Fruit Morphology of Durio zibethinus L. in Jelebu, Negeri Sembilan, Malaysia

Durio zibethinus L., commonly known as durian, is considered as King of Fruit in Southeast Asian countries viz. Malaysia, Indonesia and Thailand (Rosmaina et al., 2016; Teh et al., 2017; Husin et al., 2018). Durian is a seasonal tropical fruit and most of its varieties produce fruits twice a year, from November to February and June to September (Poerwanto et al., 2008). According to the Department of Agriculture (DOA) Malaysia, Malaysia has recorded more than 200 varieties of D. zibethinus, including 14 popular varieties, such as ‘Musang King’, ‘Hajjah Hasmah’ and D24 (DOA 2018).

       
Malaysia has concentrated on locating more D. zibethinus varieties that might serve as potential rootstock to boost fruit yield. Durian produces thorny, pungent-smelling edible fruit ranging in color from yellowish to dark green (Kothagoda and Rao 2011). Consumption of durian fruit in moderation has advantages in several aspects. As a daily dietary source, it is reported to have significant antioxidant, antibacterial and antipyretic properties and the propensity to treat chronic illness (Husin et al., 2018). The fruit of D. zibethinus has 147 calories, 5.3 g of fat, 27.1 g of carbohydrates, 3.8 g of fiber and 1.5 g of protein. It also has a high potassium content (436 mg), vitamin A (45 mg) and vitamin C (20 mg) (Durian fruit: Smelly but incredibly nutritious, Accessed on 15 November 2019, from https://www.healthline.com/nutrition/durian-fruit).
       
In view of its, great demand in the domestic and international markets, durian growers, researchers and the government rely heavily on fruit morphology to identify durian variants. This is also crucial for the official registration of new varieties in the DOA database. However, both growers and consumers are perplexed when it comes to recognizing and distinguishing the varieties (Retnoningsih et al., 2016). Thus, fruit morphological data is required for breeding effort to improve fruit quality. This research was significant for monitoring and providing information on the variability in fruit morphology of D. zibethinus to give comprehensive fruit identification criteria. The findings of this research may provide more comprehensive information for understanding the fruit quality of D. zibethinus in Jelebu and its breeding potential. The significance of plant species selection in breeding research is observed not only in D. zibethinus, but also used by Shukla et al., (2005) and Dey et al., (2018) in rice and Barbados cherry breeding.

This research was conducted from January 2020 to September 2020 in Jelebu, Negeri Sembilan, Malaysia and International Islamic University, Malaysia, Kuantan, Malaysia. A total of 53 durian accessions were procured from local durian sellers in Jelebu, Negeri Sembilan. There were three replicates for each accession. For determining the fruit characteristics, this research employed the DOA’s National Guideline for the “Conduct of Tests for Distinctness, Uniformity and Stability on Durian” (DOA 2018) and previous study by Retnoningsih et al., (2016). The length, width and circumference of the fruit parts were measured using a ruler and measuring tape. A weighing balance determined the weight of the sample data. The density of matured spines was determined by calculating the spines under quadrant 25 cm² that were randomly placed. Aril colors were determined following a standard color chart (Flutter: Material color conversion. Accessed on 10 November 2019, from https://medium.com/flutterworld/flutter-material-color-conversion-ad1574f25828.). All data were recorded and annotated.

Morphological data observation

 
A total of 26 external morphological structure and 15 internal structures were observed (Table 1) and recorded based on the fruit surface, pedicel characteristics, spines architecture, apex and base characteristics, fruit size, fruit segments, aril characteristics and number of seeds per fruit.
 

Table 1: Fruit morphology of D. zibethinus.

A) External structure

 
Fruit surface
 
There are six fruit shapes: circular, elliptic, oblate, oblong, obovate and ovate. The fruit colors could be yellow-green, light green, dark green, green-yellow, brown-green and brown. The fruit shape and color variations are prevalent among D. zibethinus variants, as reported by Sawitri et al., (2019) and Talip and Shamsuddin (2019). The results were similar to existing registered Malaysia’s popular varieties of (D24, D145 Beserah, D168 Hajjah Hasmah, D197 Musang King) as reported in DOA (2018). Fruit shape variation differentiates it from Durio grandiflorus (found in Peninsular Malaysia), whereas its shape is ellipsoid (Kostermans 1958). Furthermore, fruit color variation helps in distinguishing D. zibethinus from Durio dulcis (found in Malaysia). Kostermans (1958) reported that this species produced dark red to dark brown-red fruits. Fruit shape serves as significant criteria for plant identification. Several Medicago satvia (L.) cultivars were verified based on fruit shape (Moawed, 2016).
       
