Morpho-palynological assessment of the genus Terminalia L. (Combretaceae) in Egypt

Introduction

The family Combretaceae is an advanced flowering plant with a worldwide tropical distribution (Hutchinson 1969). The family has a different number of genera, ranging from 16 (Rendle 1956) to 20 (Mabberley 2017). Cronquist (1981) recognized 400 species, while Willis (1966) recognized 600 species within this family. These variations in the number of genera and species are due to a lot of synonyms and the either clumping or splitting of genera (Scott 1979). Classification of Combretaceae has been faced with many opinions since Exell (1954) as the family was divided into two subfamilies, Strephonematoideae with the monotypic genus Strephonema and Combretoideae with the rest of the genera. More recent treatments by Exell and Stace (1966) and Stace (2007), divided the family into two tribes, Laguncularieae and Combreteae. The latter tribe was subdivided into three subtribes: Combretinae, Pteleopsidinae and Terminaliinae. Terminalia is the second largest genus after the genus Combretum under the subtribe Combritenae, tribe Combreteae, family Combretaceae, suborder Myrtineae, order Myrtales, (Stace 2002, Heywood et al. 2007). Species under this genus are shrubs, trees, and creepers distributed in tropical and subtropical regions, especially in Africa and savannas (Arundhati et al. 2020). Many of its species have high economic value as they are used widely in commercial applications and Chinese traditional medicine (Zhang et al. 2019). The genus name came from the word terminal in Latin, as their leaves are crowded at the tops of the shoots. Recognition of the species under the genus Terminalia was according to fruit and leaf morphological characters (Chakrabarty et al. 2019, Rayane et al. 2022). The species under this genus differ greatly in their internal and external features in addition to their chromosome numbers (Stace 1965, Ohri 1996). Wickens (1973) and Fyhrquist (2007) found that bark, leaves, and fruit morphology can be used as diagnostic features in the identification of Terminalia species. Since species under the genus Terminalia are traditionally used in the treatment of some illnesses, precise identification and recognition of the closely related species need clarification.

Flowers in the genus Terminalia are grouped in terminal simple racemes with duplicates of five anthers alternating with five petals. Despite the numerous stamens, only a few pollen morphological studies have dealt with taxonomical issues of the genus, e.g. Erdtman (1966), Patel et al. (1984), El-Ghazali et al. (1998), Krachai and Pornpongrungrueng (2015). Other palynological articles were mostly focused on the family level. The taxonomic disagreement concerning the species in the genus Terminalia, compounded by the challenge of accurately identifying certain species based ­only on macro-morphological characteristics, is the reason for this investigation. The aims of this work are to investigate the variation in the pollen pheno-characters of ­Terminalia species, and to classify the different species ­under this genus with respect to their pollen characters. Therefore, the subject of this investigation was the pollen grain classification of eleven Terminalia cultivated species, planted in Cairo parks, streets, and botanical gardens as ­ornamental trees.

Material and methods

Mature unopened flowers of eleven species were collected from Egyptian botanical gardens and streets during the flowering season (Tab. 1). The flowers were opened carefully using two needles under a stereo-microscope to release the anthers. The obtained anthers were smeared onto cleaned glass slides with a few drops of glycerol, covered, and sealed with paraffin wax. In order to prevent the ­harmomegathic effects – which occur when chemicals are exposed to aperturate pollen grains during the acetolysis process and the grains fold in response to changes in ­humidity (Taia 2022), non-acetolysed pollen grains were ­examined and then measured and photographed using a Motic (B-150D) light microscope (LM) fitted with a USB digital-video camera and computer software with 10× and 40× objective lenses.

All the measurements were based on 20 to 30 pollen grains. A calibrated oculometer was used to obtain the minimum and maximum measures and calculate the mean and standard deviation of each parameter and the apocolpium index (polar area index), which was calculated as the ratio of the mean distance between the apices of two colpi in both the main and sub apertures to its equatorial diameter. Values are expressed in micrometers.

For scanning electron microscope (SEM) investigation, non-acetolysed pollen grains were sputtered onto double sticks fixed in cleaned, aluminum labeled stubs, then coated with 20 nm gold in a Polaron JFC-1100 coating unit, finally examined and photographed using JEOL-JSM.I T200 Series Scanning Electron Microscope made available in the electron microscope unit, Faculty of Science, Alexandria University, Egypt. The terminology used for the descriptions is that of Punt et al. (2007).

