the accumulation of Zn in leaves by inducing the improvement in root cation exchange capacity (Srivastava et al., 1994).

Leaf Nutrients Composition Leaf analysis is considered to be an effective guide in diagnosing nutritional problems and as a basis for evaluating the fertilizer response in citrus (Srivastava et al., 1999), since it integrates the coordination between underground root and above-ground shoot system. The concentration of the three nutrients (N, P, and Zn) was highest with treatment T 2 registering increase in N from initial value of 2.12 to 2.46%, (mean 2.37%), P from initial value of 0.08 to 0.12% (mean 0.11%), and Zn from 18.1 ppm to 28.1 ppm (mean 26.4 ppm), significantly superior (P < 0.05) over rest of the treatments whether soil or foliar application. The leaf N content on equivalent rate, was 2.00–2.16% (mean 2.16%) with T 1 , higher than 2.00–2.08% N (mean 1.99%) with T 4 at lowest dose of Zn fertilization or 2.08–2.42% (mean 2.37%) with T 3 still higher than T 6 with 2.06–2.22% (mean 2.16%) at highest level of foliar applied Zn. In earlier studies on site specific nutrient management in ‘Nagpur’ mandarin on Haplustert soil type, the requirement of N doses was higher on Zn deficient sites compared to sites having optimum supply of available Zn (Srivastava et al., 2006). There was a comparatively higher concentration of P in trees showing Zn deficiency symptoms due to phenomenon known as ‘concentration effect’ allowing the reduction in dry matter production without concomitant and equal reductions in mineral nutrient uptake conditioned further by loss of cell membrane integrity leading to passive nutrient uptake via transpiration stream (Welch et al., 1982). Significant response to Zn fertilization at different levels on the leaf Zn status, indicated the presence of strong sink for Zn acquisition due to structural disorganization at various levels of Zn-deficiency. Zinc content

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in leaf with treatment T 1 improved from 19.0 ppm to 26.4 ppm with treatment T 2 , statistically on par with treatment T 3 on the basis of pooled analysis (Table 3) . While no response was noticed involving foliar treatments suggesting that Zn is not easily translocated from top to the roots, and thus cannot correct Zn deficiency in the roots (Bukovac and Witter, 1957). The lack of response of mildly Zn-deficient trees to Zn foliar sprays is an indication of efficiency of Zn application method rather than the tree tolerance of mild Zn deficiency. Elevating Zn concentration only in the tops of Zn-deficient trees with foliar sprays partially restored normal root growth, but clearly was not as effective as the roots absorbing Zn directly from high Zn-concentration solutions. These observations suggested the involvement of two mechanisms operational at two tiers of structural organization: one in the roots and the other in the shoots (Swietlik and Zhang, 1994).

Fruit Quality Response Quality response of citrus fruits is assessed on the basis of changes of three most important parameters (Srivastava and Singh, 2000). These are: juice content, total soluble solids (TSS), and acidity, which responded significantly with differential doses of Zn application during all the three seasons, irrespective of method of application (Table 4). Pooled data analysis showed much higher juice (43.8–47.7%) and TSS (8.9–9.7%) with soil applied Zn (T 1 –T 3 ) than juice (43.1–45.1.0%) and TSS (8.7–9.2%) with foliar applied Zn (T 4 –T 6 ). On the other hand, acidity was significantly (P < 0.05) lower (0.61–0.66%) with soil application of Zn (T 1 –T 3 ) than the acidity (0.68–0.71%) with foliar treatments (T 4 –T 6 ). These treatments at equivalent rates also displayed the similar response, T 2 being much superior over T 4 and T 3 over T 6 with reference to all the three fruit quality parameters. In earlier correlation studies on ‘Nagpur’ mandarin (Srivastava and Singh, 2000), the fruit quality indices such as TSS and juice content showed positive correlations with leaf N (r = 0.611 and 0.546, P = 0.01), P (r = 0.506 and 0.521, P = 0.01), and Zn (r = 0.801 and 0.621, P = 0.01). While juice acidity reduced with higher concentration of leaf N(r = −0.582, P = 0.01), P(r = −0.408, P = 0.05), and Zn (−0.614, P = 0.01).

