Polygonum, Linnaeus, 1753

Polygonum View in CoL —from semi-shrubs to annuals

The genus Polygonum s.str. has also a Type I shoot system with indeterminate growth or, rarely, determinate growth of reproductive shoots. In mountainous areas of South-West and Central Asia, Polygonum species are mainly small shrubs and semi-shrubs. On the plains and sea shores, in regions with arid or temperate climates, the plants accelerate development and exist as annuals, some being cosmopolitan weeds. The shoot system is formed either by creeping reproductive shoots with intercalary thyrses or by orthotropic shoots with terminal thyrses ( Yurtseva et al. 2010, Yurtseva et al. 2012).

Among species with creeping reproductive shoots, there are perennial herbs, semi-shrubs or dwarf shrubs with short innovation zones (i.e., P. alpestre , P. biaristatum , P. cognatum , P. fibrilliferum , and P. maritimum ), and annuals without innovation zones (i.e., P. calcatum , P. plebejum ). The synflorescence includes a continuously growing, spikelike frondose thyrse, and paracladia alternating with axillary cymes without any particular order ( Kuznetsova et al. 1992: 59).

Among species with orthotropic reproductive shoots, there are perennial herbs and semi - shrubs with a short innovation zone (i.e., P. equisetiforme , P. luzuloides , P. thymifolium ), and annuals without an innovation zone (i.e., P. arenarium , P. bellardii ). The synflorescence is a panicle or a raceme of bracteose thyrses. The thyrses of P. equisetiforme tend to stop the growth after fruiting in Central Asia, but prolong the growth in the Mediterranean (Spätprolifikation in Troll 1964, 1969), which let to consider these thyrses pseudoterminal.

Polygonum species also tend to shorten the life span of their reproductive shoots by shortening or eliminating the innovation zone, which promotes early flowering. The orthotropy of reproductive shoots correlates with the terminal position and compactness of thyrses, as well as a decrease in the size of subtending leaves.

The intercalary thyrses transform into pseudoterminal ones if the main axis ceases its apical growth after constructing a floral zone, which withers after fruiting ( Parkin 1914). Genera with both intercalary and terminal positions of the inflorescences were found in many Angiospermae ( Sell 1969, Weberling 1981, Kusnetsova 1988, Kuznetsova & Timonin 2017).

The optimization of character states in the plastid phylogeny confirmed that the intercalary thyrse that is present in Polygonum aviculare L. s.str. and most Persepolium species is a plesiomorphic state, while the terminal thyrse found in Persepolium khajeh-jamalii , Bactria ovczinnikovii and all Atraphaxis species appeared as an apomorphy and provided determinate growth of the axes.