Tab. 1. The localities of collected specimens, geographic coordinates, and the date of collection of the investigated Terminalia species, kept at Cairo University Herbarium (CAI).

No Species Locality Date Collector
1 T. arjuna (Roxb.) Wight & Arn. Cairo: Zamalek, El-Giza street 30° 02’ N 31°13’ E 12 July 2023 Rim Hamdy
2 T. bellirica (Gaertn.) Roxb. Giza: Zoological garden 30° 01’27.4” N 31° 12’52.7” E 7 Nov. 2016 Heba Mostafa
3 T. bentzoe (L.) L. f. Giza: Zoological garden 30° 01’34.0” N 31°12’46.0” E 7 July 2016 Heba Mostafa
4 T. brownii Fresen. Giza: Mazhar botanical garden 30° 03’ N 31°08’ E 10 Dec. 2016 Heba Mostafa
5 T. catappa L. Giza: Orman botanical garden 30° 01’ N 31°12’ E 7 May 2019 Rim Hamdy
6 T. chebula Retz. Giza: Zoological garden 30° 01’34.0” N 31°12’46.0” E 7 Sept. 1963 Mohamed El-Mahdi
7 T. laxiflora Engl. & Diels Cairo: El Zohriya garden 30° 02’ N 31°13’ E 16 July 2023 Rim Hamdy
8 T. mantaly H. Perrier Giza: Mazhar botanical garden 30° 03’ N 31°08’ E 10 Feb. 2019 Rim Hamdy
9 T. muelleri Benth. Giza: Zoological garden 30° 01’35.2” N 31°12’52.4” E 7 Nov. 2016 Heba Mostafa
10 T. myriocarpa Van Heurck & Mull. Giza Zoological garden 30° 01' 34.8" N 31°12' 46.0" E 7 Dec. 2019 Heba Mostafa
11 T. sericea Burch. ex DC. Giza: Zoological garden 30° 01’34.0” N 31°12’46.0” E 7 June 1969 Mohamed El-Mahdi
Table 1.

Fifteen pollen morphological characters have been subjected to cluster analyses using PAST 3 programs V.3. designed by Hammer (1999-2013).

To generate a key, the data obtained from the description of pollen characters were subjected to automated key generation using version 4.12 of the DELTA suite of programs (Dallwitz et al. 2000).

Results

General description of Terminalia pollen grains

For the accurate identification of members of the ­Terminalia species, the morphological characteristics of pollen – such as size, shape, aperture conditions, and exine ornamentation – are crucial. The morphological properties of pollen, both quantitative and qualitative, derived by LM and SEM analysis are compiled in Tab. 2 and Fig. 1 and Fig. 2. The examined species' pollen grains are eurypalynous, meaning they vary greatly in character, shed in monads, apolar or isopolar and radially symmetric. These species are classified as either monomorphic or dimorphic. T. arjuna, T. bentzoe, T. catappa, T. mantaly, and T. muelleri are ­examples of monomorphic plants; on the other hand, the dimorphic species were represented by T. bellirica, T. brownii, T. chebula, T. laxiflora, T. myriocarpa and T. sericea; indicating that they have two distinct pollen shapes, sizes, ­apertures, and even exine ornamentation within the pollen grains that are gathered from the same flower.

Tab. 2. Pollen morphological data of 11 Terminalia species from Cairo, Egypt. AI – apocolpium index, Amb – the outline of pollen grain seen in polar view, Cir – circular, Di – dimorphic, E – equatorial axis diameter (μm), Eth – exine thickness (μm), EO – exine ornamentation, Gr – group, Hlob – hexalobate, M – mean, Mono – monomorphic, Min–Max – minimum and maximum values, No – number, P – polar axis length (μm), P/E – polar axis/equatorial axis, Pr – prolate, RG – rugate granulate, RP – rugate perforate, Sca – scabrate, SD – standard deviation, Sh – shape, SP – sub prolate, Sph – spheroidal, St – striate, StR – striate rugate, Tr – triangular.