Soil Zn Transformation Zinc applied in soil is transformed into the five major fractions (Hodgson, 1963): 1) associated with soil surface, either organic or inorganic; 2) occluded during development of new solid phases without being a principal component; 3) precipitated with other soil components forming new phase; 4) occupying sites in soil minerals either as an original constituent or by entering the crystal lattice through solid state diffusion; and 5) incorporated in biological systems

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1.94a 1.98a 1.92a 1.93a 1.98a 2.00a ns

T1 T2 T3 T4 T5 T6 CD (P = 0.05)

2.16cd 2.37a 2.36ab 1.99f 2.12de 2.16c 0.30

Final 0.22 0.39 0.44 0.06 0.14 0.16 —

Net increase 0.09a 0.08a 0.09a 0.09a 0.09a 0.09a ns

Initial 0.10c 0.12a 0.11b 0.09fg 0.09ef 0.09de 0.001

Final

P(%)

0.01 0.04 0.02 0.00 0.00 0.00 —

Net increase 16.5a 14.8a 15.2a 15.7a 14.8a 16.5a ns

Initial

19.0ef 26.4ab 26.9a 18.6fg 20.5cd 21.6c 2.8

Final

Zn(ppm)

2.5 11.6 11.7 2.9 5.7 5.1 —

Net increase

Final values are derived on the basis of average of three seasons. Means (10 samples per mean) within column followed by same letter do not differ significantly (p < 0.05) on the basis of Duncan’s multiple range test. T 1 = soil application of ZnSO 4 at 100 g/tree, T 2 = soil application of ZnSO 4 at 200 g/tree, T 3 = soil application of ZnSO 4 at 300 g/tree, T 4 = foliar application of ZnSO 4 at 100 g/tree, T 5 = foliar application of ZnSO 4 at 200 g/tree, and T 6 = foliar application of ZnSO 4 at 300 g/tree.

Initial

Treatments

N(%)

Table 3 Changes in leaf nutrients composition in response to different Zn treatments (pooled data of three growing seasons)

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Table 4 Response of Zn fertilization on the fruit quality parameters (pooled data of three growing seasons) Treatments

Juice(%)

TSS(%)

Acidity(%)

T1 T2 T3 T4 T5 T6 CD (P = 0.05)

43.8e 47.7a 46.8b 43.1ef 44.7cd 45.1c 0.68

8.9de 9.7a 9.5ab 8.7ef 9.0cd 9.2c 0.34

0.66cd 0.61f 0.64de 0.71a 0.70ab 0.68bc 0.03

Means (10 samples per mean) within column followed by same letter do not differ significantly (p < 0.05) on the basis of Duncan’s’ multiple range test. T 1 = soil application of ZnSO 4 at 100 g/tree, T 2 = soil application of ZnSO 4 at 200 g/tree, T 3 = soil application of ZnSO 4 at 300 g/tree, T 4 = foliar application of ZnSO 4 at 100 g/tree, T 5 = foliar application of ZnSO 4 at 200 g/tree, and T 6 = foliar application of ZnSO 4 at 300 g/tree.

