Effect of Fertilizers on Growth, Yield and Quality of Embryo-cultured Sap Coconuts (Cocos nucifera var. sap)

Coconut trees are grown in more than 94 nations around the world. Coconut trees are grown in sea shoreareas between 20° north latitude and 20° south latitude. Coconut trees are a livelihood for 64 million households around the world. The need for coconut products continue to increase until 2030 (APCC, 2017; Rodrigues et al., 2018). The total coconut growing area worldwide was 12,303,924 hectares with a production of 60,773,435 tons in 2018 (FAO, 2020). Over 20 million farmers and their families rely on the earnings from coconut crops (Pham, 2016; Rethinam, 2006). Flowering starts at about 3-6 years for the tall coconut group and 3-5 years for the dwarf coconut group (Mike Foale et al., 2020). Coconut trees grow well well with adequate water and nutrient supplements (Bandyopadhyay et al., 2019). According to Thai et al., (2018), coconut trees have a high salt tolerance of up to 10.
       
Today’s rare and highly economically valuable coconut varieties include sap coconut (Macapuno coconut), pineapple coconut, striped coconut, bung coconut, etc. Natural coconut trees capable of producing wax fruits are all heterozygous (Mm), but coconut wax fruit is homozygous recessive (mm). Coconut trees have only 20-25% wax fruit (Rillo and Paloma, 1992) when propagated by fruit using the traditional method. The technique for propagating sap coconut using the embryo-cultured method has > 80% wax fruits (Thuy, 2022). According to a survey by Thuy et al., (2020b), farmers cultivating embryo-cultured sap coconuts had an income of 3.9 times higher than farmers cultivating traditional sap coconuts.
       
Vietnam has about 2,00,000 hectares area under coconut and Tra Vinh province has about 25,000 ha (Thuy, 2020). According to Tra Vinh’s Agriculture and Rural Development Department, in 2020, Tra Vinh province had 500 hectares of sap coconut growing area, of which 30 hectares were dedicated to cultivating sap coconuts by embryonic culture. There is limited research on fertilizer dosage for sap coconut trees. Currently, the fertilizer dosage of other coconut trees is used to apply for sap coconut trees.
       
In Sri Lanka, application of potassium fertilizers increased the number of fruit bunches, flowers, pollination rate, total number of fruits and fruit weight. Coconut trees without potassium fertilizer had reduced productivity and signs of potassium deficiency appeared clearly on the leaves after 4-5 years. The nut yield gradually increased with the application of potassium fertilizer from 0.5 kg, 0.8 kg and 1.0 kg KCl/tree/year. Fertilization treatment with 1.0 kg KCl/tree/year achieved the highest yield of 1,000 kg of dried copra /ha/year (Ouvier and Ochs, 1980).
       
Potassium (K), chlorine (Cl), nitrogen (N), calcium (Ca), sodium (Na), phosphorus (P), magnesium (Mg) and sulfur (S) are vital elements for coconut trees. Replacing 25% of inorganic fertilizers with microbial organic fertilizers (KOMIX) increased fruit yield/tree and economic efficiency of coconut orchards by 2 times as compared to control (Hai, 2017).
       
Coconut trees fertilized with 2 tons/ha of coir dust (coconut peat) increased fruit yield by 10%, pH from 5.5 to 5.7 and improved soil density (Long, 2007a). The fertilizer dosage of 70 kg N + 40 kg P₂O₅ + 110 kg K₂O/ha/year applied to Ta and Strawberry coconut trees recorded 80 fruits/tree/year in the Mekong River Delta.
       
In hybrid coconut varieties JVA1, JVA2 the application of 110 kg N + 57 kg P₂O₅ + 140 kg K₂O/ha/year, increased productivity to 155 fruits/tree/year and the oil content was as high as 68% (Research Institute for Oil and Oil Plants, 2012).
       
Application of fertilizers twice a year helped the tree grow, develop well and bear fruit early (Long, 2007b). The optimum amount of fertilizer for juice-drinking coconut trees during the fruit production period was observed to be 0.8 kg urea + 1.2 kg super phosphate + 0.8 kg KCl/tree/year (Research Institute for Oil and Oil Plants, 2012).
       
The appropriate amount of nitrogen and potassium fertilizer was applied for in the development stage from year 1 to year 4. In the 4^th year, fertilizer/plant/year was  Application of  500 g urea + 400 kg super phosphate + 600 kg KCl/plant/year was found optimum for 1 to 4 year old hybrid coconut trees (Hanh, 1999).
       
Ben Tre Provincial Agricultural Extension Center. (2014) showed that the fruit bearing coconut trees fertilized with 0.8 kg of urea + 1.5 kg of super phosphate + 1.5 kg of  KCl/tree/year tilized twice a year  at the beginning and the end of the rainy season was found optimum. Thuy (2018) opined that  fertilizer dose, application time, season, organic fertilizer and seedlings impacted the productivity and quality of embryo-cultured sap coconuts. Thus, it is imperative to study how fertilizers affect sap coconut development, yield and quality.