No Species State P E P/E Sh Amb Eth EO Main apertures Sub apertures
No Type Length AI No Type Length AI
1 T. arjuna Mono Min - Max M ± SD 24.8 - 33.6 30.2 ± 3.4 23.8 - 30.2 28.7 ± 1.5 1.12 SP Hlob, Cir 2.6 StR 3 Colpate 23.8 - 32.6 29.8 ± 2.8 0.25 3 Colpate 21.2 - 30.2 28.4 ± 1.8 0.34
2 T. bellirica Di Min - Max M ± SD 24.8 - 32.2 29.2 ± 3.0 21.8 - 28.2 26.5 ± 1.7 1.15 SP Tr 2.0 StR 3 Colpate 22.8 - 30.2 29.2 ± 3.0 0.22 3 Colporate 21.5 - 29.8 26.2 ± 1.6 0.30
Min - Max M ± SD 25.2 - 30.4 28.2 ± 1.8 24.8 - 30.6 29.0 ± 1.6 1.0 Sph Tr 2.2 StR 3 Colpate 22.0 - 25.4 24.2 ± 1.6 0.21 3 Colporate 20.2 - 24.2 22.8 ± 1.4 0.28
3 T. bentzoe Mono Min - Max M ± SD 23.8 - 28.2 25.6 ± 2.6 22.4 - 26.0 25.2 ± 1.8 1.1 Sph Hlob 2.2 Sca 3 Colpate 21.2 - 26.4 22.6 ± 3.8 0.19 3 Colporate 20.4 -2 2.6 22.2 ± 0.4 0.22
4 T. brownii Di Min - Max M ± SD 24.2 - 31.6 28.4 ± 3.2 23.2 - 29.8 27.6 ± 2.2 1.12 Sp Tr 2.2 St 3 Colporate 23.2 - 29.6 23.8 ± 5.8 0.16 3 Colporate 22.8 - 28.2 23.2 ± 5.0 0.19
Min - Max M ± SD 25.0 - 32.2 29.8 ± 2.4 21.8 - 29.8 27.6 ± 2.2 1.26 Pr Tr 2.2 St 3 Colporate 24.2 - 31.2 29.4 ± 1.8 0.14 3 Colporate 22.8 - 30.2 26.6 ± 3.6 0.20
5 T. catappa Mono Min - Max M ± SD 25.8 - 34.6 33.6 ± 1.0 23.2 - 29.8 27.4 ± 2.4 1.25 Pr Tr 2.5 RP 3 Colporate 23.4 - 32.8 30.2 ± 2.6 0.21 3 Colporate 21.2 - 30.6 28.6 ± 2.0 0.25
6 T. chebula Di Min - Max M ± SD 22.4 - 27.6 25.2 ± 2.4 20.0 - 24.2 23.2 ± 1.0 1.12 SP Cir 2.6 Sca 3 Colpate 20.4 - 24.8 22.8 ± 2.0 0.24 3 Colporate 16.8 - 18.2 17.0 ± 1.2 0.36
Min - Max M ± SD 22.2 - 27.2 26.2 ± 1.0 22.6 - 27.8 26.0 ± 1.8 1.0 Sph Cir 2.4 Sca 3 Colpate 19.2 - 24.8 22.0 ± 2.8 0.28 3 Colporate 15.4 - 18.2 17.2 ± 1.0 0.41
7 T. laxiflora Di Min - Max M ± SD 21.8 - 26.8 24.2 ± 2.6 20.0 - 24.8 23.8 ± 1.0 1.1 Sp Tr 2.2 RG 3 Colpate 19.2 - 24.8 23.6 ± 1.2 0.25 3 Colporoidate 19.0 - 24.2 23.6 ± 0.6 0.26
Min - Max M ± SD 24.8 - 33.2 29.2 ± 4.0 19.8 - 26.4 24.2 ± 2.2 1.22 Pr Hlob 2.2 RG 3 Colpate 21.8 - 30.2 28.2 ± 2.0 0.26 3 Colporoidate 20.8 - 29.6 26.8 ± 2.8 0.28
8 T. mantaly Mono Min - Max M ± SD 24.4 - 32.2 30.2 ± 1.0 23.8 - 30.4 29.2 ± 1.2 1.12 Sph Hlob 2.2 StR 3 Colpate 20.2 - 28.2 27.8 ± 0.4 0.28 3 Colporate 19.2 - 27.2 25.8 ± 1.4 0.32
9 T. muelleri Mono Min - Max M ± SD 24.6 - 31.8 29.2 ± 2.6 23.4 - 30.8 28.2 ± 2.6 1.12 Sp Hlob 2.6 StR 3 Colpate 22.2 - 29.6 27.2 ± 2.4 0.28 3 Colporoidate 20.2 - 26.4 24.8 ± 1.6 0.32
10 T. myriocarpa Di Min - Max M ± SD 24.8 - 28.2 25.8 ± 2.4 23.2 - 26.4 24.8 ± 1.6 1.1 SP Cir 2.4 RP 3 Colpate 22.2 - 26.4 23.6 ± 2.8 0.34 3 Colporate 20.0 - 24.2 22.6 ± 1.6 0.38
Min - Max M ± SD 27.8 - 34.5 32.4 ± 2.1 24.2 - 30.8 28.6 ± 2.0 1.25 Pr Cir 2.4 RP 3 Colpate 24.2 - 30.8 28.2 ± 2.6 0.32 3 Colporate 20.2 - 27.6 25.8 ± 1.8 0.36
11 T. sericea Di Min - Max M ± SD 18.6 - 24.4 22.0 ± 2.2 17.2 - 23.4 21.2 ± 2.2 1.12 SP Hlob 2.2 RP 3 Colpate 18.8 - 20.4 19.2 ± 1.2 0.32 3 Colporoidate 15.2 - 19.8 18.6 ± 1.2 0.42
Min - Max M ± SD 20.2 - 24.4 23.2 ± 1.2 20.0 - 24.6 23.2 ± 1.4 1.0 Sph Cir 2.2 RP 3 Colpate 19.0 - 21.2 20.4 ± 1.2 0.34 3 Colporoidate 18.2 - 21.0 19.8 ± 1.6 0.41
Table 2.

