Review: Population establishment and genetic study of Java, Bali and Sumatra Salak crossing

Indonesia has a high diversity of Salak. The cultivars that are widely cultivated are Salak Pondoh, Java, Bali, and Sumatra. These cultivars have a delicious taste and different superiorities. The crossing was needed to combine their superiorities and obtain superior varieties. Crosses result in 17 hybrids Salak populations with moderate to high diversities. Salak Sidimpuan (SDP, SDS, SDM) and the progenies derived from crossing Salak Sidimpuan as female parents have a larger plant and leaflets size but have relatively few leaves. Salak Bali, Salak Sidimpuan, and the progenies derive from crossing Salak Sidimpuan as the female parents have a flesh thickness of 1.0 cm. A crossing between Salak Pondoh (non-astringent) x Salak Java (astringent) and a crossing between Salak Sidimpuan (astringent) x Salak Sanjung (astringent) produced progenies whose fruit have an astringent taste. On the other hand, the crossing between Salak Sidimpuan x Mawar produce partially astringent fruit and some non-astringent. Selection of these progenies resulted in three superior varieties, namely Sari Intan 48, Sari Intan 541, and Sari Intan 295, and an ideal candidate variety, namely PH-MW (35-13), with the advantage of thick flesh, sweet and non-astringent taste, fruit weight > 65 g, and fragrance.


Introduction
Salak is one of the tropical fruits native to Indonesia that is very popular with the public because of its sweet, sour, delicious taste and high nutritional value >1@.Every 100 g of Salak fruit contains 77 calories, 0.5 g of protein, 20.9 g of carbohydrates, 28 mg of calcium, 18 mg of phosphorus, 4.2 mg of iron, 0.04 mg of vitamin B1, and 2 mg of vitamin C >2@.The national production of Salak from 2012 to 2020 has fluctuated, although it tends to increase, reaching 1.2 million tons in 2020.The per capita consumption of Salak in Indonesia in 2021 is 1.6 kg.The demand for Salak exports, among others, from Singapore, Hong Kong, Malaysia, China, Japan, the Netherlands, and the United States.It shows that Salak has a high commercial value as a source of foreign exchange for the country >3@.
Indonesia has a high genetic diversity of Salak, which is spread in almost every province.From the exploration results, 13 species of Salak have been found, which are distributed in Sumatra, Java, Bali, and Kalimantan.Three are edible species: Salacca zalacca, S. sumatrana, and S. affinis >4@.In Indonesia, Salak has specific advantages compared to other fruit commodities; the fruit can be harvested 2-3 times per year if the management is good.Meanwhile, the demand for Salak fruit from other countries is relatively high, which can only be fulfilled if it is still needed to meet domestic consumption.
Generally, Salak fruit consumers like Salak which has thick flesh, a sweet taste, a little/nonastringent taste, and a long shelf life >5@.Varieties with characters that match consumer desires are very limited in number.Salak Pondoh has the advantage of having a sweet fruit taste, but the weakness is the thin flesh.Salak Sidimpuan has a large fruit size, thick flesh, and several cobs/bunches of 3-5 pieces, but the flesh is relatively astringent, especially if the fruit maturity is not optimal.Likewise, Salak Bali Gondok has the advantage of thick flesh, but the flesh is overly chelated.Therefore, assembling Salak varieties to combine the special characters mentioned is necessary.To assemble the high-yielding varieties, the parents must have the intended special characters, wide genetic variability, and high heritability values.The wide genetic variability of a character will provide a greater opportunity to select the best character.Characters with high heritability will be easy to inherit and can make the selection in the early generations >6@.The characteristics of fruit flesh weight, flesh thickness, fruit tannin content, and sugar content have high heritability values >7@.
A series of studies supporting the assembly of Salak varieties have been carried out, including research on the distribution of varieties, identification of varietal diversity in several production centres, and collection of Salak germplasm in Indonesia >8,9,10@.The same studies are carried out on the flowering of Salak Java and Bali >11-13@.The results of the study found that the parents had superior characteristics, including the character of thick flesh owned by the Salak Bali, the sweet taste of the fruit without astringent owned by the Salak Pondoh, the character of a large number of cobs, and thick flesh held by the Salak Sidimpuan.Crossing between several cultivars of Salak is expected to produce new superior varieties which sweet taste thick flesh, non-astringent and productive.This study aims to review the results of research on population formation and genetic studies of crosses of Salak Java, Bali, Pondoh, and Sidimpuan.

