Optimization of Copper Sulphate Levels to Enhance Yield and Quality of Aggregatum Onion (Allium cepa var aggregatum L)

Field experiments were conducted during rabi season of 2018 and summer season of 2019 at farmer’s field, western zone of Tamil Nadu at Pattiyarkovilpathy, Theethipalayam and Narasipuram villages of Thondamuthur block of Coimbatore district. Totally five treatments comprising of varied CuSO 4 levels (0, 2.5, 5.0, 7.5 & 10.0 kg ha -1 ) were replicated four times in a randomized block design under irrigated conditions. The results showed that, fresh mean bulb yield of onion in four locations ranged from 12.4 to 15.6 t ha -1 . The highest mean bulb yield of 15.6 t ha -1 was registered with recommended NPK+5 kg CuSO 4 ha -1 and the lowest mean bulb yield of 12.4 t ha -1 was recorded in NPK control with no CuSO 4 addition. Dry Matter Production ranged from 1065 to 1199 kg ha -1 . The highest DMP of 1199 kg ha -1 was recorded in the treatment that was applied with NPK+5 kg CuSO 4 ha -1 and the lowest DMP (1065 kg ha -1 ) was noticed with NPK control. Concerning copper content in bulb and foliage, it ranged from 6.09 to 12.3 and 8.46 to 13.7 mg kg -1 with the location mean of 6.16 to 12.0 mg kg -1 and 7.61 to 11.1 mg kg -1 , respectively. Copper uptake in bulb and sheath varied from 7.05 to 18.3 and 6.26 to 11.0 g ha -1 (mean of 8.55 to 12.3 and 6.52 to 10.4 g ha -1 ), respectively with a total uptake of 14.1 to 27.6 g ha- 1 . In both bulb and foliage, Cu content and uptake, increased with increasing doses of CuSO 4 and the highest Cu content and uptake was registered with the application of CuSO 4 @ 10.0 kg ha -1 . Regarding the availability of copper in soil, it ranged from 0.73 to 2.42 mg kg -1 and increasing levels of CuSO 4 application increased the DTPA Cu status in soil and application of 10.0 kg CuSO 4 ha -1 recorded the highest copper availability in soil. Quality parameters viz., TSS, titrable acidity and ascorbic acid varied from 10.25 to 14.08 o Brix, 0.28 to 0.61% and 12.9 to 20.1 (100 g -1 FW), respectively. Field experiments conducted to optimize the rates of copper sulfate application to improve the yield and quality of aggregatum onion in four locations at Coimbatore district, showed that, soil application of 5 kg CuSO 4 along with recommended NPK @ 60:30:30 kg ha -1 was found optimal for obtaining higher bulb yield (15.6 t ha -1 ) and better quality in aggregatum onion with a BC ratio of 4.63.


INTRODUCTION
Onion (Allium cepa L.), belonging to the family Alliaceae, is one of the most important and popular vegetable spice crops cultivated worldwide (Mishra et al., 2013). It is famous for its characteristics flavour and is widely used to increase the taste of foods like gravies, soups, stew stuffing, fried fish, and meat (Rashid et al., 2016). The increased productivity with high quality onion is the utmost demand of the onion growers. Lack of micronutrients addition in the balanced fertilization schedule is one of the important causes for low yield and quality (Lata Shukla et al., 2015). Onion requires a sufficient amount of plant nutrients and responds very well to the added nutrients (Alam et al.,20l0). Therefore, systematic fertilization of different micronutrients for onion cultivation is needed for the low organic matter and micronutrient deficient soils (Lata Shukla et al., 2015). Onion is more susceptible to nutrient deficiencies than most crop plants because of their shallow and unbranched root system; hence they require and often respond well to the addition of fertilizers (Brewester, 1994). Among the various factors affecting the yield of onion, adequate and balanced nutrient management plays a major role to optimising the quality and quantity of harvested plant products (Lakshmi and Sekhar, 2018). Nutrient management involves using crop nutrients as efficiently as possible to improve productivity while protecting the environment. The key fundamental principle behind nutrient management is balancing soil nutrient inputs with crop requirements (Khalid Mahmud Khokhar, 2019). Among all the essential nutrients, micronutrients viz., Fe, Mn, Zn, Cu, B and Mo are important for the optimal growth and productivity of the plants. Though these elements are required in small amounts, they are very important for plant development and for profitable crop production because they act as activators of many plant functions. Micronutrients among essential elements, plays a catalytic role in nutrient absorption and balancing other nutrients (Singh andKalloo, 2000, Poongothai andChitdeshwari, 2003). Although the requirement of a micronutrient is small compared to macronutrients, nevertheless, micronutrient deficiency can limit crop growth and production. Furthermore, micronutrients help increase the efficiency of the use of macronutrients. Micronutrients have a great role in the fertilizer program to achieve higher and sustainable crop yields (Vijay Aske et al., 2017 andJegadeeswari et al., 2019). Among the micronutrients, copper is important element for plant growth because it acts as a catalyzer of several plant processes, imparts disease resistance in plants, indirect role in chlorophyll production, intensifies the color and flavor of fruits and vegetables and playing an irreplaceable role in the function of a large number of enzymes which catalyze oxidative reactions in a variety of metabolic pathways. Cu can also be considered as a toxic element whose deleterious effects usually arise at high concentrations (Donghua Liu et al., 1994). The copper deficiency interferes with protein synthesis and causes a buildup of soluble nitrogen compounds. Normal plants contain 8 to 20 mg kg -1 copper and deficient plants usually contain less than 6 mg kg -1 . Copper deficiency in many plants shows up as wilting or lack of turgor and development of a bluish green tint before leaf tips become chlorotic and die. Onion bulbs show poor pigmentation because of copper deficiency (El-Hadidi et al., 2016). Looking at these facts, the present research was undertaken to optimize the dose of copper sulfate for increasing the productivity and quality of onion.

