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 : 47-48

Atraphaxis —early flowering of specialized reproductive shoots (Type II)

A half of Atraphaxis species from the section Tragopyrum have a Type II shoot system, in which the perennial axial system is formed by vegetative shoots. Differentiation of shoots into vegetative and reproductive ones led to further specialization in different functions. The specialization of long vegetative shoots for supporting and assimilation functions led to their lignification and preservation, which allowed acro- and mesosympodial growth of innovation shoots and resulted in the formation of woody forms, such as dwarf shrubs or shrubs ( Serebryakov 1955). Vegetative shoots create a perennial axial system that accommodates the numerous short reproductive shoots that will develop next spring as a result of catalepsis.

Parkin (1914: 549) and Weberling (1983: 920) noted that many trees and shrubs transfer flowering structures from orthotropic shoots to lateral shoots, allowing them to continue growing in height. In woody plants, we often find a differentiation of the shoots into long shoots that contribute to the expansion of the entire branching system, and axillary short shoots that are sitting on the long vegetative shoots and produce flowers. For early flowering, the axillary reproductive shoots reduce their innovation zones and become shorter. The long vegetative shoots are lignified and entirely preserved in the perennial axial system.

Most Atraphaxis species with Type II shoot systems are small shrubs 20–80 cm tall. The exceptions are mesophytic A. muschketowi from the Tien Shan and xerophytic A. pyrifolia from the Pamirs and Tien Shan, which reach the heights of 1–2 m ( Yurtseva et al. 2016a: Supplementum 18).

The specialization of reproductive shoots for seed production, combined with selection for attractiveness to pollinators, led to a decrease in the size of shoots and their innovation zones, as well as the compactization of thyrses and a decrease in the size of bracts. Additionally, the bright color and large size of inner perianth tepals increased the attractiveness of the thyrses ( Yurtseva et al. 2016a, Yurtseva et al. 2017). Being fully preformed in the buds, the small reproductive shoots can unfold very quickly, which allows to flower early in spring. Due to the small size of thyrses, the relative loss of biomass is minimal compared to plants with Type I shoot systems, which lose their long branched synflorescences and great portions of their innovation zones.

Atraphaxis pyrifolia and A. pungens , having the shortest reproductive shoots with compact thyrses, bloom twice a season, in early spring and autumn. In spring, these shoots develop on the last-year’s vegetative shoots due to catalepsis. In autumn, they may arise proleptically on branched vegetative shoots that have developed during the current growing season. The small size of the reproductive shoots is compensated by their large number ( Yurtseva et al. 2016a: Supplementum 19, Yurtseva et al. 2022b).

Atraphaxis species with Type II shoot systems inhabit steppe, forest-steppe, and shrubby communities in low and middle mountain belts at elevations of 1200–1500 m a.s.l. All of them are locally distributed in the montainous regions of South-West and Central Asia. The examples include A. kopetdagensis in the Kopet Dag, A. seravschanica , and A. pyrifolia in the Pamirs and Tien Shan; A. muschketowi in the Tien Shan; A. laetevirens in Dzungaria, A. pungens and A. kuvaevii in Altai, Tuva and Mongolia; A. davurica in Transbaikalia. In Asia Minor and the Caucasus, there are also A. caucasica , A. billardierei , A. tournefortii , and A. daghestanica . Like species with Type I shoot systems, these species grow in a cold, semi-arid steppe сlimate with hot and dry summers (Bsk), and in humid variants of the continental climate, with hot or warm summers (Dfa, Dfb, and Dwb), however, they are absent from the steppe zone of Eurasia. Despite the fact that species with a type II shoot system also grow in arid conditions, they have a different set of adaptive morphological features. These species maintain the meristems in the vegetative state and build a maximally branched perennial axial system with a lot of reproductive buds, which unfold early in the following growing season and bring fruits before the onset of summer drought.

Atraphaxis teretifolia from the section Physopyrum also has a Type II shoot system, but is locally distributed in extremely arid, semi-desert areas of Central Kazakhstan (Bsk). This dwarf shrub has prostrate vegetative shoots and numerous short axillary reproductive shoots ending in short frondose thyrses. This rare species is in danger due to its low seed production and germination energy ( Ivlev 2008).