Figure 1.

Fig. 1. Light microscopy micrographs of the studied Terminalia species. (1-2) T. arjuna, (3-6) T. bellirica, (7-8) T. bentzoe, (9-10) T. brownii, (11- 12) T. catappa, (13- 16) T. chebula, (17-19) T. laxiflora, (20) T. mantaly, (21-22) T. muelleri, (23-25) T. myriocarpa, (26-28) T. sericea. (A amb, C colpus, E exine, P pore, bar = 10 µm).

Shape and size

In their polar view, the pollen grains were triangular, hexa-lobed or circular (Fig. 1 and Fig. 2). In the equatorial view, they were spheroidal (T. bellirica, T. bentzoe, T. chebula, T. mantaly and T. sericea), subprolate (T. arjuna, T. bellirica, T. brownii, T. chebula, T. laxiflora, T. muelleri, T. myriocarpa and T. sericea)and prolate (T. brownii, T. catappa, T. laxiflora and T. myriocarpa). T. catappa had the largest pollen dia­meter on the polar axis (P) at 34.6 μm, while T. sericea had the smallest (18.6 μm). Similarly, T. muelleri and T. myriocarpa had the biggest pollen diameter on the equatorial axis (E) (30.8 μm), whereas T. sericea had the smallest diameter (17.2 μm). Based on the ratio between the lengths of the ­polar axis (an imaginary straight line connecting the two poles) to the equatorial diameter (P/E ratio), pollen sizes and shape variations were examined; T. brownii had the highest P/E value (1.26), while T. bellirica, T. chebula, and T. sericea had the lowest values (0.1).

Figure 2.

Fig. 2. Scanning electron microscope microphotographs showing the different pollen characters within the studied Terminalia species. (29-31) T. arjuna, (32-34) T. bellirica, (35) T. bentzoe, (36-37) T. brownii, (38-39) T. catappa, (40-41) T. chebula, (42-46) T. laxiflora, (47-49) T. mantaly, (50-52) T. myriocarpa. (a - amb, c - colpus, e - exine, p - pore)

Aperture conditions

Apertures are the first characteristic to take into account when identifying pollen. In T. arjuna, T. brownii, and T. catappa, the apertures are iso-colpate; in the other investigated species, the apertures are hetero-colpate; they have two or more types of colpi, one of which varies in length and/or in whether endoapertures are present. The six apertures seen on the pollen grains of the Terminalia species under study are composed of three primary, long apertures that alternate with three smaller, possibly distinct, but somewhat shorter apertures.

