Introduction
Litchi belongs to the family Sapindaceae and sub-family Nepheleae comprising about 150 genera and 2000 species [
1] and is native to Southern China. By virtue of its delicious taste, excellent flavour, pleasant fragrance, attractive appearance and high nutritional values, it has gained popularity in many parts of the world opening up new vistas for accelerated export opportunity. The principal chemical components of litchi fruits are carbohydrates, organic acids, vitamins, pigments, protein and fat. The fruit is very much enjoyed as a table fruit as well as in dried and canned forms. Jam, jelly, squash and cordial are also prepared from this fruit. Generally, the harvest maturity of the fruit is judged by the development of red colour on epicarp and flattening of tubercles [
2].
The foliar application of nutrients and bioregulators in specific plant developmental and critical stages improves the yield and quality. Foliar application of micronutrients has gained importance because it is a well-established operation to complete and boost plant nutrition. Foliar application can meet the expense of nutrients where absorption of nutrients from the soil is unavailable due to plant stress or in adverse soil conditions [
3].Application of plant bioregulators results in increased flowering, fruiting and fruit retention. Yield and quality of litchi fruit have been positively influenced by application of nutrients and plant growth regulators [
4].Reduced flower induction and fruit set, leading to inferior fruit quality are caused by zinc and boron deficiency [
5].The fruit quality is mainly affected by calcium via calcium pectate formation, which is associated with increase in the strength of cell wall and middle lamella [
6]. Micronutrients applied in optimum concentrations results in better plant growth which causes better flowering, higher fruit set that ultimately leads to higher yield [
7]. Zinc and boron take part in various enzymatic reactions and helps in sugar accumulation in fruit from source to sink [
8]. Boron is the only micronutrient not only specifically associated with either photosynthesis or enzyme function, but it is also associated with the carbohydrate chemistry and reproductive system of the plant.
Bioregulators are the product of natural and organic origin that stimulates plant to achieve their maximum growth and yield potential. Humic acid is especially beneficial in release of nutrients in the soil so that they are made available to the plant when needed. Humic acids play several important roles such as enhanced soil physical and biochemical activities by improving structure, texture, water holding capacity and microbial population [
9]; increase soil nutrients availability, especially micronutrients by chelating and co-transporting micronutrients to plants [
10].
Seaweeds are qualified to be used as biofertilizers, not only because they have a biological impact, but also because of their biocompatibility as they contribute to common biological compounds with plants. Seaweed extract are known as bio-stimulants due to presence of multiple growth regulators (cytokinin, auxins, gibberellins), macronutrients (Ca, P and K) and micronutrients (Fe, Cu, Zn, B, Mn, Co and Mo), which are necessary for plant growth and development [
11]. The foliar spray of seaweed extract is a common method to increase yield in many commercial crops [
12]. Multiple growth regulators have been found in seaweed extracts, including cytokinins, auxins, and gibberellins [
13]. Seaweed extracts have currently gained much beneficial significance because they can induce speedy growth and yield in cereals, vegetables, and fruit orchards, as well as in horticultural plants [
14]. Despite the unique and desirable characteristics, the major constraint in litchi cultivation causing lower yield and return are: poor fruit set [
15], heavy fruit drop [
16] [
4], fruit cracking [
16] and inferior fruit quality [
17]. Fruit drop in litchi is often thought to be associated with interruption in the endogenous hormonal level [
18], failure of fertilization, embryo abortion, internal nutrition and hormonal imbalance and internal factors like high temperature, low humidity and strong winds [
19]. Fruit cracking of developing fruits is a worldwide problem in litchi cultivation. High temperature, low humidity and low soil moisture conditions during fruit development promote this disorder [
20]. Due to the above constraints, there is a massive gap between demand and production of litchi, which increases the market price. To reduce this gap, many nutrients and bio-regulators are applied with recommended irrigation. Several investigations have been reported different benefits of seaweed extracts on growth, development and increased yield of many crops [
21,
22].
