Studies on foliar application of micronutrients and bio stimulants on storage attributes of rose onion

Introduction

Onion (Allium cepa L.) is an important culinary vegetable that belongs to the Alliaceae family, with a chromosome number of 2n = 16. It is native to South West Asia, from where it has spread around the world. This crop is mainly grown for local consumption and export. Onions are a versatile, bulbous crop with shallow roots. Although they are botanically classified as herbs, they are grown as annuals for bulb production and as biennials for seed.

Rose onion production heavily relies on mineral fertilizers, as increasing NPK levels has been shown to enhance mineral uptake by the plants. Yet, this benefit comes with a steep price: continuous fertilizer use causes environmental pollution and jeopardizes the sustainability of agricultural practices (Bagali et al., 2012) [2] . Despite these risks, farmers frequently apply excessive amounts of fertilizer to boost their output. Such over use of chemical fertilizers creates significant damage to our ecosystem, atmosphere and human health (Mostafa et al., 2021) [15] , compounding the problems with higher costs and the degradation of soil health. The foliar application of micronutrients like Zinc sulphate (ZnSO 4 ) and Ferrous sulphate (FeSO 4 ), combined with humic acid, has shown to promote better growth and higher yields in various vegetable crops (Zandonadi et al., 2007) [23] . Zinc and Iron supports healthy plant growth and development (Atiyeh et al., 2002) [1] . Humic acid offers many advantages for plants. It helps with nutrient uptake in vegetable crops (Cimrin and Yilmaz, 2005) [6] , adjusts hormone levels by acting as a growth regulator, improves soil quality (Serenella et al., 2002) [19] , boosts photosynthesis and cell elongation. One effective strategy to improve crop productivity and enhance rose onion quality is the foliar application of micronutrients paired with bio stimulants, such as seaweed extract (Datta et al., 2003; Saravanan et al., 2003) [7,18] . Applying bio stimulants directly to leaves has shown to promote vegetative growth, increase yield and improve the quality of bulbs. Seaweed extract, a modern natural organic fertilizer, is exceptionally nutritious and positively impacts plant growth and development (Battacharyya et al., 2015) [3] , subsequently leading to higher yields (Dhargalkar and Pereira, 2005) [8] . Furthermore, seaweed is recognized as a rich source of both micronutrients and macronutrients (Shehata et al., 2011) [21] . Specifically, seaweed extract serves as a fertilizer supplement containing modest amounts of macronutrients (nitrogen, potassium and phosphorus), alongside vital organic substances like amino acids, key plant growth regulators (auxin, cytokinin and gibberellins) and various trace elements (Begum et al., 2018) [4] .

Materials and Methods

Experimental site was conducted in the Late Kharif season during 2022-2023 and 2023-24 at All India Network Research Project on Onion and Garlic (AINRPOG) at the Main Agricultural Research Station (MARS), University of Agricultural Sciences (UAS), Dharwad The site is located at Northern Transitional Zone (Zone VIII) of Karnataka state, situated at 15 0 26' North latitude, 75 0 07' East longitude and at an altitude of 678 meter above the mean sea level. The soil is deep black and well drained with uniform texture. The research was executed in a Complete Randomized Design (CRD) with three replications and twelve treatment combinations. The rose onion local variety was used, known for its deep scarlet red colour and high pungency. Harvesting was done uniformly at 110 days after sowing. Bulbs were harvested for storage assessments. two kg per treatment of bulbs were used for storage studies. Additionally, soil factors were evaluated both prior to and following harvest. Observations were recorded at 20, 40 and 60 days after storage on physiological loss in weight (%), sprouting percentage, rotting percentage and marketable bulb (%).

The cumulative loss in weight of the bulbs at different DAS was calculated and expressed as percent physiological loss in weight of bulbs using the formula given below:

(P 0 -P 1 ) or (P 0 -P 2 ) or (P 0 -P 3 ) PLW (%) = × 100 P 0 Where, P 0 = Initial weight P 1 = Weight after 30 days P 2 = Weight after 40 days P 3 = Weight after 60 days

During storage of bulbs which are showing at least one sprout longer than 0.5 mm size was counted as sprouted bulbs and sprouting percentage was calculated by following formula as mentioned below: Number of bulbs sprouted Sprouting percentage (%) = × 100 Total number of bulbs stored

