Zingiber barbatum Wall. is an aromatic, medicinal and endemic species for Myanmar [1]. The biology of Z. barbatum is poorly known. The species is characterized as a geophyte, flowering plant, varying in height and size, with horizontal tuberous rhizomes. It is a perennial but grows as an annual with a forced dormancy period. The plant goes dormancy and loses all the vegetative aboveground parts with the onset of winter when the habitat temperature decreases below 15˚C to 10˚C in the night. The plant grows wild in hilly areas from 75 m up to 1050 m altitudes of the Dipterocarp and Shorea forests, and prefers light, but moderate shade and high humid environment; the major pollinators are bees and moths [2]. Nowadays, the local population keeps Z. barbatum as a backyard plantation and grows it in small places near fences or levees, more for personal consumption as a medicinal plant, than for commercial production or food.

Z. barbatum is one of the most troublesome taxa due to the very variable morphological features and Myanmar considered as a center of its diversification [3]. Genetic diversity studies based on morphological and molecular markers revealed high intraspecific genetic variability among Z. barbatum genotypes [4]. Z. barbatum is well known by its vernacular names “Pwe-au” or “Meik-thalin” in Myanmar [3]. However, there is still ambiguity that occurs in the description of Z. barbatum due to the lack of comprehensive investigations. The specific epithet “Meik-tha-lin” is also used as a common name for Z. montanum by the local population of Myanmar [5], bringing confusion on characterizing these two different taxa. Therefore, the inflorescence habit and flower morphology remain the most common parameters used to discriminate Zingiber taxa at the first stage of the taxonomic description. Although the general morphology of Z. barbatum has been described by Wicaksana [6], the description of the flower morphology was not included in the study through the absence of flowers.

Z. barbatum belongs to the section Cryptanthium Horan. of the monophyletic genus Zingiber [7] and characterized by inflorescence consisting of spike on a short procumbent peduncle [8]. Taxonomically, Zingiber classified into four sections based on the inflorescence habit [8] [9] [10]. The fusion of two sterile stamens into a labellum and presence of single anther with a horn-like appendage embracing the upper part of the style [11] are the distinguishing features of the flowers in Zingiber. The complexity of characterizing Zingiber flowers lies in the fact that flowers are delicate and ephemeral, and liable to rot if exsiccate in the bracts; thus the herbaria specimens have a little value for study of flower morphology [12].

The objective of this study was to characterize the reproductive morphology of under-exploited Z. barbatum species with the focus on the inflorescence architecture and phenotypic features of the flower. This a first comprehensive study on the characterization of flower biology of Z. barbatum from Myanmar, which complements a cycle of previous studies done in the Gene Research Center of the University of Tsukuba (Tsukuba, Japan) (GRC UT) on the assessment of existed genetic diversity in Z. barbatum.

The two accessions of Z. barbatum, ZO113 and ZO223, formed inflorescences and flowered from the beginning of July up to begin of September of 2019. The accession ZO223 (Figure 2) produced one and accession ZO113 produced two inflorescences respectively (Figure 3). The plant height and inflorescence length and width measurement showed the growth of both the plant itself and the inflorescence continued throughout the entire flowering period (TableS1). The summary of comparative morphological characteristics of similarities and/or differences in Z. barbatum based on quantitative and qualitative assessment has shown in TableS2.

Myanmar is situated in Indo-Burma biodiversity hotspot and possesses various types of ecosystems (evergreen rainforests, montane forest, mixed deciduous, savanna and alps) due to climate, topography, and wide latitudinal range from tropical to subtropical [15] [16]. Myanmar is also known as a country with a strong cultural heritage possessing more than a hundred ethnic groups with their own dialects and traditions [5]. The traditional agricultural approach perhaps is one of the factors affecting the distribution and diversification of the Z. barbatum species in Myanmar.

Z. barbatum for the first time was described by Wallich in Plantae Asiaticae Rariores [17]. According to Wallich [17], the species was abundantly found in Rangoon (current Yangon of Myanmar) in the hills along the Irrawaddy River near Prome (current Pyay city).

The detailed revision of the available literature confirmed scarcity of information regarding Z. barbatum, especially on flower biology for comparison with the current study. Z. barbatum produced flowers in radical inflorescences on erect peduncle and confirms the former studies of taxonomic affiliation of Z. barbatum to the Zingiber section Cryptanthium Horan. [8] [9]. According to Wallich [17], inflorescences are forming from the creeping roots near the stem or from the base of the stem and are barely elevated above the ground surface. Our study confirms that inflorescences arise directly from a rhizome near the stem; however, they were formed on a long enough erect peduncle, which also corresponds to the description of the inflorescence habit of Z. barbatum reported by Theilade [18]. According to Theilade [18], Z. barbatum produces inflorescence on the erect peduncle of 2.0 - 6.0 cm length, while according to Aung [3], Z. barbatum belonging to Type A, produces inflorescence on the erect peduncle of approximately 3.0 cm length.

There was not observed notable differences among quantitative parameters assessed regard to inflorescences and flowers among two accessions, except the time span that inflorescences undergo senescence, while the qualitative parameters showed some variation regarding the shape of inflorescences at the stage of appearance and the shape of the labellum (bifurcation of the central labellum lobe in ZO113) between two examined accessions. The reported distinguishing feature of Z. barbatum inflorescence, corresponding to its etymology (“barbatum”—having long, weak hair), is the presence of high pubescence [3] [17]. However, we did not observe the high pubescence of inflorescence among two examined accessions; on the contrary, the observed pubescence decreased in the process of inflorescence growth.

