Polygonum (Komarov, 1936)

Yurtseva, Olga V., Severova, Elena E. & Mavrodiev, Evgeny V., 2017, Persepolium (Polygoneae): A new genus in Polygonaceae based on conventional Maximum Parsimony and Three-taxon statement analyses of a comprehensive morphological dataset, Phytotaxa 314 (2), pp. 151-194 : 183-184

f. Polygonum subsection Spinescentia is likely a self-pollinated taxon

According to Ronse De Craene & Smets (1991), a well-developed perianth and nectar-secreting tissue lining the receptacle and also present at the bases of filaments indicate entomophyly.On the other hand, simple leafy inflorescences with widely spaced cymes of flowers, an urceolate perianth, inner stamens which reach stigmas, the enclosure of the androecium and stigmas by the segments that do not widely open, and the lack of trichomes at the bases of the filaments are considered as arguments in favor of self-pollination.

The genus Persepolium is characterized by frondose or frondulose thyrses with well spaced cymes of 10–20 cymes including 2–3 flowers, an inconspicuous urceolate or campanulate perianth with equal-sized tepals and a rather short perianth tube, a spacious receptacle lined inside with nectar-secreting tissue also covering the bases of the filaments, linear poorly developed stigmas, and pollen with poorly sculptured foveolate, foveolate-perforate, to microreticulate-foveolate sporoderm surface. This suite of morphological characters indirectly indicates self-pollination. Since the perianth segments enclose the stamens, which remain hidden inside the perianth throughout flowering, these species are likely visited by small non-specific pollinators that facilitate self-pollination.

Persepolium has a purplish-green perianth with sepal-like oblong-elliptical, oblong-ovate, or oblong-spatulate segments, which are entirely covered with short simple trichomes on the outside. The sepal identity of the tepals is inferred from the rectangular or irregular epidermal cells and the numerous stomata interspersed with the epidermal pavement cells with a deep longitudinal or random cuticular striation, and some of the cells forming short simple linear trichomes. This resembles the epidermis of the sepals as described in Arabidopsis ( Pelaz et al. 2000) , and the leaf blades and annual shoots of Persepolium . A similar type of epidermal cells covers the inner surface of the lobes in Pteropyrum ( Hong et al. 1998) . The trichomes covering the outside of the perianth might perform a protective function in an arid climate by preventing superfluous transpiration, or may contribute to the inflow of water to the flower buds. In addition, these trichomes may play a tactile role for the recognition or grasping action by pollinators.

In terms of habit, structure of shoots, inflorescences and ocreas, Atraphaxis ariana , A. atraphaxiformis , A. toktogulica , and A. tortuosa are Atraphaxis s.s. They have the same shoots, ocreas, thick leaf blades with undulating edges, and striate-perforate sporoderm ornamentation typical of the rest of Atraphaxis (except for A. toktogulica , which has a sporoderm surface transitioning from reticulate-perforate to striate-perforate). These species are locally distributed and geographically well separated from each other with A. ariana growing in the Badghyz Semi-Desert in Central Asia, A. toktogulica and A. atraphaxiformis occuring in Tian-Shan, and A. tortuosa in Inner Mongolia, China. These species likely retained plesiomorphic traits in perianth morphology, present in Persepolium , but possibly were less reproductively successful than the rest of Atraphaxis .

Similarly to Persepolium , these species share the campanulate perianth with five equal-sized oblong-elliptical or oblong-ovate segments, the funnel-form perianth tube of medium size, and the filaments inserted rather deeply in the receptacle. Their perianth segments are not rotund or cordate as in many other species of Atraphaxis (see more in Yurtseva et al. 2016a), but they are already petaloid, white or brightly coloured, demonstrating a transitional state to the specialized perianth typical of Atraphaxis .

