Rauwolfia tetraphylla L. – Bee Supporting Plant during Nectar and Pollen Dearth

Species: Rauwolfia tetraphylla L. (synonym Rauvolfia canescens L.)Family: Apocynaceae Common names: English (Garden Rauwolfia, American serpentwood, Wild snake root, Devil pepper, Snake weed); Hind...


Pollen and nectar
The flowers are a good a source of pollen (2300-9280 pollen grains/flower) and nectar (2-3 μl) which attract honey bees and other pollinators. The sugar concentration measured by refractometer was 18 ± 2%. Nectar secretion starts about one hour before the opening of flower and continuous throughout anthesis. The nectar secretion shows a marked autonomous rhythm that corresponds to the periodicity of the pollination process (Akare & Chaturvedi, 2018). The flower has a longevity of 1 day.

Honey
Fresh honey is light whitish with a thick consistency having fine flavor and strong aroma.

Medicinal use
The plant is extensively exploited in traditional (folk), Ayurvedic and Unani System of Medicines in Asian countries. About 30 different types of alkaloids have been reported from the different parts of the plant (Pandey et al., 2016). Various parts of R. tetraphylla are shown to exhibit a range of pharmacological activities such as antimicrobial, antioxidant, anti-inflammatory, cytotoxic, antihemolytic, antihypertensive, anxiolytic, insecticidal, allelopathic, platelet antiaggregant, cardioprotective, antipsychotic and antiparasitic activity (Khan, 2013). Roots extracts are used as sedative for treating the blood pressure, hypochondria, disorders of the central nervous system diarrhea, colic, dysentery, cholera, and fever, anti-inflammatory, syphilis, tranquilizing actions diuretic, stomatitis, expectorant, swelling, and for intestinal troubles. The leaf extract is employed for the treatment of cholera, intestinal disorders, diarrhea, dysentery, fever and eye disease (Mahalakshmi et al., 2019). The crushed leaves are applied over snakebite site. Leaf juice is used in eye troubles and decoction prepared from leaves is used for toothache, cough and cold. The latex from the plant is reported to be cathartic, emetic, and expectorant besides its use in the treatment of dropsy.
Fruits of this plant are used to cure spleen disorders. The juice of the fruit is used as a substitute for ink.

Cultural notes
R. tetraphylla is becoming critically endangered due to wide indiscriminate collection from wild, poor seed germination and lack of sufficient commercial plantation. Rauwolfia grows well in clays, acidic (pH 4-6) and well-manured soil. Propagation is carried out by planting seeds, root-culture or stem-cutting. The establishment of botanical gardens may be the best ex-situ conservation strategy that can be adopted for sustainable utilization of plant resources of the area.

Association with bees
As the plant is one of the very few floras that bloom during summer season, it is very important for the development of colonies during that season. The flowers attract many kinds of bees (Figure 1, Supplementary material, Table S1). Anoosha et al. (2019) reported insects belonging to 4 orders, 10 families, 16 genera and 17 species as pollinators of R. serpentina. A very rich diversity of pollinating bees belonging to five major families of order Hymenoptera, viz. Apidae, Megachilidae, Andrenidae, Colletidae and Halictidae has been reported by Abrol et al. (2012) from various agri-horticulture plants. Hymenopterans were recorded as most abundant pollinators of Rauwolfia. A rich diversity and abundance (54.57%) of non-Apis bees and Apis bees (45.40%) while other insect visitors constitute only 0.03% of total insect pollinators. Peak abundance of major insect pollinators was recorded at 0800-1000 h while minimum abundance was recorded at 1200-1400 h ( Figure 2).

Plants for Bees
Visitation of insect pollinators to R. tetraphylla flowers is shown in Figure 3. Among solitary bees, P. viridissima and N. viridiscens were the major pollinators recorded throughout the flowering season (see Supplementary material, Figure  S1) while A. dorsata was the dominant forager among all the insect-pollinators on R. tetraphylla and Apis bees ( Figure S2, Supplementary material).
The relative abundance of bees in crops varied with flowering through the season, with the greatest percentage of pollinators recorded in June when the plant was in full bloom. The foraging activity number of bees foraging during different times on the day ranged between 0.20 and 3.80/ plant/2 min ( Figure 2). The amount of time bees takes to forage on flower is an important component determining foraging behaviour. The time spent by Apis bees on flowers ranged between 5.5 s/flower (A. dorsata) to 15.0 s/flower (A. mellifera) while Pithitis sp. maximum time (17.5 s/flower) and A. zonata spent minimum time (2.3 s/flower) on the flowers. All the pollinators started foraging early in the morning. The maximum A. mellifera population was observed during 10.00-12.00 h (2.80 bees/plant/2 min). The A. cerana and A. dorsata population was observed during 8.00-10.00 h (3.80 and 1.80 bees/plant/2 min, respectively). The differential distribution of flower visitation showed a clear demarcation on resource utilization patterns which correspond to the nectar/pollen production rhythm in Rauwolfia flowers. Each pollinator has specific ecological threshold for foraging activity which differ inter and intra specifically depending upon the level of adaptation of a given species in an environment (Burrill & Dietz, 1981;Sihag, 1982). Very limited information on the contribution of solitary bees in pollination of crops is available in the literature. The solitary bees are major contributors to crop pollination in R. tetraphylla and must be considered in management efforts aiming at adequate crop pollination. The ecosystem services rendered by the pollinators, especially non-Apis bees, should be promoted and included in conservation planning of environment.