Field experiment and laboratory analysis
Four field experiments were conducted, two each during rabi season of 2018 and two during summer season of 2019 at farmer's fields, western zone of Tamil Nadu at Pattiyarkovilpathy, Theethipalayam and Narasipuram villages of Thondamuthur block, Coimbatore district, Tamil Nadu, India. Initial soil sample was collected from 0-15 cm depth and analyzed for various physico-chemical and chemical characteristics (Table 1). Onion variety local was used as a test crop and cropping system was irrigated.  (Lindsay and Norvell, 1978) Cu Triple acid digestion and AAS method (Lindsay and Norvell, 1978) Quality parameters - Ranganna, S. 1997 Experimental design Totally five treatments comprising of varied Cu levels (0, 2.5, 5.0, 7.5 & 10.0 kg CuSO 4 ha -1 ) were replicated four times in a randomized block design. The recommended dose of NPK fertilizers at 30:60:30 kg ha -1 was applied as basal, 30 kg N alone was applied as a top dressing on 30 DAS to all the treatments. Urea as N source, Single super phosphate for P, Muriate of Potash for K and Copper sulfate for Cu were used as inorganic fertilizer sources. A similar trend was observed with respect to the dry matter production of onion in four locations and the DMP in all the locations ranged from 1031 to 1241 kg ha -1 with a mean range of 1065 to 1199 kg ha -1 . The highest DMP of four locations were 1227,1163,1241,1165 kg ha -1 with a mean of 1199 kg ha -1 , which was recorded in the treatment that was applied with NPK+5 kg CuSO 4 ha -1 and the lowest DMP of 1093,1031,1100,1036 kg ha -1 with a mean of 1065 kg ha -1 was noticed with NPK control with no CuSO 4 (Table.3). Response to CuSO 4 was found to be very high in the soils having deficient Cu availability. This could be attributed to the vital functions of copper in plant growth and metabolism. Application of CuSO 4 at higher levels up to 5 kg per ha resulted in the development of good growth of sheath, bulbs and accelerated protein synthesis, bulb formation The crop was grown to maturity with routine cultural operations by imposing the treatments and at harvest bulb yield, dry foliage yield, yield attributing characters and quality parameters like total soluble sugars, ascorbic acid and titrable acidity were recorded besides collecting plant and post-harvest soil samples for analyzing the Cu availability, its content and uptake in bulb and leaves.

Statistical analysis
The experimental data collected relating to different parameters were statistically analyzed using the procedure given by Panse and Sukhatme, 1967. The data were subjected to Fisher's method of analysis of variance and the level of significance used in F test was P = 0.05. The critical difference was calculated at 5 per cent probability level whenever F value was found to be significant.