In case of fruit segments, the ridges along the dividing sections are either absent or weak, medium and strong. According to DOA (2018), D197 Musang King has a significant presence of ridges along the line of dividing sections, allowing enthusiasts and durian growers to readily identify the variety. Several accessions have small short spines along the line of dividing sections, representing an additional characteristic for durian fruit classification that varies from absent to few, moderate and many.

Spines’ architecture
 
There are six dominant spines and several accessions exceptionally have two kinds of dominant spines. Concave, pointed-concave, hooked, conical, convex and pyramidal spines have been identified. The observation of spine variations is not peculiar in botany research, as it is also utilised to characterise Artocarpus integ, as described by Lestari et al., (2020). The height and width of matured spines ranged from 0.70 cm to 1.73 cm and 1.10 cm to 2.43 cm, respectively. Retnoningsih et al., (2016) classified spine height into three categories for further identification: short (less than 1.00 cm), intermediate (between 1.10 cm and 1.50 cm) and long (above 1.50 cm). The density of matured spines is another quantitative characteristic that is assessed. The density of spines as one of the external morphologies of durian fruit and reported that the variations distinguished durian fruit varieties (Effendi, 2013).

B) Internal structure

 
Fruit size
 
According to the findings, length of durian fruits in Jelebu ranges from 11.27 cm to 50.60 cm, the width from 11.60 cm to 18.10 cm and circumference from 37.77 cm to 59.90 cm. The overall weight of durian fruit ranges from 665 g (the lightest) to 2.7 kg (the heaviest). Fruit weight may vary due to genetic as well as environmental factors (Effendi, 2013). Fruit weight has been one of the characteristics that drive market demands for various registered cultivars, such as D168 Hajjah Hasmah at 1.5 kg and D175 Udang Merah at 1.5 kg^-3 kg (DOA, 2018). Fruit weight has also become a vital observation element in other fruit productions, such as dragon fruit. According to Siddiq and Nasir (2012), dragon fruit size determined the fruit grade before it is marketed.
       
In fact, clients are more concerned with arils quality than with the size of the fruit. In Jelebu, the percentage of aril and seed to total weight of durian varies from 20.00 to 50.00 per cent. In contrast, the percentage ratio of fresh aril weight to a total of the fruit ranges from 12.00 to 26.00 per cent. Based on this parameter, arils can be divided into three categories: Grade A quality (premium quality with more than 25.00%), Grade B quality (average quality between 19.00-24.99%) and Grade C quality (low-grade quality with less than 18.99%). Shamin-Shazwan et al., (2021) mentioned that division of fruit grade was an appropriate method for determining the fruit price range. They noted that Durian Kampung has potential comparable to registered popular varieties such as Hajjah Hasmah and Udang Merah. 
 
Segments characteristics
 
Study revealed that durian have four to seven segments. Sawitri et al., (2019) employed this parameter as additional data in the morphological research as one of the characteristics to analyze the phylogeny of durian variants. Fruit chamber length spans from 8.50 cm to 17.00 cm and width from 3.90 cm to 6.26 cm. White color of the pericarp was present in all accessions. The pericarp thickness ranged between 0.20 cm and 1.20 cm. Furthermore, it is vital in fruit characterization due to the fact that Aziz et al., (2016) utilize it to provide morphological data of jackfruit (Artocarpus heterophyllus L.) to producers and consumers.
 
Aril characteristics and number of viable seeds per fruit
 
Aril color is the most often utilized characteristics in differentiating the fruit quality of durian varieties. This study found six aril colors viz. creamy white, light yellow 1, middle yellow 2, middle yellow 3, bright yellow 4 and bright yellow 5 (Fig 1).
 

Fig 1: Aril colour.

This research found that external and internal morphological data of durian fruits were valuable for recognizing and categorizing durian variants. At least five external morphological factors, such as shape, skin color, apex, base shape and dominant spine type, may be utilized in initial durian fruit identification. The internal structure of the fruit may be utilized to assess its quality. Furthermore, two critical parameters, the percentage of fresh aril weight over total fruit weight and aril thickness, must be considered while selecting the best fruit quality. Durian Jelebu has shown the ability to be at par with registered popular cultivars such as Musang King. Several Durian Jelebu accessions have a high percentage of fresh aril weight above 25.00 per cent and a fairly thick aril of more than 1.0 cm. However, more research should be conducted by increasing the sample quantity for each accession in order to evaluate the consistency of fruit data.

Special thanks to Ministry of Higher Education for supporting this research through grant research FRGS 19-108-0717.