During storage number of rotten bulbs was counted and decay percentage was calculated by following formula as mentioned below: Number of bulbs rotted Rotting percentage (%) = × 100 Total number of bulbs stored

At the end of storage period (60 DAS), the rotted and sprouted bulbs was separated and discarded. Only weight of healthy bulbs was recorded. The marketable bulb percentage was calculated by using the following formula as mentioned below:

Weight of the healthy bulbs obtained Marketable bulb (%) = × 100 Initial weight of bulbs stored

Results and Discussion

At 20 DAS (days after storage), significantly lower percentage of physiological weight loss of 2.87 and 2.48 percent for 2023 and 2024 respectively with an average of 2.67 percent was accounted with T 12 (T 1 + humic acid 2 kg ha -1 + FeSO 4 + ZnSO 4 @ 25 kg ha -1 each) followed by T 8 (T 1 + Seaweed extract of Ascophyllum nodosum 1.5 ml l -1 + FeSO 4 + ZnSO 4 @ 25 kg ha -1 each) with 3.12, 2.66 and averaging 2.89% during 2023 and 2024. The higher percentage of physiological weight loss of 4.15 and 3.46 percent for 2023 and 2024 respectively with an average of 3.80 percent was accounted with T 1 (RDF). At 40 DAS, the treatment T 1 exhibited the higher percentage of physiological loss of weight in both 2023 and 2024 season, with values of 12.13 and 10.37 percent, averaging 11.25 percent. T 12 had the lower physiological loss of weight, with values of 9.43 and 8.98 percent, resulting with an average of 9.20 percent followed by T 8 (10.19, 9.18& 9.69%) and T 4 (10.33, 9.44 and 9.89%) respectively at 40 DAS. At 60 DAS, T 12 maintained supremely by registering significant lower physiological weight loss of 15.39 and 14.97 percent for 2023 and 2024 respectively with an average of 15.18 percent. However, treatment T 1 noticed with higher physiological weight loss of 18.07 and 16.67 percent with an average of 17.37 percent respectively at 60 DAS. This implied the critical role of micronutrients and bio stimulants with their management in maintaining plant health and minimizing weight loss during the growth period. Absence of adequate fertilization, plants are more susceptible to stress factors such as drought and disease, leading to increased respiration rates and moisture loss, which contributes to a higher physiological weight loss in onion (Jinkovic et al, 2016) [13] . The reduced PLW under these treatments is attributable to higher dry matter content, firmer bulb structure and thinner neck diameter, which collectively minimize water loss and respiration rates. Similar findings were reported by Sharma et al. (2014), who observed that balanced nutrient supplementation improved bulb firmness and reduced PLW in stored onions. At 20 DAS (days after storage), significantly lower sprouting percentage of 0.58 and 0.56 percent for 2023 and 2024 respectively with an average of 0.57 percent was accounted with T 12 (T 1 + humic acid 2 kg ha -1 + FeSO 4 + ZnSO 4 @ 25 kg ha -1 each) followed by T 8 (T 1 + Seaweed extract of Ascophyllum nodosum 1.5 ml l -1 + FeSO 4 + ZnSO 4 @ 25 kg ha -1 each) with 0.65, 0.59 and averaging 0.62% during 2023 and 2024. The higher sprouting percentage of 1.29 and 1.23 percent for 2023 and 2024 respectively with an average of 1.26 percent was accounted with T 1 (RDF). At 40 DAS, the treatment T 1 exhibited the higher percentage of sprouting percentage in both 2023 and 2024 season, with values of 3.18 and 2.77 percent, averaging 2.98 percent. T 12 had the lower sprouting percentage with values of 1.53 and 1.45 percent, resulting with an average of 1.49 percent followed by T 8 . At 60 DAS, T 12 maintained supremely by registering significant lower sprouting percentage of of 7.86 and 7.68 percent for 2023 and 2024 respectively with an average of 7.77 percent. However, treatment T 1 noticed with higher sprouting percentage of 8.98 and 8.51 percent with an average of 8.75 percent respectively at 60 DAS. The sprouting reduction can be linked to improved bulb dormancy, delayed physiological aging and enhanced accumulation of inhibitory compounds under humic acid and micronutrient-enriched conditions. Feibert et al. (2023) [11] reported that foliar application of humic substances reduced sprouting incidence in onion during storage. Nahar (2021) [16] similarly confirmed that balanced nutrient management delayed sprouting by improving bulb dormancy and reducing stress metabolism. Gupta et