Racemose inflorescences of monocots: structural and morphogenetic interaction at the flower/inflorescence level

Abstract

Background: Understanding and modelling early events of floral meristem patterning and floral development requires consideration of positional information regarding the organs surrounding the floral meristem, such as the flower-subtending bracts (FSBs) and floral prophylls (bracteoles). In common with models of regulation of floral patterning, the simplest models of phyllotaxy consider only unbranched uniaxial systems. Racemose inflorescences and thyrses offer a useful model system for investigating morphogenetic interactions between organs belonging to different axes.

Scope: This review considers (1) racemose inflorescences of early-divergent and lilioid monocots and their possible relationship with other inflorescence types, (2) hypotheses on the morphogenetic significance of phyllomes surrounding developing flowers, (3) patterns of FSB reduction and (4) vascular patterns in the primary inflorescence axis and lateral pedicels.

Conclusions: Racemose (partial) inflorescences represent the plesiomorphic condition in monocots. The presence or absence of a terminal flower or flower-like structure is labile among early-divergent monocots. In some Alismatales, a few-flowered racemose inflorescence can be entirely transformed into a terminal 'flower'. The presence or absence and position of additional phyllomes on the lateral pedicels represent important taxonomic markers and key features in regulation of flower patterning. Racemose inflorescences with a single floral prophyll are closely related to thyrses. Floral patterning is either unidirectional or simultaneous in species that lack a floral prophyll or possess a single adaxial floral prophyll and usually spiral in the outer perianth whorl in species with a transversely oriented floral prophyll. Inhibitory fields of surrounding phyllomes are relevant but insufficient to explain these patterns; other important factors are meristem space economy and/or the inhibitory activity of the primary inflorescence axis. Two patterns of FSB reduction exist in basal monocots: (1) complete FSB suppression (cryptic flower-subtending bract) and (2) formation of a 'hybrid' organ by overlap of the developmental programmes of the FSB and the first abaxial organ formed on the floral pedicel. FSB reduction affects patterns of interaction between the conductive systems of the flower and the primary inflorescence axis.

Keywords: Bracteole; flower; flower-subtending bract; inflorescence; inhibitory field; pattern formation; prophyll; regulation of development; vasculature.

Fig. 1.

Schematic drawings of selected patterns of flower arrangement to explain the terminology used in the text (see also Endress, 2010). All drawings show the distal part of a shoot; the proximal part (developing during the same season or during the previous season or seasons) contains vegetative leaves, lateral renovation bud(s) and sometimes reduced leaves. (A) Bracteate raceme/botryoid without floral prophylls; (B) single terminal flower; (C) bracteate raceme/botryoid with a single floral prophyll per pedicel; (D) bracteate thyrse/thyrsoid with lateral monochasia; (E) bracteate raceme/botryoid with two floral prophylls per pedicel; (F) bracteate thyrse/thyrsoid with lateral dichasia. Closed circles indicate flowers that are invariably present; open circles represent terminal flowers that can be either present (botryoids, thyrsoids) or absent (racemes, thyrses).

Fig. 2.

Inflorescences of Tofieldiaceae. (A) Single terminal flower (Harperocallis); (B) botryoid (Isidrogalvia); (C) raceme (Tofieldia); (D) thyrsoid (Triantha).

Fig. 3.

Orientation of lateral flower in monocots shown on diagrams of anthetic flowers. (A) Bracteate flower without floral prophyll (e.g. Melanthiaceae s.s.); (B) non-bracteate flower (e.g. Acorus); (C) bracteate flower with a single lateral floral prophyll – all variants can be found within the same inflorescence (e.g. Nartheciaceae, Petrosaviaceae, some Liliaceae); (D) bracteate flower with a single adaxial floral prophyll (Iridaceae); (E) bracteate flower with two lateral floral prophylls (e.g. Tricyrtis hirta); (F) flower with three calycular phyllomes on the pedicel (Tofieldiaceae); (G) non-bracteate flower (some basal Araceae). Closed circles indicate inflorescence axis, black arcs represent the flower-subtending bract (FSB) and pedicel phyllome(s), dark grey arcs represent outer tepals and light grey arcs represent inner tepals.

Fig. 4.

Patterns of floral orientation and development established by inhibitory fields of FSBs and floral prophyll(s). (A) Bracteate flower without floral prophyll; (B) bracteate flower with a single lateral floral prophyll; (C) bracteate flower with a single adaxial floral prophyll; (D) bracteate flower with two lateral floral prophylls. Closed circles indicate inflorescence axis, black arcs represent the FSB and pedicel phyllome(s). Grey areas represent inhibitory zones. Numbers indicate the sequence of initiation of outer tepals.

Fig. 5.

Orientation of lateral flowers in racemose monocot inflorescences with distichous and spiral phyllotaxy shown. (A) Bracteate inflorescence with distichous phyllotaxy, floral prophylls absent; (B) bracteate inflorescence with spiral phyllotaxy, floral prophylls absent; (C) bracteate inflorescence with distichous phyllotaxy and adaxial floral prophyll; (D) bracteate inflorescence with spiral phyllotaxy and lateral floral prophyll. Closed circles indicate inflorescence axis, black arcs show the FSB and pedicel phyllome(s), dark grey arcs represent outer tepals and light grey arcs represent inner tepals. Numbers indicate relative position of the flower within the inflorescence and sequence of their initiation and opening.

Fig. 6.

Patterns of flower-subtending bract (FSB) reduction in the order Alismatales. (A) Species of Potamogeton with bracteate inflorescences; (B) species of Potamogeton with non-bracteate inflorescences where FSBs are suppressed; (C) Potamogeton densus with ‘hybrid’ organ formed by the FSB and outer tepal; (D) species of Triglochin with non-bracteate inflorescences where FSBs are suppressed; (E) Triglochin maritima with ‘hybrid’ organ formed by the FSB and outer tepal; (F) species of Tofieldia with bracteate inflorescences; (G) Tofieldia pusilla with ‘hybrid’ organ formed by the FSB and calyculus phyllome. Closed circles indicate inflorescence axis, black arcs show morphologically expressed FSBs, dashed-lined arcs indicate suppressed FSBs and grey arcs show ‘hybrid’ organs. Numbers indicate the sequence of organ initiation.

Fig. 7.

Three-dimensional diagrams of nodal vasculature in monocot inflorescences lacking floral prophylls. (A) Tofieldia cernua, bracteate raceme; (B) Isidrogalvia robustior, bracteate botryoid; (C) Triglochin maritima, non-bracteate botryoid; (D) Triglochin palustre, non-bracteate botryoid.

Fig. 8.

Three-dimensional diagrams of nodal vasculature in monocot inflorescences possessing floral prophylls. (A) Metanarthecium luteo-viride; (B) Narthecium ossifragum; (C) Petrosavia stellaris.