Flowering and hybridization in Salak
Salak is a fruit plant with dioecious, monoecious, and hermaphrodite flowering types.Generally, zalacca var.zalacca is a dioecious plant because male and female flowers are found on different plants.Salak plants with only male flowers never produce fruit.So far, S. zalacca var.amboinensis is classified as a monoecious plant because it has separate male and female flowers but is present in one plant >14@.The Salak Bali plant (S.zalacca var.amboinensis) is andromonoecious, namely hermaphrodite flowers, and male flowers are present in one plant and not monoetic >12@ The varieties of hermaphrodite salacca include Salak Bali Gula Pasir and Gading >15@.Salak Bali contains Salak plants whose pollen is sterile, so they cannot produce fruit, often called "muani" or male Salak >12,16@.As in the Salak Bali, inflorescences that produce fruit are called "female" inflorescences >17@.The Salak Sidimpuan plant is not dioecious because the panicles contain hermaphrodite flowers with both females and males on the same plant >18@.S. sumatrana, for example, salak Sidimpuan, is selfpollinating, and climatic conditions strongly influence the success of pollination >19, 20@.
No insects are found in Salak Bali flowers, so Salak Bali is said not to need a pollinator to produce fruit.The formation of seeds and fruits of Salak Bali is thought to be through apomixis or an asexual reproduction system >21@.At Salacca zalacca var.zalacca, such as Salak Condet and some varieties in Salak Java, flowers are often surrounded by insects as pollinators that help the continuity of the pollination process.Insects are found on both male and female flowers.The types of insects found are Trigona sp.(Hymenoptera), Rhynchophora palmarum L. (Coleoptera), and Curculionid beetles (Curculionidae) >16@.
The flowering nature of the dioecious Salak plant with male and female flowers located on different individuals causes it to naturally cross-pollinate so that many new allele combinations are formed in its offspring.This factor could cause Salak plants to have a wide genetic diversity.A broad genetic background is an invaluable asset in a plant breeding program.A study of population diversity in Salak plants is needed to determine the extent to which superior phenotypes can be selected.

Population formation of Salak crosses
In collaboration with the Tazagawa Bogor Foundation, Indonesian Tropical Fruit Research Institute has hybridized the elders of salak Pondoh, salak Sidimpuan, and salak Java.In 2003, as many as 13 new hybrids were obtained from crosses between Sidimpuan x Mawar, Sidimpuan x Sanjung, Pondoh x Sanjung, Pondoh x Mawar, and Pondoh x Java, Mawar.x Sidimpuan, Mawar x Afinis; two old hybrids, namely Salak Mawar and Salak Sanjung, and four local varieties for comparison, namely Salak Sidimpuan Merah (SDM), Salak Sidimpuan Semburat (SDS) and Salak Sidimpuan Putih (SDP), and Salak Gula Pasir and they have been planted at Aripan research station in 2004.
Flower formation and fruit set are strongly influenced by the conditions of the surrounding.In the case of Salacca species, the most influential environmental factors for fruit set are air temperature, air humidity, rainfall, and light intensity >20@.The percentage of fruit sets resulting from crosses is low, ranging from 21.82% -59.09% >22@.It is due, among others: 1).female flowers do not bloom simultaneously in one cob and bunches.Usually, the flowers located at the base and centre bloom earlier than the tips of the cobs, so the flower tips are not ready to be pollinated; 2).Flowers on female and male plants sometimes bloom at different times, and the locations are far apart, so they must store the pollen; 3).During storage and transportation, pollen's viability is lower than fresh pollen; 4).The rainy season causes many of the flowers that have been pollinated to rot.Snake fruit rot is caused by at least two types of fungi, namely the Fusarium and Marasmius groups >23@.The production or quantity of salak fruit during the rainy season is less than during the dry season >24@.
Level of viability can determine the quality of pollen >25@.Factors that affect salak pollen viability are variety, storage method, and environment.At eight weeks of storage, the viability of salak Bangkalan, Suwaru, and Kersikan pollen are 53.30%,56.70% and 55.91%, respectively.Pollen from salak Bangkalan during 1 to 8 weeks of storage shows the lowest viability compared to salak Suwaru and Kersikan.On storage for eight weeks, the viability of salak pollen stored in the freezer is 74.59% and more significant than the viability of salak pollen stored in the exicator, which is 36.02%>26@.The storage of Pondoh pollen in the refrigerator and freezer for up to 3 weeks can form fruit sets similar to what farmers usually do >27@.Humidity and temperature are the storage factors affecting salak pollen viability >28@.
The seed germination percentage for each accession also varies, ranging from 3.64% to 98.38%.The low percentage of seed growth is mainly caused by the failure of the seeds to germinate due to the seeds being attacked by fungus and eventually rotting.This fungal attack is caused by the remaining pulp of the seeds and the presence of several rotten fruits in a bunch >22@.

