Floral morphology and pollination mechanism of Salvia liguliloba, a narrow endemic species with degraded lever-like stamens

doi:10.17521/cjpe.2015.0072

Abstract

Abstract: Aims Diverse stamen structures and interactions with pollinators make Salvia the model plants for studying evolutionary changes in plant pollination mechanisms. The dorsal pollination mechanism of lever-like stamens has been extensively investigated as a classic example for precise interactions between plants of Salvia genus and their pollinators. However, there are many atypical lever-like structures and pollination mechanisms among Salvia species. For example, Salvia liguliloba has floral organ structures and a pollination mechanism characterized by degenerated lever-like stamens. The aim of this work was to understand the selection pressure and ecological significance of Salvia plants that have the atypical staminal level mechanism. Methods In this study, we described the morphological features of S. liguliloba, a plant species endemic to the Tianmu Mountains, and investigated its pollination ecology in detail. Various components of the flower of S. liguliloba were measured, which included the corolla, corolla tube, corolla entrance, filament, connective and pistil. Flower-visiting insects, pollinators, and the pollination process were observed and recorded by a digital video camera. Furthermore, we focused on comparing the floral organ structures and the pollination characteristics of S. liguliloba with those of S. digitaloides, which has a short-lever stamen. The relative frequency of insects, visiting time per flower, activity rate and visitation rate were measured and compared with the data from our previous study of S. digitaloides, for which the flower structure and pollination features were well concluded. Important findings Salvia liguliloba has smaller corolla length, tube width, and shorter filament and pistil than S. digitaloides (p < 0.05). The only effective pollinator was Bombus trifasciatus, and its average relative visiting frequency and the visiting time per flower were (0.959 ± 0.065) and (1.54 ± 0.60) s. The degenerated lower arm and limited moving space in the upper arm of the stamen restrict the pollinating insects from pushing the stamens in a lever-like motion. Thus, bumblebees completed pollination with the aid of their heads. Compared with the structure of the lever-like stamen and the pollination mechanism of S. digitaloides, the structural features of the floral organs of S. liguliloba make it adapt to a more specific pollinator with shorter visiting time and higher activity rate. The results suggested that the species with degraded lever-like stamens might be different from other typical Salvia species in their evolution direction and reproductive strategy.

Key words: adaptive evolution, forehead pollination, short-lever stamen, Salvia liguliloba, Bombus trifasciatus

HUANG Yan-Bo,WEI Yu-Kun,WANG Qi,XIAO Yue-E,YE Xi-Yang. Floral morphology and pollination mechanism of Salvia liguliloba, a narrow endemic species with degraded lever-like stamens[J]. Chin J Plan Ecolo, 2015, 39(7): 753-761.

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URL: https://www.plant-ecology.com/EN/10.17521/cjpe.2015.0072

https://www.plant-ecology.com/EN/Y2015/V39/I7/753

Figures/Tables 8

Fig. 1 Photographic images of Salvia liguliloba and its visiting insects. A, Habitat. B, Plant. C, Lateral view of flower. D, Longitudinal section of flower. E, Pistil. F, Front view of flower. G, Stamens. H, Fruits. I, Bombus trifasciatus (pollinator). J, Ventral visiting by a species of Halictidae. K, Dorsal visiting by a species of Halictidae. L, A visitor of Sphingidae species. Bar = 2 mm.

Fig. 2 Diagram of Salvia liguliloba flower measurement. A, Corolla length. B, Corolla width. C, Corolla height. D, Tube length. E, Entrance height. F, Entrance width. G, Filament length. H, Connective length. I, Pistil length. Bar = 2 mm.

Fig. 3 Comparisons of morphometric data between Salvia liguliloba (A to D) and S. digitaloides (E to H). A, E, Lateral view of a flower. B, F, Longitudinal section of a flower. C, G, Front view of a flower. D, H, Stamens. Bar = 5 mm.

Table 1 Comparison between Salvia liguliloba and S. digitaloides on flower features (mean ± SD) (mm)

	花冠长 Corolla Length	花冠宽 Corolla width	花冠高 Corolla height	冠筒长 Tube Length	冠口高 Entrance height	冠口宽 Entrance width	花丝长 Filament length	药隔长 Connective length	雌蕊长 Pistil Length
舌瓣鼠尾草 S. liguliloba	23.67 ± 0.74^b	5.31 ± 0.34^b	7.30 ± 0.67^b	19.49 ± 0.8^b	4.87 ± 0.61^a	4.41 ± 0.52^b	2.39 ± 0.14^b	5.19 ± 0.33^a	23.74 ± 0.43^b
毛地黄鼠尾草 S. digitaloides	39.80 ± 2.60^a	12.52 ± 1.49^a	22.98 ± 2.62^a	28.99 ± 2.01^a	4.33 ± 0.68^b	6.80 ± 0.72^a	7.33 ± 0.46^a	5.10 ± 0.43^a	34.89 ± 2.30^a

Table 2 Morphometric data of visiting insects from Salvia liguliloba in the study (mean ± SD) (mm)

访花昆虫 Visiting insect	体长 Body length	胸宽 Thorax width	胸厚 Thorax thickness	喙长 Tongue length
三条熊蜂 Bombus trifasciatus (n = 10)	21.8 ± 1.29	7.35 ± 0.41	5.10 ± 0.24	11.0 ± 1.32
隧蜂科昆虫 Halictidae (n = 1)	6.88	1.91	1.66	0.73

Fig. 4 The pollination process of Bombus trifasciatus, an effective pollinator of Salvia liguliloba. A, Approaching to the corolla. B, Accessing to flower tube and the pollinator’s tongue is extending. C, B. trifasciatus is sucking nectar and its forehead touching the fertile anther. Bar = 2 mm.

Fig. 5 The daily dynamic of flower-visiting times for Salvia liguliloba by Bombus trifasciatus.

Table 3 Flower-visiting traits of pollinators from Salvia liguliloba and S. digitaloides (mean ± SD)

	访花昆虫 Visiting insects	相对频次 Relative frequency	单花访问时间 Visiting time per flower (s)	活跃度 Activity rate	访花频率 Visitation rate	传粉方式 Pollination mode
舌瓣鼠尾草传粉昆虫 Pollinator of S. liguliloba	三条熊蜂 Bombus trifasciatus	0.959 ± 0.065 (n = 7)	1.54 ± 0.60 (n = 1 035)	18.2 ± 3.12 (n = 10)	17.45	F
毛地黄鼠尾草传粉昆虫 Pollinators of S. digitaloides	拟熊蜂属昆虫 Psithyrus sp.	0.57	8.68 ± 0.90	-	3.94	D
	小雅熊蜂 B. lepidus	0.20	15.57 ± 1.20	-	0.77	D/V
	B. infrequens	< 0.20	-	-	-	D

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