An experiment was conducted at the sap coconut farm of 5 years age at Luong Hoa commune in Chau Thanh district of Tra Vinh province, Vietnam, between January 2018 and December 2020. The experimental plot was located at 9°53'32"N latitude and 106°18'3"E longitude at an average altitude of 0.1 meters and experienced a tropical climate. The physico-chemical properties of experimental soil are presented in Table 1.
 

       
The experiment comprised of 5 treatments viz., inorganic fertilizer, applied once a month (T₁), inorganic fertilizer, applied every 3 months (T₂), inorganic fertilizer, applied every 6 months (T₃), replace inorganic nitrogen fertilizer with 100% manure, applied once a month (T₄) and replace 50% inorganic nitrogen fertilizer with 50% organic from decomposed cow manure, applied once a month (T₅). It was conducted in a randomized block design replicated 5 times. Each replication had 2 sap coconut trees.
       
Organic fertilizer comprised of decomposed cow manure. The organic fertilizer had 0.45% N, 0.23% P₂O₅, 0.5% K₂O  and 30% moisture. The amount of fertilizer was applied according to tree age (Table 2).
 

       
Fertilizer was applied evenly around the trunk base of the tree. The manure was dispersed across the soil’s surface in the shape of a circle, the size of which varied according to the tree’s age and leaf canopy. The fertilizer was mixed into the soil with a hoe. 
       
The circumference of the trunk base was measured uniformly at the scar of the 13^th leaf (Fig 1). The number of coconut leaves/tree, bunches/tree and fruits/bunch were collected once a month for 12 months. Data on fruit weight and the thickness of the coconut meat were gathered once a year.
 

       
The data collected was statistically analyzed using IBM SPSS Statistics 16 and Microsoft Office Excel 2010 as per Duncan test at 5% and 1% significance levels.

Effect of fertilizer on sap coconut growth and production
 
The trunk base circumferences of the 7-year-old sap coconut trees after 3 years of experiment in all the treatments were not significantly different from month 1 to month 11 and the growth rate between months too was not much different. The trunk base circumference of a 7-year-old embryo-cultured coconut tree ranged from 139-166 cm. This value was higher than the research of Duong (2013) in Tra Vinh and Ben Tre provinces, where the trunk base circumference of 7-year-old coconut trees ranged from 103 cm to 107 cm. The circumference of the coconut trunk base and trunk did not change in size due to the diversion of nutrients for growing flowers and fruits (Thuy et al., 2020a). The circumference in this study was lower than the findings of Mike Foale et al., (2020). The trunk base circumferences were generally 200-250 cm and remained the same as the trunk extended.
       
The circumferences of 5-year-old embryo-cultured sap coconut trees and 6-year-old trees had statistically significant differences in some treatments (Table 3). But by the 3^rd year, the trunk base circumference of the 7-year-old coconut tree was not different and the growth rate of the trunk base circumference was prolonged. The reason may be that the amount of fertilizer for 2 consecutive years did not meet the growth needs of embryo-cultured sap coconut trees. It is shown by the soil analysis values of total and available N, P and K, which are lower than the analysis results at the beginning of the season before the experiment was set up (Table 1). The circumference of the trunk base did not increase and there were no differences between treatments. It may be attributed to reduction in nutrients and energy to support the trunk when the plant started to flower.
 

 
Number of new leaves
 
There was no increase in the number of newly formed leaves/trees over the 3 years of the experiment (Table 4). The number of new leaves for embryo-cultured sap coconut trees ranged from 23.0-25.3/tree/year and there were no significant differences among the treatments. The rate of new leaf growth was speedy on an average of 14.3-15.9 days/new leaf. Duong (2013) reported that in (not embryo-cultured) sap coconut trees in Tra Vinh and Ben Tre provinces, on an average, there were 14-16 leaves/year (or one new leaf/24-26 days) for the tall coconut group and 16-18 leaves/year (or one new leaf/20-22 days) for the dwarf coconut group. The life cycle of a leaf on a healthy coconut tree is 2 years. If the coconut tree produces a new leaf each month, the coconut tree will have 12 new leaves/year. However, the frequency of leaf production depends on the tree’s age, climate and farming techniques (Mike Foale et al., 2020).
 

 
Flowering rate of the embryo-cultured sap coconut tree
 
The flowering rate of embryo-cultured sap coconut ranged from 19.8 to 21.7 days/inflorescence (Table 5) and there were no significant differences among treatments. The results were similar as the inflorescence growth rate/month as shown in Fig 2. Meanwhile, the average number of inflorescences/tree/year of 6-year-old Sap coconut trees was lower than that of 5 and 7-year-old trees. It may be attributed to the fact that the fertilizer did not meet the nutritional needs of the embryo-cultured sap coconut tree.
 