Pollens of the species T. arjuna are hexa-colpate in which the main apertures and the sub-apertures are of the same type. The pollen grains of the species T. bellirica, T. bentzoe, T. chebula, T. mantaly and T. myriocarpa are hexa-colpate in which the main apertures are colpate and the sub-apertures are colporate. Also, hexa-colpate pollen grains are observed in T. laxiflora, T. muelleri and T. sericea in which the main apertures are colpate and the sub-apertures are colporoidate. In T. brownii and T. catappa the pollen grains are hexa-colporate in which the main apertures and the sub-apertures are of the same type.

The colpi are not connected at the poles and the apocolpium indices varied between the studied species from 0.14 in T. brownii to 0.42 in T. sericea. The aperture membranes are either psilate or granulate. In the case of composite apertures, the pores are either small and in the same colpi direction (lolongate) or oval shaped covered by granulated exinous layer (colporoidate). According to the state of the pollen grains, momo-or di- morphic and aperture types the studied species are grouped mainly in two main groups and four subgroups (Tab. 2).

Exine ornamentation

Exine sculpturing is the most distinguishing character of the species; in this study, the species have some variation in exine ornamentation; the exine is generally tectate, scabrate in T. bentzoe, and T. chebula; rugate in T. laxiflora; rugate perforate in T. catappa, T. myriocarpa and T. sericea with thin ectexine and thick endexine and intine layers as observed by light microscope investigation. Striate ornamenta­tion is observed only in T. brownii while striate-rugate orna­mentation is recorded in T. arjuna, T. bellirica, T. mantaly and T. muelleri (Tab. 2). Exine thickness ranges from the minimum (2.0 μm) in T. bellirica to the maximum (2.6 μm) in T. arjuna, T. chebula and T. muelleri (Fig. 1).

Clustering analysis

The phenogram resulting from the cluster analysis of fifteen palynological characters (On-line Suppl. Tab. 1) after coding according to their states (On-line Suppl. Tab. 2) divided the studied taxa into three groups at similarity distance 0.936. Group 1 has one species only, T. sericea (11). Group 2 holds eight species, subdivided into two subgroups at similarity distance 0.952, 2A with three species; T. brownii (4), T. laxiflora (7), T. myriocarpa (10); and subgroup 2B with five species; T. catappa (5), T. mantaly (8), T. muelleri (9), T. arjuna (1), T. bellerica (2). Group 3 includes two species, T. bentzoe (3) and T. chebula (6) (Fig. 3).

Computer-generated key

1. Pollen shape subprolate or prolate …………………………………………… 2

Pollen shape subprolate or spheroidal ……………………………………….. 3

Pollen shape prolate ……………………………………………….. T. catappa

Pollen shape spheroidal ……………………………………………………… 4

Pollen shape subprolate ……………………………………………………… 5

2. Amb shape triangular or hexalobate; Exine ornamentation rugate-granulate ………………………………………...………………………..…. T. laxiflora

Amb shape triangular; Exine ornamentation striate ……………….. T. brownii

Amb shape circular; Exine ornamentation rugate-perforate ….... T. myriocarpa

3. Amb shape circular or hexalobate; Exine ornamentation rugate-perforate …………………………………………………………………..… T. sericea

Amb shape triangular; Exine ornamentation striate-rugate ………... T. bellirica

Amb shape circular; Exine ornamentation scabrate …………….…. T. chebula

4. Exine ornamentation scabrate …………………………………...…. T. bentzoe

Exine ornamentation striate rugate …………………………….….. T. mantaly

5. Amb shape circular or hexalobate; Sub aperture type colpate …....... T. arjuna

Amb shape hexalobate; Sub aperture type colporoidate ……….… T. muelleri

Figure 3.

Fig. 3. Phenogram based on 15 pollen characters showing the possible relationships among the studied Terminalia species using percent similarity distances.