and their residues in soil. A comprehensive understanding on these transformations made the chemistry of Zn availability more meaningful in order to regulate the Zn nutrition in relation to fruit yield. However, the poor mobility of Zn in soil has discouraged extensive experimentation with soil Zn application (Swietlik, 1989) especially using citrus as a test crop. Application of Zn in present studies was associated with significant changes in different soil Zn fractions (Table 5). The relative distribution of different soil Zn fractions in untreated soil was observed as Resi.-Zn > Cryst.-Zn > Carbonate-Zn > Org.- Zn > Comp.-Zn > Exch.-Zn > WS-Zn, which upon Zn-fertilization (T 2 treatment), these fractions transformed in the order of: Resi.-Zn > Carbonate-Zn > Comp. Zn > Cryst. –Zn > Org.-Zn > Exch.-Zn > WS-Zn, contrary to no such distinct changes with any of the foliarly applied Zn treatments. Out of 7 fractions of soil Zn, 4 fractions viz., Exch.-Zn, Comp.-Zn, Org.bound Zn, and Carbonate-Zn were largely influenced, with corresponding changes from 0.25 to 0.60 mg kg−1 , 2.71 to 4.86 mg kg−1 , 0.86 to 2.0 mg kg−1 , and from 2.56 to 4.96 mg kg−1 upon increasing levels of soil application of ZnSO 4 from T 1 to T 3 (Table 5). Interestingly with all the foliarly applied Zn treatments, most of the soil Zn fractions remained more or less unaffected. These observations suggested: 1) the restricted movement of Zn via leaves compared to roots, accountable for triggering effect on movement of Zn-auxin complex synthesized within the roots on the sites acting as sink, a pre-requisite to produce the favorable growth and consequently the improved yield response and 2) soil applied Zn improved the potential reserve fractions of soil Zn which could be later mobilized with the use of organic manure based soil amendments.

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0.09 0.14 1.08 0.31 1.48 3.82 73.28 80.20

Initial values

0.11a 0.25ef 2.71c 0.86de 2.56cd 4.18de 76.64a 87.31c

T1 0.16a 0.48b 4.52ab 1.52b 3.89b 4.22cd 77.83a 92.60b

T2

Soil application

0.20a 0.60a 4.86a 2.01a 4.96a 4.31a 79.86a 96.10a

T3 0.13a 0.28de 1.98ef 0.86de 2.48ef 4.16ef 74.35a 84.24de

T4 0.12a 0.32cd 2.01de 0.91cd 2.52de 4.28ab 73.22a 83.83ef

T5

Foliar application

0.14a 0.36c 2.02cd 0.92c 2.60c 4.26bc 74.98a 85.26d

T6

0.03 0.10 0.94 0.48 0.73 0.10 ns 1.72

CD (P = 0.05)

Means (10 samples per mean) within column followed by same letter do not differ significantly (P < 0.05) on the basis of Duncan’s multiple range test. T 1 = soil application of ZnSO 4 at 100 g/tree, T 2 = soil application of ZnSO 4 at 200 g/tree, T 3 = soil application of ZnSO 4 at 300 g/tree, T 4 = foliar application of ZnSO 4 at 100 g/tree, T 5 = foliar application of ZnSO 4 at 200 g/tree, and T 6 = foliar application of ZnSO 4 at 300 g/tree.

i. WS -Zn ii. Exch.-Zn iii. Comp.-Zn iv. Org.-Zn v. Carbonate- Zn vi. Cryst.-Zn vii. Resi. -Zn viii. Total Zn

Soil Zn fractions mg kg−1

Treatments

Table 5 Changes in soil Zn fractions (water soluble Zn, exchangeable Zn, complex Zn, organically bound Zn, Zn bound to carbonates and amorphous oxides, Zn bound to crystalline oxides, residual Zn and total Zn) as influenced by differential Zn application (pooled data of three growing seasons)

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Table 6 Path coefficient analysis amongst soil Zn-fractions and fruit yield of ‘Nagpur’ mandarin