RESULTS AND DISCUSSION
The soil reaction of the experimental initial soil was neutral in all the locations with pH values of 7.30, 7.40, 7.0 and 7.7 with nil salinity status having the electrical conductivity of 0.40, 0.31, 042 and 0.50 dSm -1 . The organic carbon content was 0.45, 0.40, 0.54 and 0.60 g kg -1 in all four locations. Available nitrogen and phosphorus contents of the soils were low 204, 185, 154, 160 kg ha -1 and 15.8, 16.8, 14.4 and 18.1 kg ha -1 respectively. The values for available K and S were 288, 309, 240, 275 kg ha -1 and 22.3, 260, 28.2 and 20.3 mg kg -1 respectively. The nutrient status of all the farm soils showed low to medium in organic carbon content, low to medium in available N and P, high in available K and S. Micronutrient availability in the soils revealed sufficient status of Mn and Fe, while many of the farms were deficient in Zn and Cu availability. DTPA Copper status in the soils were 0.72, 0.66, 0.57 and 0.81 mg kg -1 . The experimental soil was sandy loam.

Yield and DMP
The results showed that, fresh bulb yield of onion in all four locations ranged from 11.5 to 16.4 t ha -1 and a mean variation of 12.4 to 15.6 t ha -1 . Application of CuSO 4 at graded levels increased the onion bulb yield up to 5 kg CuSO 4 ha -1 and thereafter gets decreased. Highest bulb yield of 14.6, 15.3, 16.4 and 16.0 t ha -1 was registered in Location 1, 2,3 & 4, respectively with the a mean of 15.6 t ha -1 . It was followed by 7.5 kg CuSO 4 ha -1 in all the locations with the mean yield of 15.0 t ha -1 and the lowest bulb yield of 11.8, 12.6, 13.5,11.5 t ha -1 with a mean of 12.4 t ha -1 was recorded in the NPK control with no CuSO 4 in four locations (Table.3 & Fig.1).  Figure.1. Optimizing the rate of copper dose for onion of copper as essential for plant metabolism, one of the constituents of certain oxidizing reduction enzymes. Therefore, its role in plant metabolism and participation in oxidation-reduction action, check in certain diseases and there by improved growth of plant and yield. The benefit-cost ratio ranged from 3.72 to 4.63, a maximum of 4.63 was observed in the treatment with the application of NPK + 5.0 kg CuSO 4 ha -1 and the lowest was noticed in NPK control with no CuSO 4 ( Table.3).

Cu content and Copper uptake
The copper content in both onion bulb and foliage was estimated and the uptake was computed. The results showed that application of copper sulfate resulted in increased Cu content in onion bulb and sheath and it was increased with increasing levels of CuSO 4 . In onion bulb, copper content in all four locations varied from 6.09 to 12.3 mg kg -1 with a mean variation of 6.16 to 12.0 mg kg -1 . In all the locations, the mean value showed the highest Cu content of 12.3,11.4,11.9,12.2 mg kg -1 with an overall mean of 12.0 mg kg -1 in the treatment that received NPK+10 kg CuSO 4 ha -1 which was followed by 7.5 kg CuSO 4 ha -1 addition and the lowest mean values was noticed in the NPK control. With regard to copper content in foliage, the values ranged from 8.46 to 13.7 mg kg -1 with the location mean of 7.61 to 11.1 mg kg -1 (Table 4). A similar trend was observed in the foliage also with the increased values for the increased levels of CuSO 4 application. In all the locations, the highest copper content of 13.1,13.4,12.9,13.7 mg kg -1 and a mean 13.3 mg kg -1 was recorded in the treatment applied with NPK + CuSO 4 @ 10.0 kg ha -1 which was followed by 7.5 kg CuSO 4 addition and the values were 11.6,12.4,11.8,12.7 mg kg -1 with a mean of 12.1 mg kg -1 . Similarly, the lowest copper content of 8. 65, 8.57, 8.51, 8.46 mg kg -1 and a mean of 8.55 mg kg -1 were observed with NPK control with no CuSO 4 . Copper uptake in bulb and sheath also increased with increasing dose of copper sulphate and the values varied from 7.05 to 18.3 and 6.26 to 11.0 g ha -1 (mean of 8.55 to 12.3 and 6.52 to 10.4 g ha -1 ) respectively. In both bulb and foliage, the highest uptake was registered with the application of CuSO 4 @ 10.0 kg ha -1 . In all the four locations, in bulb the highest Cu uptake of 17.2,16.2,18.3,17.0 g ha -1 with a mean of 17.2 g ha -1 (Table 5) was recorded with NPK+10 kg ha -1 CuSO 4 addition, which was followed by NPK+7.5 kg ha -1 CuSO 4 addition and the values were 14.2,14.4,15.6,14.7 g ha -1 with a mean of 14.7 g ha -1 . In bulb, the lowest copper uptake of 7. 19,7.77,8.42,7.05 g ha -1 with a mean of 7.61 g ha -1 which was recorded in the NPK control with no CuSO 4 . Similarly, in foliage, also similar trend was observed and in all four locations, the highest Cu uptake of 10.4,11.0,9.7,10.5 g ha -1 with a mean of 10.4 g ha -1 was recorded with NPK+10 kg ha -1 CuSO 4 addition, which was followed by NPK+7.5 kg ha -1 CuSO 4 and the values were 10.0,10.8,9.5,10.3 g ha -1 with a mean of 10.1 g ha -1 . In foliage, the lowest copper uptake of 6.76,6.78,6.26,6.29 g ha -1 with a mean of 6.52 g ha -1 was recorded in the NPK control.