Discussion

Morpho-palynological studies have proved to be of great importance for plant identification and taxonomic classification. In this study, different palynological features such as variations in pollen characters in the same flower (morphism), polar and equatorial outline and their ratio (P/E), type of aperture, presence or absence of pores, beside exine thickness and ornamentation were investigated in eleven Terminalia (Combretaceae) species belonging to 7 sections from Egypt using LM and SEM. Based on palynological features, a taxonomic key was created for simple identification and species delimitations.

Mignot et al. (1994) pointed to the importance of aperture type in angiosperm recognition at any taxonomic level, even down to the intra individual level. Based on the results, Terminalia species may be divided into two main groups: monomorphic and dimorphic. Each group has a variety of pollen shapes, ranging from spheroidal to prolate, with many shapes seen in the pollen sampled from the same flower. Pollen hydration status and environmental conditions may contribute to heterogeneity in pollen shape and size within the same taxon (Grant-Jacob et al. 2022, Taia et al. 2023). El Ghazali (2022) noted differences in the sizes and forms of pollen among the same taxa within the family Combretaceae. Despite this, he defined their pollens as stenopalynous, meaning that their characteristics are either stable or slightly changed. According to this investigation, the Terminalia under study exhibit significant variety in their pollen morphological characteristics, and as a result, they can be classified as eurypalynous.

The highest PA length is seen in T. myriocarpa (27.8-34.5 µm), whereas T. sericea has the smallest PA length (18.6-23.2 µm). El Ghazali et al. (1998) have documented these differences in pollen size and shape. Given that multiple shape types have been observed in a single flower, shape class is determined to have less diagnostic value within the taxa in the material under examination. Taia et al. (2023) have noted this investigation; as a result, it does not play a significant role in the taxonomic discrimination of the examined taxa, except in the case of T. sericea, which is grouped separately in the clustering analysis (group A), this separation being due to it having the smallest PA and ED lengths and the smallest main aperture and sub-aperture lengths. This result supports the classification of this species as it belongs to section Psidioides Exell.

Four monomorphic species (T. catappa, T. mantaly, T. muelleri, and T. arjuna) were gathered by the mono-morphic character within the same taxa and placed in subgroup 2B in close proximity to the dimorphic taxa (T. bellerica). This grouping strengthens the taxonomic status of T. catappa, T. mantaly, T. muelleri as they belong to the same section Eucatappa Engl. & Diels. Despite T. arjuna being a separate section Pentaptera (Roxb.) Engl. & Diels, it is grouped with the former three species according to the mono-morphic pollen character.

According to El Ghazali and Krzywinski (1989), the apertures can be classified as iso- or hetero-colpate, meaning that they have two or more types of colpi, with one type differing in length and/or having endoapertures or not. This previously acknowledged observation was discovered in ­Rosa sericea (Ullah et al. 2022). It was also discovered in ­Fagonia species (Taia et al. 2021). El-Ghazali et al. (1998) noted the heterocolpate pollen grains within the taxa under study. This study noted variations in the length and width of the colpi as well as the existence and dimensions of endo-apertures; this result is in line with previous studies mentioned.

Since T. brownii, T. laxiflora, and T. myriocarpa have triangular or circular amb, the triangular amb gathered them in subgroup 1B. Amb shape was determined to be a valuable character in the building of the identification key. This result is partially in agreement with the taxonomy of T. brownii, T. laxiflora as they belong to the same section, Platycarpae Engl. & Diels, but does not agree with the classification of T. myriocarpa as a separate section Myriocarpae. A triangular amb is also present in T. bellirica, although it is distinct from the preceding three species in terms of other pollen characteristics and is located in subgroup 2B, albeit remaining closely linked to them. Certain species could be distinguished using the hexagonal, triangular, or circular amb, as the identification key illustrates. The two species; T. bellirica and T. bentzoe are belonging to section Billiricae Engl. & Diels, however, the study separated them in different groups; this separation is due to the difference between them in pollen state and amb shape.