Evaluation of vegetative growth
Salak breeding population test or multilocation test is carried out in three planting locations: Tanjung Pinang-Riau Islands, Kampar district-Riau, and Aripan Solok research station (West Sumatera) through the Participatory Breeding pattern.Environmental factors, genetics, and interactions between environment and genetics influence the growth of Salak plants.The growth of Salak plants in Aripan can be seen in Table 1.
At Aripan-Solok research station, the number of leaves observed range from 13-22 leaves three years after planting.Generally, Salak Bali Gula Pasir (SBGP), Salak Sidimpuan (SDP, SDS, SDM), and accessions from crossing use Salak Sidimpuan (SDP, SDS, SDM) and Salak Mawar (MWR) as female parents have the least number of leaves.Otherwise, crosses that used Salak Pondoh (PH) as female elders have the most leaves number.Salak Pondoh Hitam seedlings produced a total of 12 leaves in 13 months so that new leaves will emerge at 33-day intervals >29@.
The length and width of the terminal leaflets of the observed accessions also varied.Genotypes which short and narrow terminal leaflets are SBGP, MWR, and accessions from crossing use Salak Pondoh (PH), MWR as female parents.Otherwise, Salak Sidimpuan (SDP, SDS, SDM) and crossing use Salak Sidimpuan as females have long and wide terminal leaflets.Likewise, for the length and width of the leaflets.Salak Sidimpuan and crossing use Salak Sidimpuan as females have the longest and the widest leaflets.Usually, Salak Bali has the smallest leaves than Salak Pondoh and S. sumatrana (Salak Sidimpuan).Terminal leaflet's size can be used as a differentiator between Salak plants >30@.The numbers in one column followed by the same letter are not significantly different according to the Scott-Knott test p : 0.05 In the Kampar district, all accessions possess a similar growth pattern to the Solok research station.Salak Sidimpuan (SDP, SDS, SDM) or crossing use salak Sidimpuan as female parents generally have the largest leaves sizes (terminal leaflets and leaflets), but they have the least number of leaves (Table 2).
Salak plant which propagated from seed requires 3-4 years from sowing till flowering >33, 34@.Well-sized suckers will flower after 2-3 years.Environmental factors such as temperature, humidity and nutrients also influence the start of the generative phase >29@.At 36 months after planting all the accessions tested had flowered, although the percentage of flowering plants varied between accessions (Table 3.).Salak accessions that have flowered more than 50% are PH-MW, SJG, and PH-MJ.Their accessions come from crosses with Salak Pondoh (PH) as female parents.Otherwise, accessions come from Salak Sidimpuan (SDP, SDS, SDM) and crosses with Salak Sidimpuan as female parents have low percentage of flowering plants (less than 10%).If related to the number of leaves, it turns out that PH-MJ, PH-MW, and SJG have the highest number of leaves which statistically have the same value, namely 30 -32 leaves.There is a correlation between the number of leaves and the number of plants that have flowered (r = 0.92).It means that the more the number of leaves, the faster the plant will flower or the more mature the plant will be..10 a Sources : >32@ Note: The numbers in one column followed by the same letter are not significantly different according to the Scott-Knott test p : 0.05 At 36 months after planting, the plants that flowered generally are male plants, or the percentage of male plants is more than that of female plants (Table 3).It shows that male plants flower faster than female plants.At 28 months after planting, Salak plants that flower are male >29@.