Figure.2. Effect of Copper sulphate application on DTPA Cu availability in soil
However, the uptake of plant nutrients depends on several factors, such as cultivar, crop environment, soil fertility, fertilization methods (Lee et al., 2009;Yoldas et al., 2011).

Quality parameters
The quality parameters of onion were significantly influenced by the application of copper sulfate at different levels. The data on total soluble solids, titrable acidity and ascorbic acid content in the bulbs were assessed and reported the table 6. TSS, titrable acidity and ascorbic acid varied from 10.25 to 14.08 o Brix, 0.28 to 0.61% and 12.9 to 20.1(100 g -1 FW) respectively. The highest TSS (14.08 o Brix), titrable acidity (0.61%) and ascorbic acid 20.1(100 g -1 FW) content was recorded with the application of CuSO 4 @5 kg ha -1 and the lowest was observed in NPK control with no CuSO 4 addition. Quality parameters were found to be good under association with the CuSO 4 application at increasing levels up to CuSO 4 @ 5 kg ha -1 while the reduction in quality attributes were observed with increasing doses of copper sulfate. The improvement in TSS content in onion bulbs with the application of micronutrients might be attributed to the enhanced metabolic process involved in the biosynthesis of TSS, sugars, ascorbic acid, amino acid and other inorganic constituents. (Acharya et al., 2015). Similar results were reported earlier by Alam et al., 2010, Aske et al., 149 2017& Tariq et al., 2018.

Cu availability
Regarding the availability of copper in soil, it ranged from 0.73 to 2.42 mg kg -1 and the availability was found to be sufficient at all levels of CuSO 4 addition (Fig.2). Increasing levels of application increased the soil Cu status and in all four locations, application of 10.0 kg CuSO 4 ha -1 recorded the highest DTPA Cu of 1.50, 1.24, 1.30, 5.65 42 mg kg -1 with a mean of 2.42 42 mg kg -1 in soil. The copper availability in post harvest soil showed the sustained status of copper except NPK control, which recorded the lowest value of 0.73 mg kg -1 . This might be due to the effect of the addition of Cu through fertilisers and also available from inherent properties of soil like, parent material, the effect of organic matter and presence of clay minerals. Similar findings was reported by Kamble and Kathmale (2015).

CONCLUSION
Relevant Field experiments were conducted to optimize the dose of copper sulfate for aggragaum onion to increase the yield and quality under irrigated conditions. The highest bulb yield of 15.6 t ha -1 with the addition of recommended NPK+5 kg CuSO 4 ha -1 and the lowest bulb yield of 12.4 t ha -1 was recorded in NPK control. Mean DMP ranged from 1065 to 1199 kg ha -1 . In addition, the application of copper sulfate increased the Cu content both in sheath and bulb thus increased Cu uptake was noticed with increasing levels of CuSO 4 and the highest content and uptake was recorded with NPK+10 kg CuSO 4 ha -1 . Regarding the availability of copper in the soil, the availability was found to be sufficient at all levels of CuSO 4 addition. Increasing levels of copper application increased the soil Cu status and application of 10.0 kg CuSO 4 ha -1 recorded the highest copper availability in soil. Application of CuSO 4 @ 5 kg ha -1 also registered better quality attributes viz., total soluble solids, ascorbic acid and titrable acidity in onion bulb. The above field experiments conducted to optimize the rates of copper sulfate application to improve the yield and quality of aggregatum onion in four locations at Coimbatore district, showed that, soil application of 5 kg CuSO 4 along with recommended NPK @ 60:30:30 kg ha -1 was found optimal for obtaining higher bulb yield (15.6 t ha -1 ) and better quality in aggregatum onion with a BC ratio of 4.63.