 

 
Fruits yield
 
On average, the number of fruits/tree  of a 7-year-old tree was higher than that of 6-year- old and 5-year-old trees by 26.7 and 16.9 fruits, respectively (Table 6). Upon comparision of treatments, it was found that 100% inorganic fertilizer once a month had the highest efficiency, a significant difference was observed compared to the remaining treatments on 5, 6 and 7  old sap coconut trees. The coconut-growing soil belonged to the sandy loam soil group, so the nutrient mineralization rate was fast. Therefore, fertilizing once every 1-2 months was more effective than every 3 or 6 months (Table 6). Application of  50% (T₄) or 100% (T₅) of decomposed manure was not different from other treatments (Fig 3). It may be due to slow nutrient availability in manure and it did not provide enough nutrients in time for coconut trees. Coconut trees grow continuously and bear fruits all year round. Therefore, if fertilized many times a year, the tree will grow well, flower continuously increase the number of inflorescences and have high productivity. If divided and applied many times, the same amount of fertilizer will help trees absorb it more effectively and avoid losses (Tran and Nguyen, 2011). A high yielding year for coconut trees frequently translate  into a decreased yield the following year, according to Abeywardena (1962), as the tree’s internal energy reserve get deplete due to the previous year’s high production. Coconut productivity is influenced by fertilizer, soil quality and irrigation water. It is reported that fertilizing for 3 consecutive years for embryo-cultured sap coconut trees of 5 years of age or older with the fertilizer dose of 118 N- 42 P₂O₅-54 K₂O applied evenly 12 times/year, recorded the highest efficiency in achieving productivity ranging from 72.3-93 fruits/tree/year. Bandyopadhyay et al., (2019) reported that fertilizer can be reduced by 25% if coconut farming uses drip irrigation with 3.2 times more water than traditional irrigation methods. On a 7-year-old coconut tree, fertilizing with 90 kg N + 60 P₂O₅ + 130 kg K₂O/ha/year recorded a higher number of leaves, fruit set rate and high yield than other treatments under saltwater intrusion conditions (Nguyen et al., 2020). In particular, 4 times/year fertilized plots recorded high yields with a statistically significant difference as compared to 2 times/year fertilized (Nguyen et al., 2020).
 

 

 
Average weight of fruit components of embryo-cultured sap coconut trees in 3 years of 5, 6 and 7-year-old trees
 
The highest efficiency was recorded with inorganic fertilizer, applied once a month (T₁) and it was   significantly different with other treatments about the average fruit weight in 3 years (Table 7). It  may be due to frequent fertilization (once a month), well timed to facilitate good growth and production of sap coconut trees. Duong (2013) reported that the average weight of sap coconuts in Tra Vinh and Ben Tre provinces ranged from 1.73 to 1.94 kg/fruit.
 

 
Effect of fertilizer on wax fruit ratio of embryo-cultured sap coconut trees in 3 years of 5, 6 and 7-year-old trees
 
The wax fruit/bunch ratio was 94-96% (Table 8). The results are consistent with the findings of Ramirez and Mendoza (1998), who who also reported a ratio of wax fruit of  98-100%.
 

Coconut trunk base and trunk are usually stable in size when the embryo-cultured sap coconut tree is over 7-years-old. The number of newly formed leaves ranged from 24.7-25.3 leaves/tree/year and the average number of newly formed inflorescences fluctuated from 16.1-18.5 inflorescences/tree/year. Fertilizer and its application time did not affect the wax fruits/bunch ratio on embryo-cultured Sap coconut trees.
       
A fertilizer dose of 118N-42 P₂O₅ - 54K₂O applied at one-month intervals on embryo-cultured sap coconut trees over 5 years old recorded the highest yield ranging from 72.3-93 fruits/tree/year and the ratio of wax fruits/tree from 94-96%.
       
It is necessary to continue studying many other fertilizer formulas for embryo-cultured sap coconut trees under different soil types, watering techniques, tree age and planting density to improve the productivity and quality of embryo-cultured sap coconut trees.

The Tra Vinh’s Department of Agriculture and Rural Development, the Vietnam Ministry of Agriculture and Rural Development and Tra Vinh University greatly appreciate the authors’ support of this research.
 
Author contributions
 
Conceptualization: Thuy P.T.P. and Nhu C.T.; Methodology: Thuy P.T.P. and Nhu C.T; Software: Thuy P.T.P., Nhu C.T., Nga S. T. and Tron T.T.T.; Formal analysis: Thuy P.T.P. and Nhu C.T; Investigation: Thuy P.T.P.; Resources: Thuy P.T.P. and Nhu C.T; Data curation: Thuy P.T.P.; Writing and original draft preparation: Thuy P.T.P. and Nhu C.T.; Writing, reviewing and editing: Thuy P.T.P. and Nhu C.T.; Visualization: Thuy P.T.P. and Nhu C.T.; Project administration: Thuy P.T.P.; Fund acquiring: Thuy. P.T.P. Each author has reviewed and approved the published version of the work.
 
Funding
 
This study was financed by the Vietnamese Ministry of Agriculture and Rural Development (Project number: HÐ: 11/2017).

The authors have disclosed no conflict of interest.