Stamen development and winter dormancy in apricot (Prunus armeniaca)

Abstract

Background and aims: In temperate woody perennials, flower bud development is halted during the winter, when the buds enter dormancy. This dormant period is a prerequisite for adequate flowering, is genetically regulated, and plays a clear role in possibly adapting species and cultivars to climatic areas. However, information on the biological events underpinning dormancy is lacking. Stamen development, with clear differentiated stages, appears as a good framework to put dormancy in a developmental context. Here, stamen developmental changes are characterized in apricot (Prunus armeniaca) and are related to dormancy.

Methods: Stamen development was characterized cytochemically from the end of August to March, over 4 years. Developmental changes were related to dormancy, using the existing empirical information on chilling requirements.

Key results: Stamen development continued during the autumn, and the flower buds entered dormancy with a fully developed sporogenous tissue. Although no anatomical changes were observed during dormancy, breaking of dormancy occurred following a clear sequence of events. Starch accumulated in particular places, pre-empting further development in those areas. Vascular bundles developed and pollen mother cells underwent meiosis followed by microspore development.

Conclusions: Dormancy appears to mark a boundary between the development of the sporogenous tissue and the occurrence of meiosis for further microspore development. Breaking of dormancy occurs following a clear sequence of events, providing a developmental context in which to study winter dormancy and to evaluate differences in chilling requirements among genotypes.

Fig. 1.

Anther development in Prunus armeniaca. (A) Anther differentiation before dormancy, showing a homogenous appearance in the summer. (B) A well-defined stamen structure, with early cell layers (arrowheads) and sporogenous tissue (ST) in the autumn. (C) During winter dormancy, all anther layers were differentiating: epidermis (Ep), endothecium (En), middle layers (ML) and a single-layered tapetum (T), and archesporal cells (AC) were distinguishable. (D) After dormancy, the pollen mother cells (PMC) were layered with a thick cellulosic wall (arrowheads). (E) Following meiosis, young microspores (arrowheads) appeared. (F) Mature anther at anthesis with pollen grains (arrowheads). Transverse 2-μm sections of anthers embedded in JB4 and stained with calcofluor. Scale bars: (A–C) = 20 µm; (D) = 40 µm; (E, F) = 100 µm.

Fig. 2.

Timing of chilling fulfilment, microsporogenesis and flowering in Prunus armeniaca in 4 years of experiments.

Fig. 3.

Mitotic activity and vascular differentiation in the stamen at breaking of dormancy in Prunus armeniaca. (A, B) Pre-vascular area (PA) void of starch during endodormancy. (C, D) Once chilling requirements were fulfilled, starch grains (arrowheads) began to accumulate around the pre-vascular area. (E, F) Starch progressively vanished from the pre-vascular area. (G, H) Concomitantly, mitotic activity (arrowheads) was apparent in the pre-vascular area (white arrow). (I, J) Establishment of young xylem vessels (arrowheads) following mitotic activity. Young (K) and (L) well-developed vascular connections (arrowheads) following breaking of dormancy. Transverse sections of stamens embedded in JB4 and stained with PAS and Toluidine blue (A, C, E); with PAS (B, D, F); with DAPI and acridine orange (G–J), and with calcofluor (K, L). Scale bars: (A, C, E, G, I, K, L) = 40 µm; (B, D, F) = 20 µm; (H, J) = 10 µm.

Fig. 4.

Microsporogenesis in Prunus armeniaca. (A) Starch (arrowheads) accumulated progressively in the anther layers and in the pollen mother cells (PMC). (B) Callose deposition (arrowheads) surrounding the PMC and (C) nuclei division (arrowheads) of tapetum cells indicate the starting of microsporogenesis. (D) Starch vanished from the PMC, which were initially connected by cytomictic channels (arrowheads) and (E) chromosomes (arrowheads) became evident at the end of prophase I. (F) At telophase I (arrowheads), (G) cytomictic channels disappeared isolating each PMC. (H) Following telophase II, a second callose deposition (arrowheads) isolated each microspore forming the tetrad (T). Transverse 2-μm sections of anthers embedded in JB4 and stained with PAS and Toluidine blue (A, D, G); with calcofluor (B, H); with DAPI and acridine orange in (C, E); with DAPI superimposed with phase contrast (F). Scale bars = 20 µm.

Fig. 5.

Diagram representing the sequence of developmental events in the stamen, specifying those occurring at the breaking of dormancy.