al. (2021) [12] also highlighted that the use of bio stimulants and micronutrients lowered sprouting incidence by strengthening bulb structure and delaying sprout initiation. At 20 DAS (days after storage), significantly lower rotting percentage of 2.87 and 2.48 percent for 2023 and 2024 respectively with an average of 2.67 percent was accounted with T 12 (T 1 + humic acid 2 kg ha -1 + FeSO 4 + ZnSO 4 @ 25 kg ha -1 each) followed by T 8 (T 1 + Seaweed extract of Ascophyllum nodosum 1.5 ml l -1 + FeSO 4 + ZnSO 4 @ 25 kg ha -1 each) with 3.12, 2.66 and averaging 2.89% during 2023 and 2024. The higher rotting percentage of 4.15 and 3.46 percent for 2023 and 2024 respectively with an average of 3.80 percent was accounted with T 1 (RDF). At 40 DAS, the treatment T 1 exhibited the higher percentage of rotting percentage in both 2023 and 2024 season, with values of 12.13 and 10.37 percent, averaging 11.25 percent. T 12 had the lower rotting percentage with values of 9.43 and 8.98 percent, resulting with an average of 9.20 percent followed by T 8 (10.19, 9.18& 9.69%) and T 4 (10.33, 9.44 and 9.89%) respectively at 40 DAS. At 60 DAS, T 12 maintained supremely by registering significant lower rotting percentage of of 15.39 and 14.97 percent for 2023 and 2024 respectively with an average of 15.18 percent. However, treatment T 1 noticed with higher rotting percentage of 18.07 and 16.67 percent with an average of 17.37 percent respectively at 60 DAS. The reduced rotting can be attributed to firmer bulb texture, improved neck closure, higher antioxidant content (ascorbic acid) and enhanced plant defense mechanisms under humic acid and seaweed extract supplementation. El-Sawy et al. (2021) [10] reported that humic acid applications improved disease resistance in onions by enhancing bulb firmness and defense responses. Similarly, Gupta et al. (2021) [12] demonstrated that the combined use of seaweed extracts reduced storage rots in onions by enhancing systemic resistance. The treatment T 12 yielded significantly higher marketable bulb in 2023 and 2024 seasons with values of 57.15 and 59.05 percent, averaging 58.10 percent respectively, T 8 was statistically parallel with former and registered marketable bulb of 55.95 and 58.47 percent, averaging 57.21 percent. T 1 (RDF) was recorded lower marketable with the values of 46.46, 51.34 percent averaging 48.90 percent respectively in 2023 and 2024 and average of both years compared to all other treatments. The addition of micronutrients such as zinc and iron likely played a critical role in enhancing the physiological health of the plants, contributing to improved bulb quality and weight as documented by El-Hadidi et al. 2016 [9] , which is in line with the result of current study. The synergistic effect of nutrient application helps in producing bulbs with better marketable quality. Next best treatment was T 8 with average marketable bulb content, indicating that the inclusion of seaweed extract along with the other nutrient strategies can also lead to favorable outcomes. The humic acid provides additional nutrients or bioactive compounds that enhance growth and quality. RDF was recorded lower of marketable bulb (average of 48.90%) as compared to all other treatments, which might be due to less number of additive factors (only NPK fertilizers). RDF recorded the lower number of marketable bulbs. The substantial disparity between the RDF and the treated groups underscores the critical role of nutrient management which plays important role in the production of marketquality onions and statement was endorsed with earlier researcher (Kumar et al., 2018) [14] .

Conclusion

The study clearly demonstrated that both micronutrients and bio stimulants significantly influence the storage attributes of rose onion. Minimum loss in physiological weight, sprouting percentage, rotting percentage and higher marketable bulb were achieved with 100% RDF (T 1 ) + Humic acid 2 kg ha -1 + FeSO 4 + ZnSO 4 @ 25 kg ha -1 each. Thus, for optimizing rose onion production. a combination of micronutrients and humic acid with 100% RDF is recommended to achieve the best balance between high productivity, superior quality and maximum marketable bulb. Future research could further explore the role of integrated nutrient management with organic sources to enhance sustainability in rose onion production.

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