Bioregulators and nutrients are being used by the growers to increase the yield by improving the yield attributing parameters in litchi. The effect of nutrients in enhancing yield and fruit quality of litchi is a predetermined conclusion, but the valuable effect of nutrients in combination with bioregulators was yet to be fully explored. Under western part of Odisha, India, although the litchi growers are facing problem unstable and lower marketable yield and quality due to higher incidence of fruit cracking, fruit drop, low sugar content, higher acidity etc but no research has been conducted yet to study about the effect of bioregulators and nutrients on litchi. Based on this context, an experiment was conducted to assess the influence of bioregulators and nutrients on yield and fruit quality of Litchi cv. Bombai in western part of Odisha in India.
4.0. Discussion
The study provides well defined results than those carried out previously, since our research predicted that foliar application of nutrients and bioregulators exhibited positive responses in enhancing yield and physico-chemical parameters of litchi fruits.
The total number of fruits per plant was recoded highest in plants sprayed with ZnSO
4 @ 0.5% followed by those treated with humic acid @ 1%. This result corroborates the findings of [
26], who obtained maximum number of fruits per tree with spraying of zinc sulphate (0.6 %) in litchi. Similar findings were also reported by [
27]. Humic acids might act as a medium for transporting nutrients from the soil to the plant because they can hold onto ionized nutrients, preventing them from leaching away. When they arrive at the roots, they bring along water and nutrients to the plant. This would have helped the better availability and utilization of nutrients [
28]. The production of plant hormones and enzymes as well as the increase in root and shoot weight, chlorophyll content, and photosynthetic rate following humic acid application, might have also been shown to improve yields [
29].
The highest total fruit yield was recorded in plants with foliar feeding of 0.3% Borax which was found to be statistically similar to plants treated with seaweed extract @ 0.1% and seaweed extract @ 0.5%. This result was in closeness with the findings of [
30] who recorded enhanced total fruit yield over the control due to application of seaweed extract in Kiwi fruit. The essential nutrients contained in seaweed extracts viz. nitrogen, potassium, phosphorous, calcium, magnesium, sulphur, iron, sodium, zinc, and copper [
31], might reduces the production losses caused by cracking without compromising either the quality or the yield in crops. The results are also in agreement with those reported higher yield per plant using humic acid on peach [
32]; peach and apple [
33], pear [
34]. The increase in yield by boron application may be accredited to the positive effect of boron on increasing the rates of carbohydrate and RNA metabolism [
35] as well as on accelerating the transport of photosynthates from the leaves to the developing fruits [
36].
Among the treatments better response for higher number of marketable fruits and marketable yield was recorded in plants treated both 0.3% borax followed by 0.5% Zinc sulphate. Application of 1% Humic acid and 0.1% CaCl
2 also enhanced the marketable yield in litchi. This result was in line with the findings of [
37] who reported higher yield of marketable fruits per plant due to application of boron. The beneficial role of boron may be assigned to its optimal level which might have played important role in flowering and fruiting processes, nitrogen metabolism, hormone synthesis and cell division [
38]. It might helped in mobilization of food material to the fruits, thereby increasing the yield of healthy fruits. The findings are also in accordance with the reports of [
39]. Boron is also related to biosynthesis of auxin in the meristem of plants. Boron deficiency leads to a decreased level of bound auxin and a reduction of IAA oxidase activity [
40]. The marketable fruits represent the number of fruits excluding the cracked, pest and disease affected fruits. The data presented on fruit cracking in litchi in our experiment was fully supported with the findings of [
41] who concluded that extent of fruit cracking was reduced significantly with the application of boron. Reduction in fruit cracking with the application of boron has also been reported in litchi by [
42] and [
43]. Uptake of water and solutes are governed by the presence of boron, zinc and other micronutrients. In case of enhanced water uptake, solutes accumulate in the fruits and minimize the pressure on the skin resulting in less cracking. Auxin stimulation both due to application of bioregulators might be the reason for accumulation of building block at a faster rate and better execution of source-sink relation registering higher fruit setting, retention and less cracking.