We observed a notable change in the visual shape of inflorescences (spikes) among two accessions during growth. The comparative assessment of the observed phenotypic variation regarding the shape of the spike during growth revealed similarity corresponding with the description of the spike given for both Z. barbatum (spike shape at the stage of blossom begun, current study) and Z. montanum (spike shape at the last stage of blossom, current study) [3]. We assume it might be the variation of the morphological traits among Z. barbatum species or it might be a completely different species (e.g. Z. montanum and/or its variation) which the local people of Myanmar are not able to distinguish easily due to the phenotypic similarity of plants and utilized similar vernacular name, of what was reported by Wicaksana [6]. The comprehensive investigations are required to elucidate this confusion, such as the whole genome sequencing or the sequences of highly conserved genes and intergenic spacers of organellar DNA.

From another side, the observed differences regarding the shape of inflorescence and shape of labellum can be either genetically determined feature, when the activity and position of the shoot apical meristem (SAM) is determining a degree of the inflorescence architecture [19], or as a result of phenotypic plasticity, when due to various environmental condition the variation of the same traits among the species of the same taxa could be inducing [20]. The visual reduction of indumentum (pubescence) also could be the result of phenotypic plasticity or/and as the consequence of the terminated trichomes metabolic activity, after which they die, may either persist or be shed [20] [21].

Z. barbatum has zygomorphous, bisexual, ephemeral, epigynous flowers, with a light fragrance, with the presence of a horn-like anther crest embracing the upper part of the style and leaving the stigma free. The observed life span of flowers was less than one day. Also, Wallich [17] reported the plants introduced into Calcutta Garden blossom freely during the cold and rainy season. We observed flowers mainly during August when the average daily temperature was above +30˚C and the average humidity above 80%. The result of study corresponds with those reported by Aung [3] for Z. barbatum Type A when the flowers and fruits were observed in July-September. Although the plants were kept in a greenhouse, we observed about 5 - 6 days gap in flowering in ZO113 accession (TableS3). According to Ravindran et al. [22] rarity of flowering in Zingiber is influenced by photoperiodic and climatic factors. Thus, the observed trend probably was triggered by temperature differences on those days, when the average daily temperature ranged between +21˚C ... + 28˚C, and the weather was mostly cloudy and cool.

We did not observe the formation of seeds in Z. barbatum. The absence of seeds could be due to the high pollen sterility of many Zingiber species resulting in hence no sexual reproduction [22] influenced by a single or the complex of factors. The dominant xenogamy, entomophily and different types of breeding systems have been reported for Zingiberaceae [23] [24] [25] [26]. The heterostyly with a gametophytically controlled self-incompatibility system has been reported for Z. officinale [27] and a partial self-incompatibility reported for Z. densissimum [28]. Given the above, it can be assumed that several factors have been influenced by the absence of seeds in Z. barbatum in our study:

1) The “failed” xenogamy due to absence of pollinators when the plants are grown in a greenhouse.

2) Due to heterostyly of flowers which may be a contributing factor of sterility. According to our observation, Z. barbatum flowers belong to longistylous morphotype in which anthers are situated below the slender longer style that protrudes out of the flower parts, and hence the pollen grains cannot reach the stigma.

3) Due to self-incompatibility, when the growth of the pollen tube can be enzymatically inhibited in the style to prevent inbreeding [29].

4) Since Z. barbatum flowers are monoclinous, a dichogamy (protandry or protogyny) can be another reason influenced the absence of seeds. The protandry as one type of the breeding system has been reported in Zingiberaceae [25], however, if more than two flowers of an individual asynchronously bloom, the geitonogamy can occur [30].

Elucidation of the above-mentioned trend requires further detailed studies on pollination ecology, palynology, and mechanism of a breeding system that will allow better understand of the reproductive biology of Z. barbatum species.

The study allows concluding that phenotypic variation exists among Z. barbatum species. Two Z. barbatum accessions showed phenotypic variation that can be differentiated by the inflorescence shape and morphological feature of the central labellum lobe. The study confirmed the taxonomic affiliation of Z. barbatum at the intraspecific level based on the inflorescence habit. The conventional taxonomic description based on inflorescence habit and flower morphology accepted as a useful method primarily identify taxa and elucidate an occurred ambiguity at the intraspecific level. However, we think that a comprehensive investigation of reproductive biology (flower biology, pollen ecology, palynology, breeding system) joint with molecular biology is required to understand existed variability among Z. barbatum species. The eco-geographical factors, artificial selection, and/or traditional agricultural approaches probably influenced the reproductive isolation of Z. barbatum leading its diversification and variability. The study will be useful in different areas of biology as taxonomy, botany, plant systematics, biodiversity and conservation purposes since this species is found in the wild and up to now only in Myanmar.

The authors declare no conflicts of interest regarding the publication of this paper.

Shukurova, M.Kh., Myint, D., Gilani, S.A. and Watanabe, K.N. (2020) Description of Flower Biology of Under-Exploited Species, Zingiber barbatum (Wall.) from Myanmar. American Journal of Plant Sciences, 11, 1031-1048. https://doi.org/10.4236/ajps.2020.117074

Abbreviation: inf., Inflorescence.

Abbreviation: inf., Inflorescence.