Fruit evaluation on several Salak accessions
The characterization results showed that the fruit quality of each accession varied.Variations also occurred between individuals in one cross (inter population) (Table 4).Fruit weight, and total soluble solid characters have a broad genotype variability prediction values and phenotype variability >35@.This broad genetic variability may be caused by salak open pollination system.The similar individual performance on the population might not reflect similar genetic properties.Moreover, the significantly different morphological performance must come from different genetic properties.The Salak plants evaluated are derived from generative propagation (seeds), so individuals in one cob do not necessarily have the same genetic composition and phenotype.Therefore, in Salak plants, the selection is carried out per individual plant, and the selected candidates are propagated vegetatively >36@.
Consumers like fruit having thick flesh.The observations show that the accessions should have flesh thickness ranging from 0.47 to 1.20 cm.Accession with the thickest flesh is Mwr-Sp-35.13(1.20 cm).The flesh thickness positively correlates with Rubisko activity on Salak leaves.Rubisko activity has a direct and quite large effect on the thickness of the fruit flesh.Crosses between Salak Pondoh x Salak Bali and between Salak Bali x Salak Pondoh and crosses Pondoh x Pondoh have lower Rubisko activity than crosses between Salak Bali x Salak Bali.It indicates that crosses using Salak Pondoh as both female and male parents produce thin fruit [37].Salak Sidimpuan (SDP, SDM), crosses in which one of the parents used Salak Sidimpuan have the most-thick flesh (Table 4).Super has a sugar content of 18.28•brix >11@.Salak Pondoh Hitam is the best general combining parent for sweetness >39@.The parents who displayed good general affinity became a heterosis phenomenon in a cross so that the appearance of the firstgeneration offspring (F1) was better than the two parents >40].
One of the characteristics of Salak fruit that consumers like less is the relatively high astringent taste in Salak fruit.The tannin content of the fruit influences the astringent taste in Salak fruit.Salak fruit with an astringent taste has a higher tannin content than non-astringent.Salak Gula Pasir and Salak Pondoh, which are non-astringent, have tannin content of 0.1870% and 0.2007%, respectively.In contrast, Salak Sidimpuan and Sanjung, whose fruit has an astringent taste, contain tannins of 0.5108% and 0.3219%, respectively >41@.The evaluation results showed that a crossing between the Salak Mawar (Mwr) x Sidimpuan Putih (Sp) and Salak Pondoh x Salak Pondoh produced nonastringent Salak fruit.Meanwhile, the crossing between Salak Pondoh (PH) x Salak Java (K, M, MJ) and Salak Sidimpuan x Salak Sanjung (Sjg) produce fruit with an astringent taste.One of the characteristics of Salak Pondoh is that the fruit does not have an astringent taste, even though the fruit is still young.Salak Java, Salak Sanjung, and Salak Sidimpuan (SDP, SDM) used in this study has astringent taste fruit >36, 42@.A cross between Pondoh Hitam (non-astringent) x Bali Putih (astringent) have a higher leaf tannin content.It was significantly different from a cross between Pondoh Hitam (non-astringent) x Salak Gula Pasir (non-astringent) >39@.
The result of the evaluation and selection of Salak crosses has released three new superior varieties: Salak Sari Intan 48, Sari Intan 295, Sari Intan 541, and one superior variety candidate, namely PHMW .These varieties have the advantage of thick flesh, sweet and non-astringent taste, fruit weight > 65 g, and fragrance.

Conclusion
Salak species have different types of flowering.Terminal leaflets' size can be used as a differentiator between Salak plants.Male plants flower faster than female plants.Cross-parents influence the quality and quantity of their progeny's fruit.

Table 1 .
Average number of leaves, length and width of terminal leaflets and leaflets on several accessions of Salak in Aripan-Solok research station at three years after planting.

Table 2 .
Average number of leaves, length and width of terminal leaflets and leaflets on several accessions of Salak in Kampar district at three years after planting.

Table 3 .
Percentage of flowering plants, male and female plants on several Salak accessions at 36 months after planting in Kampar district.