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 : 46-47

Atraphaxis —annual renewal of reproductive shoots (Type I)

In all species of the genus Atraphaxis , the flowering and fruiting times are shifted to spring. The thyrses stop growing after they have formed a certain number of cymes, so they exibit a determinate growth of axes. Depending on the type of shoot system, different groups of species ensure successful seed reproduction in various ways.

Approximately half of the Atraphaxis species from the section Tragopyrum have a Type I shoot system formed by orthotropic reproductive shoots with long innovation zones, which raise the synflorescences above ground. The synflorescence includes a long, terminal thyrse and often paracladia of the first and second order, which increase the seed production. The plants rarely develop sylleptic floriferous shoots within the innovation zone. Proleptic development of floriferous shoots in the innovation zone was detected only in A. frutescens if the mother shoot lay down to the ground or the apex of the main axis was damaged ( Fig. 6B ).

The lifespan of reproductive shoots is short. After fruiting, synflorescences and weakly lignified distal parts of reproductive shoots die off completely, leaving only short basal areas of the shoots, which form short perennial axes. Winter buds located on the basal areas ensure the cataleptic development of new reproductive shoots. As a result, plants exist mostly as semi-shrubs or dwarf semi-shrubs 20–80 cm tall. The exceptions are A. virgata from the Northern Tien Shan and Dzungaria ( Yurtseva et al. 2016a: Supplementum 17), and A. bracteata from China, which are shrubs 1–1.2 m tall with acro- and mesosympodial growth of skeletal axes.

Most species with Type I shoot systems are distributed very locally. Atraphaxis avenia is a local endemic to the Khatlon region, Shuroabad district, village Bag in Southern Tajikistan, where it grows along with Bactria ovczinnikovii . Atraphaxis ariana is a local endemic to Badkhyz in Southern Turkmenistan, which experiences the same continental variation of the Mediterranean climate, Köppen Bsk ( Ashirova 2006).

Atraphaxis angustifolia , A. aucherii , A. radkanensis , and A. suaedifolia are endemics of South-West Asia, Transcaucasia and Asia Minor with a cold semi-arid (steppe) climate (Köppen Bsk); A. alaica , A. atraphaxiformis , and A. toktogulica are local endemics of the Pamirs and Tien Shan with a hot-summer humid continental climate (Köppen Dsa).

Atraphaxis schischkinii is a local endemic to Xinjiang ( China) with mid-latitude desert climate (Köppen Bwk); A. bracteata and A. manshurica grow in China in regions with mid-latitude steppe climate (Köppen Bsk); A. selengensis grows in Buryatia (Transbaikalia) with humid variant of continental climate with warm summers (Köppen Dwb). Only A. frutescens , with its depressed race A. decipiens , is wider distributed in the steppe zone of Eurasia from the Middle Volga to Southern Siberia and Mongolia in a cold, continental, climate with hot summers (Köppen Dfa). Atraphaxis virgata replaces A. frutescens in the more southern desert areas on the foothills of the Tien Shan (Köppen Dsa).

Figures

FIGURE 6. Arrangement of thyrses in Atraphaxis species with Type I (A–D), Type II (E–F, J), and Type III (G–I) shoot systems. A. A. frutescens, a semi-shrub with numerous orthotropic reproductive shoots extending from the caudex and ending with thyrses. B. A. frutescens, a reproductive shoot with a vegetative apex of the main axis and sylleptic floriferous shoots, some branching. C. A. alaica, a semi-shrub with three reproductive shoots extending from a caudex and ending in thyrses. D. A. alaica, three shoots: reproductive with underdeveloped thyrse (left) and two vegetative shoots (right). E. A. muschketowii, last-year’s vegetative shoot with annual lateral reproductive shoots. F. A. pungens, annual reproductive shoot with sylleptic vegetative shoot below the terminal thyrse. G. A. replicata, annual reproductive shoot with a terminal thyrse and paracladia repeating the structure of mother axis. H. A. replicata, perennial axis with numerous short annual reproductive shoots (brachyblasts). I. A. replicata, reproductive shoots sitting on last-year’s axis: a shoot ending in a thyrse (arrow), and two shoots with vegetative apexes (crosses) and axillary cymes. J. A. ledebourii, annual shoots sitting on last-year’s shoot: reproductive shoots ending in thyrses with axillary cymes (arrows on the left), and the shoot with vegetative apex (×) and axillary cymes (on the right). Arrows indicate thyrses, crosses indicate vegetative apexes of prolificated thyrses that bear only axillary cymes.