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Soil Zn-fractions 1. Water soluble Zn(WS-Zn) TOTAL EFFECT Direct effect Indirect effect via Exch.-Zn Indirect effect via Comp.- Zn Indirect effect via Org.- Zn Indirect effect via Carbonate-Zn Indirect effect via Cryst.- Zn 2. Exchangeable Zn(Exch.-Zn) TOTAL EFFECT Direct effect Indirect effect via WS-Zn Indirect effect via Comp.- Zn Indirect effect Org.-Zn Indirect effect via Carbonate- Zn Indirect effect via Cryst.-Zn 3. Complex Zn(Comp.-Zn) TOTAL EFFECT Direct effect Indirect effect via WS- Zn Indirect effect via Exch.- Zn Indirect effect via Org.-Zn Indirect effect via Carbonate-Zn Indirect effect via Cryst.- Zn 4. Organically bound Zn (Org.-Zn) TOTAL EFFECT Direct effect Indirect effect via WS - Zn Indirect effect via Exch.-Zn Indirect effect via Comp-Zn Indirect effect via Carbonate-Zn Indirect effect via Cryst. - Zn 5. Zn-bound to carbonates and amorphous oxides (Carbonate-Zn) TOTAL EFFECT Direct effect Indirect effect via WS-Zn Indirect effect via Exch.- Zn Indirect effect via Comp.- Zn Indirect effect via Org.- Zn Indirect effect via Cryst. - Zn 6. Zn-bound to crystalline oxides (Cryst.-Zn) TOTAL EFFECT Direct effect Indirect effect via WS - Zn Indirect effect via Exch.- Zn Indirect effect via Comp. - Zn Indirect effect via Org. - Zn Indirect effect via Carbonate- Zn

Correlation coefficient (r) 0.578 0.108 0.717 0.482 0.054 −0.401 −0.382 0.618 0.268 0.080 0.402 −0.368 0.108 0.128 0.718 0.528 0.30 0.760 −0.389 0.041 0.052 0.423 −0.559 0.016 0.102 0.758 0.043 0.063 0.510 0.086 0.036 0.109 0.504 −0.606 0.381 0.462 0.104 0.034 0.047 0.438 −0.503 0.342

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Improvement in total Zn content of soil was observed from 87.31 to 96.10 mg kg−1 with increase soil application of ZnSO 4 from treatment T 1 to T 3 (Table 5). The bulk of the native and applied Zn remained in the residual fraction, which was not easily extracted by the reagents. Residual-Zn, the major fraction of the total Zn constituted 87.7% of total Zn at T 1 level reduced to 83.1% with treatment T 2 against much higher initial value of 91.1%. These observations lend support that Zn application helped in bringing a significant fraction of Resi.-Zn into other fractions having a strong vulnerability to being more assimilated by plants. Linear correlation between the different soil Zn fractions and fruit yield (Figure 1) showed highest influence of Comp.-Zn (r = 0.718, P = 0.01) followed by Exch.-Zn (r = 0.618, P = 0.01), WS-Zn (r = 0.578, P = 0.01), and Carbonate-Zn (r = 0.510, P = 0.01). Significantly positive correlations between different fractions of soil Zn further confirmed the presence of dynamic equilibrium. Edward Raja and Iyengar (1986) observed that major fraction of applied Zn accumulated in Resi.-Zn fraction. The other four fractions viz., Exch.-Zn, Org-Zn, Carbonate-Zn, and Cryst.-Zn were found to become available through complex formation revealed by path coefficient analysis. Partitioning of total effect of various soil Zn fractions on fruit yield into direct and indirect effect (Table 6) using path coefficient analysis showed that indirect effect of WS-Zn via Exch.-Zn was much higher (r = 0.717, P = 0.01) than total effect (r = 0.578, P = 0.01) on fruit yield. Similarly, total effect of Exch.-Zn on fruit yield was observed significantly positive, which upon partitioning into direct and indirect effect on fruit yield, both the effects were non-significant. The influence of Exch.-Zn through Org.-Zn fraction on the yield was negative, indicating that available zinc was reduced due to stable organic complex formation. Org-Zn showed positive correlation on fruit yield, but direct effect was negative, indicating that available Zn was reduced due to strong bonding force of element with organic complexes. Its direct effect through water soluble, exchangeable and Zn-bound to carbonates and acid soluble mineral fraction was negligible and its main influence was through Comp.-Zn. Indirect effect of Carbonate-Zn (r = −606, P = 0.01) and Cryst.Zn (r = −503, P = 0.01) via Org.-Zn was negative. Thus, in soils with low Comp-Zn, Exch.-Zn forms the major pathway by which applied Zn passes between different forms which finally becomes accessible to improve supply level of Zn to the plant.

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