Exine thickness ranged from 2.0 µm to 2.6 µm; however, there were significant differences in exine ornamentation. The rugate-perforate pattern was seen in T. myriocarpa and T. sericea, while the striate or striate-rugate pattern was seen in the majority of the investigated species. Only T. laxiflora has rugate-granulate ornamentation, but T. bentzoe and T. chebula have scabrate exine ornamentation; for this ­reason, the former two species were grouped together in group C although they belong to different sections; T. bentzoe sec. Belliricae Engl. & Diels, T. chebula sec. Myrobalanus (Gaertn.) Eichl. As a result, exine thickness varies very little among the taxa, whereas exine ornamentation, displayed by the key, is more helpful in identifying the taxa. This observation is consistent with every palynological study conducted since that of Erdtman (1966).

Conclusion

Based on the study's findings, three groups and two subgroups can be identified based on the unique characteristics of the pollen morphology of the Terminalia species under investigation. The pollen morphism, aperture homogeneity, amb form, and exine ornamentation are the most crucial characteristics in classifying the investigated taxa.

Acknowledgment

The authors are grateful to the Electron Microscope Unit, Faculty of Science, Alexandria University for completing this work.

References

Arundhati, D., Akshayakumar, B., Ranjan, R. G., 2020: Molecular characterization and genetic diversity assessment of ten plant species under family Combretaceae using ISSR markers. Research Journal of Biotechnology 15(12), 11–18.

Chakrabarty, T., Krishna, G., Rasingam, L., 2019: Taxonomic notes on Indian Terminalia (Combretaceae). Plant Science Today 6(3), 281–286. https://doi.org/10.14719/pst.2019.6.3.539

Cronquist, A., 1981: An integrated system of classification of flowering plants. Columbia University Press, New York. https://doi.org/10.2307/2806386

Dallwitz, M. J., Paine, T. A., Zurcher, E. J., 2000: Onwards. Principles of interactive keys. Website http://delta-intkey.com/www/interactivekeys.htm [accessed 16 December 2014].

El Ghazali, G. E., Krzywinski, K.,1989: An attempt to clarify the term heterocolpate. Grana 28(3), 179–186. http://dx.doi.org/10.1080/00173138909427429.

El Ghazali, G. E., Tsuji, S., El Ghazaly, G. A., Nilsson, S., 1998: Combretaceae R. Brown. World Pollen and Spore Flora 21, 1–40.

El Ghazali, G. E., 2022: The genus Buchenavia Eichl. and its taxonomic affinity to the genus Terminalia L. (Combretaceae): Insight from pollen morphology-a review. Review of Palaeobotany and Palynology 301, 104644. https://doi.org/10.1016/j.revpalbo.2022.104644

Erdtman, G., 1966: Pollen morphology and plant taxonomy. Angiosperms. (An Introduction to Palynology I). Hafner Publishing Company, New York and London.

Exell, A. W., 1954: Combretaceae. Flora Malesiana 4, 533–589.

Exell, A. W., Stace, C. A., 1966: Revision of the Combretaceae. Boletim da Sociedade Broteriana sér. 2, 40: 5–26.

Grant-Jacob, J. A., Praeger, M., Eason, R. W., Mills, B., 2022: Generating images of hydrated pollen grains using deep learning. IOP SciNotes 3(2), 024001. https://doi.org/10.1088/2633-1357/ac6780.

Heywood, V. H., Brummitt, R. K., Culham, A., Seberg, O., 2007: Flowering Plant Families of the World. Royal Botanic Gardens, Kew. Ontario: Firefly books. https://doi.org/10.1111/j.1467-8748.2007.00585.x.

Hutchinson, J., 1969: Evolution and phylogeny of flowering plants: Dicotyledons: facts and theory, with over 550 illustrations and maps by the author. (No Title). Academic Press, London.

Fyhrquist, P., 2007: Traditional medicinal uses and biological activities of some plant extracts of African Combretum Loefl., Terminalia L. and Pteleopsis Engl. Species. Ph.D Thesis, 183p. http://ethesis.helsinki.fi.

Krachai, P., Pornpongrungrueng, P., 2015: Pollen morphology of Combretaceae from Thailand and its taxonomic significance. Thai Forest Bulletin (Botany) 43, 4–14.