Conical papillae covering the perianth tubes and the segment bases of these species are as long as the linear trichomes covering the perianth of Persepolium , but are twice as wide and have a more subtle tightly pressed cuticular striation. Conical papillae are present also on the perianth of some Polygonum species, Oxygonum , and Fagopyrum ( Hong et al. 1998) , that are distant from Atraphaxis in molecular phylogenetic trees ( Sanchez et al. 2011; Schuster et al. 2011b, Yurtseva et al. 2016a). These papillae likely play a tactile role for recognition by pollinators, or allow them to grip the flower surface, or have effects on petal colour, petal reflexing, scent production, or petal wettability ( Noda et al. 1994, Hong et al. 1998, Martin & Glover 2007, Ojeda et al. 2009, Whitney et al. 2009, 2011).

Most of Atraphaxis species have more specialized flowers with glabrous petaloid segments arranged in two whorls of different size. The segments are broadly ovate, rotund, cordate, or reniform in shape, white, yellow, or brightly pink in colour. During flowering, the segments are spread out widely, providing access to the stamens and ovary for pollinators. By the time of fruiting, the two outer segments of the perianth are smaller and often reflected towards the pedicel, the inner segments are enlarged, upright and hide the mature achene.

In Atraphaxis spinosa and A. fischeri , the inflated bases of the filaments are densely covered with conical thin-walled papillae. The trichomes associated with nectar-secreting tissue were detected also in Koenigia Linnaeus (1767: 3) , Fallopia , Oxygonum Burchell (1822: 548) , Calligonum , and Pteropyrum ( Ronse De Craene & Smets 1991) , in which they supposedly hold the nectar by capillarity action, or maintain nectar consistency. However, the thin-walled papillae present on the filaments of A. spinosa and A. fischeri likely serve as aids for grasping by pollinators, may play a tactile role in species-specific recognition, or could increase an inflow of water to the bases of the filaments during flowering, promoting the opening of the flower.

The showy compact bracteose thyrses and open flowers, the fairly specialized perianth with a long fifliform basal part of the tube and accrescent inner segments, the nectar-secreating tissue lining the receptacle inside and the filament bases adaxially, the fimbriate-capitate stigmas, and the papillae at the filaments indicate that the majority of Atraphaxis species are insect-pollinated, which is consistent with the opinion by Ronse De Craene & Smets (1991). These features possibly give advantages for autcrossing, successful seed reproduction and wider distribution in comparison with Bactria , Persepolium , and some Atraphaxis species with less specialized perianth, which are local endemics.

The diversity of flower and inflorescence structure observed in Atraphaxis possibly indicates the different pollination modes in the species with accrescent inner segments and long tubes, as compared with part of Atraphaxis with a less specialized perianth. The brightly coloured or white accrescent petaloid segments are more attractive for pollinators and promote cross-pollination. The filiform basal portion of the tube contributes to the elongation of a straight pedicel, both serving to exert the perianth out of the compact thyrse formed by congested cymes of flowers. In Atraphaxis , the long tube fused to a gynophore does not contain nectar but has a purely mechanical function. The same feature is seen in Polygonum subsection Polygonella and Reynoutria Houttuyn (1777: 639) with spike-like bracteose thyrses. In fruiting, the long pendulous pedicels elongated by the filiform bases of the tubes may facilitate movement of fruits equipped with persistent perianth segments and may promote anemochory.

The distinct shape of the stigmas, which are linear in Persepolium , capitate in part of Atraphaxis , and fimbriate-capitate in the majority of Atraphaxis also points to different pollination modes.

One might assume that the differences in epidermis of the petaloid segments of Bactria lazkovii and Atraphaxis , or the sepaloid segments of Persepolium and B. ovczinnikovii might correspondingly be the result from the expression of the genes responsible for the identity of petals or sepals (e.g., Coen & Meyerowitz 1991, Pelaz et al. 2000, Theissen & Melzer 2007, Irish 2009, Landis et al. 2012, Chanderbali et al. 2016). Also the lack of papillae from the perianth surface of the majority of Atraphaxis species could correlate with MIXTA mutation ( Noda et al. 1994, Martin et al. 2002, Martin & Glover 2007) as well as with the shift of pollination mode of these taxa in comparison to the species with equal-sized segments and conical papillae on the tube. However, more work is necessary to describe and understand the developmental patterns of the flower of Atraphaxis and related taxa, as well as to establish the proper context of such studies.