Atraphaxis (Kostina & Yurtseva, 2021)

Yurtseva, Olga V. & Kostina, Marina V., 2024, Flower arrangement and plant architecture in Atraphaxis, Bactria, and Persepolium (Polygonaceae, Polygonoideae, Polygoneae) and their systematic implications, Phytotaxa 671 (1), pp. 12-58 : 48

Section Atraphaxis —flowering from early spring to autumn (Type III)

Species from the section Atraphaxis have a Type III shoot system and differ by strongly reduced bracteose thyrses which contain 4–5 closely spaced, 1(2)-flowered cymes and resemble short racemes (see also Yurtseva 2006). This group also shows a differentiation of shoots into vegetative and reproductive. Both types of shoots form perennial axes, which accommodate numerous innovation buds. Long vegetative shoots often branch, perform supporting and assimilation functions, and accommodate reproductive buds, that give rise to small reproductive shoots (brachyblasts) in the autumn of the current year or in the following spring.

The group of species with a Type III shoot system is characterized, first, by a strong reduction of thyrses, which allows them to unfold quickly in favorable weather conditions; second, by diverse structure of reproductive and vegetative shoots. The reproductive shoots vary the length from 1 to 60 cm, what ensures their long-lasting flowering during almost entire growing season. The reproductive shoots 1–2 cm long (brachyblasts) have extremely reduced innovation zones, end in short thyrses and can unfold in few days with the onset of favourable conditions early in spring. Their small size is compensated by their large number. Longer reproductive shoots end in a synflorescence (a thyrse, or a thyrse and paracladia) and can develop a floral zone of sylleptic axillary floriferous shoots. In some cases, they maintain a vegetative apex, but have a floral zone. The short reproductive shoots blossom earlier than longer shoots. That confirms the well-known correspondence between the size of the reproductive shoots with terminal flowering units, and the time of flowering ( Serebryakov 1952, Filatova et al. 1989, Mikhalevskaya & Libatskaya 1991, Krylova & Belyanina 1995, Kostina 2005).

At last, brachyblasts of A. spinosa and A. replicata demonstrate the accelerated development in late summer due to the partial replacement of the cataleptic axillary branching with proleptic ( Hallé et al. 1978, Serebryakova 1983, Kuznetsova & Timonin 2017).

Thus, in this group, only a portion of apical or axillary meristems switches to floral identity at any time, as a response to favorable weather conditions. The variety of reproductive shoots blooming at different times leads to a maximum seed production and a higher fitness value. These features can lead to better adaptation to different climatic conditions in the widest range. Indeed, the common range of A. canescens , A. compacta , A. karataviensis , A. replicata and A. spinosa covers the distributional area of all other species of Atraphaxis in Eurasia from the Middle East to the Middle Volga Region, Mongolia and China. These species inhabit the mountainous regions of South-West and Central Asia with warm and temperate (humid), continental climates (Dfa, Dfb, and Dsa). They also inhabit desert zone with a cold desert climate (Dwk) and steppe zone with a cold semi-arid climate (Bsk). The flat, dimeric perianths hiding flat, digonous fruits contribute to the long-distance dispersal.

Species from the section Atraphaxis are shrubs 50–80 cm tall. The perennial branched axes of A. spinosa persist for 2–3 years. After all reproductive buds are exhausted, the perennial axes die off and are replaced by new ones. The life span of the bush of A. spinosa is 10–15 years in the desert climate ( Nikitin 1966), while A. replicata lives up to 40 years in the steppe zone of Eurasia.