Mabberley, D. J., 2017: Mabberley's plant-book: a portable dictionary of plants, their classification and uses. Cambridge University Press, Cambridge, UK. https://doi.org/10.1017/9781316335581

Mignot, A., Hoss, C., Dajoz, I., Leuret, C., Henry, J. P., Dreuillaux, J. M., Heberle-Bors, E., Till-Bottraud, I., 1994: Pollen aperture polymorphism in the angiosperms: importance, possible causes and consequences. Acta Botanica Gallica 141(2), 109–122. https://doi.org/10.1080/12538078.1994.10515144

Ohri, D., 1996: Genome size and polyploidy variation in the tropical hardwood genus Terminalia (Combretaceae). Plant Systematics and Evolution 200, 225–232. https://doi.org/10.1007/BF00984937

Patel, V. C., Skvarla, J. J., Raven, P. H., 1984: Pollen characters in relation to the delimitation of Myrtales. Annals of the Missouri Botanical Garden 71(3), 858–969. https://doi.org/10.2307/2399170

Hammer, Ø., 1999-2013: PAST paleontological statistics version 3.0: reference manual. University of Oslo.

Punt, W., Blackmore, S., Nilsson, S., Thomas, A., 2007: Glossary of pollen and spore terminology. Review of Paleobotany and Palynology 143(1–2), 1–81. https://doi.org/10.1016/j.revpalbo.2006.06.008

Rayane, T., Ribeiro, M., Maria, I., Loiola, B., Margareth, F. DE. S., 2022: Look at the trees: synopsis of Terminalia ss in the Brazilian Amazon with conservation and distribution implications. Anais da Academia Brasileira de Ciências 94(4), 1–19. https://doi.org/10.1590/0001-3765202220210265

Rendle, A. B., 1956: The Classification of Flowering Plants-Dicotyledons 2. Cambridge University Press, Cambridge, UK. https://doi.org/10.5962/bhl.title.95518

Scott, A. J., 1979: A revision of Anogeissus (Combretaceae). Kew Bulletin 1, 555–566. https://doi.org/10.2307/4109799

Stace, C. A., 1965: The significance of the leaf epidermis in the taxonomy of the Combretaceae: I. a general review of tribal, generic and specific characters. Journal of the Linnean Society of London, Botany 59(378), 229–252. https://doi.org/10.1111/j.1095-8339.1965.tb00060.x

Stace, C. A., 2002: Proposal to conserve Terminalia nom. cons. (Combretaceae) against an additional name Bucida. Taxon 51(1), 193–194. https://doi.org/10.2307/1554985

Stace, C. A., 2007: Combretaceae. In: K. Kubitzki (ed.), The families and genera of vascular plants, 9, 68–82. Springer Verlag, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-32219-1_11

Taia, W. K., 2022: Pollen grain diversity and application in taxonomy and evolution. Taeckholmia 42(1), 27–42. https://doi.org/10.21608/taec.2022.258213

Taia, W. K., Ibrahim, M., Hassan, S., Asker, A., 2021: Palynological study of the genus Fagonia L. (Zygophyllaceae, Zygophylloideae) in Libya. Libyan Journal of Science and Technology 13(1), 29–37. https://doi.org/10.37376/ljst.v13i1.2272

Taia, W. K., Hamdy, R. S., Abd El-Maged, A. M., 2023: Morphological diversity of pollen grains within the flowering stages of Tecoma stans (L.) Juss. ex Kunth. GSC Advanced Research and Reviews 16(1), 280–287. https://doi.org/10.30574/gscarr.2023.16.1.0320

Ullah, F., Gao, Y. D., Zaman, W., Gao, X. F., 2022: Pollen morphology of Rosa sericea complex and their taxonomic contribution. Diversity 14(9), 705–722. https://doi.org/10.3390/d14090705

Wickens, G. E., 1973: Combretaceae. In: Polhill, R.M. (ed.), Flora of Tropical East Africa, 1–99. London, Crown Agents for Oversea Governments and Administrations, United Kingdom.

Willis, J. C., 1966: A dictionary of the flowering plants and ferns. A dictionary of the flowering plants and ferns 7. Cambridge University Press, Cambridge, UK. https://doi.org/10.5962/bhl.title.1428

Zhang, X. R., Kaunda, J. S., Zhu, H. T., Wang, D., Yang, C. R., Zhang, Y. J., 2019: The genus Terminalia (Combretaceae): An ethnopharmacological, phytochemical and pharmacological review. Natural Products and Bioprospecting 9, 357–392. https://doi.org/10.1007/s13659-019-00222-3