Application of 1% Humic acid followed by 1.5% Humic acid enhanced the fruit set (%) and fruit retention (%) along with reduction in fruit drop (%) in litchi. The application of humic acid works as a chelating agent for nutrients already present in the soil and make them available to plant. The results obtained through this study is in line with findings of [
44] who studied the effect of humic acid in pomegranate and registered higher fruit set and fruit retention percentage with increasing rate of humic acid application. They also reported reduction in fruit drop percentage on higher humic acid rates. Positive and significant effect of humic acid has also been reported on grapes [
45] and on pear [
46,
47].
The enhanced fruit size (fruit length and fruit breadth) and fruit weight was obtained in plants treated with 0.3% Borax in litchi. This increase in length of litchi fruit may be due to boron application which have direct role in hastening the process of cell division and cell elongation due to which size and weight would have improved. The increase in the size of fruits was due to the rapid fruit development and the greater mobilization of food materials from the site of production to storage organs under the influence of applied nutrients. A similar result found with treatment of boron had also been reported in litchi [
48]. The present finding also corroborates with the finding of [
49] who reported no obvious differences in fruit vertical diameter, transverse diameter, lateral diameter and fruit shape index among fruits of all treatments and control. These results are in conformity with those reported by [
50] in guava and [
51] in litchi. The increase in fruit weight might be due to the rapid increase in the size of cells or it is also due the fact that foliar application of boron increased the fruit weight eventually by maintaining lighter level of auxins in various parts of the fruits which helped in increasing the fruit growth [
52].
The highest aril weight and lower seed weight was observed in plants treated with 0.3% Borax. The application of boron enhanced the aril weight and reduced the seed weight which as a consequence gave high aril /seed ratio. These findings are in line with the findings of [
51] in litchi and [
18] in apricot. The aril weight also depends on the fruit and seed size [
53] but is affected by the plant nutrition [
54]. Boron produced fruits with smaller seed. This may be due to involvement of boron in IAA metabolism which reduces seed size. The decrease in seed weight may be due to the fact that auxins induced parthenocarpic effect to some extent there by resulting lesser seed weight [
55].
There were no significant differences observed in plants treated with nutrients and bioregulators with respect to pericarp weight in litchi. Similar findings in mango were also opined by [
56] who reported that borax (1.0%) resulted no significant variation among treatments on peel weight of mango cv. Amrapali. Similarly, [
57] concluded a non significant effect of borax (0.5 and 1.0%) and calcium nitrate (2.0 and 3.0%) on peel weight of litchi fruits cv. Rose Scented.
Aril: seed ratio is the ratio between the weight of aril and weight of seed. The maximum aril: seed ratio was recorded in the fruits harvested from plants treated with boron and the minimum was observed in untreated fruits. It pertains to the fact that application of boron enhanced the pulp weight and reduced the stone weight which as a consequence gave high pulp/stone ratio [
58]. Similar findings are obtained by [
18] and [
59] in litchi.
The foliar application of borax had resulted in higher TSS and lower acidity in litchi cv. Bombai. Total sugars and reducing sugars were also improved due to application of borax. This is similar to the findings of [
60] who reported that pre-harvest application of boric acid resulted in higher TSS, total sugar, reducing sugar and lower titratable acidity. The data presented on acidity of litchi was fully supported with the findings of [
61] who observed that foliar spray of boric acid reduced acid levels in fruits of litchi. Lower acidity in fruits might be owing to increased sugar build up, improved sugar release into fruit tissues, and the conversion of organic acids to sugars [
62]. Another possible cause for limiting the titratable acidity might also be due to fast acid consumption of organic acid in respiration.The higher TSS : acid ratio and ascorbic acid content were obtained in litchi due to foliar application of 0.3% borax followed by humic acid @ 1.5% and humic acid @1% in litchi. Similar findings were also reported by [
63] who concluded that spraying of borax @ 0.5% or 1% increased TSS and decreased the acidity content in litchi fruits. [
59] also reported that spray of 0.4 per cent borax increase TSS, sugar and ascorbic acid content in litchi cv. Purvi while acidity was lowest.