Terminology of inflorescence diversity has often been used in a confusing way in the literature, partly because it was based on uncritical and outdated definitions. In particular, the terms cyme, thyrse, and panicle have been misused. Although a more critical classification worked out by several authors is available, it is unfortunately not in general use because most of the relevant publications are written in German. In addition, some terms have not been used in the same way by morphologists and developmental geneticists. The present review attempts to remedy the situation with a simple outline of a classification based on: (i) different branching patterns; (ii) differential elongation of axes of different orders; and (iii) repetition of basic ramification patterns in different ways. Racemose and cymose branching are two extreme patterns; the former with limitation of axial orders to two, the second with limitation of lateral axes of each order to two. In a branching system, a sequence of racemose → cymose and, within the cyme, of dichasial → monochasial is common, but the reverse sequence generally does not occur. Systematic and evolutionary aspects of inflorescences are briefly discussed. Branching patterns are often stable in larger clades. Inflorescences of mutants studied in developmental genetic studies are mainly altered in flower or branch numbers or relative branch length, but not in branching patterns. This is also a contribution towards the goal of a unified terminology for the different fields of biology dealing with inflorescences.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1759-6831.2010.00087.x

Disentangling confusions in inflorescence morphology: Patterns and diversity of reproductive shoot ramification in angiosperms

Grass inflorescences are diverse, developmentally complex, and provide many of the taxonomic characters used to differentiate the estimated 10,000 grass species. Here we review grass inflorescence development in detail and discuss which genes are involved at each developmental stage. We demonstrate that grass inflorescence development is complex, with multiple structures that are not present in Arabidopsis . New and published phylogenetic analyses of genes involved at each developmental stage indicate that the maize FASCIATED EAR2 ( FEA2 ) and Arabidopsis CLAVATA2 ( CLV2 ) genes are the sole remaining co‐orthologs following multiple rounds of whole‐genome duplication. Analyses of BARREN STALK1/LAX PANICLE ( BA1 / LAX1 ) , FRUITFULL ( FUL ) , INDETERMINATE SPIKELET1 ( IDS1 ) , KNOTTED1 ( KN1 ) , LEAFY HULL STERILE1 ( LHS1 ), and RICE CENTRORADIALIS1/2 ( RCN1/2 ) indicate that these genes are members of grass‐ or monocot‐specific small gene families. The complex pattern of gene relationships mirrors a complex pattern of functional evolution. Maize FEA2 and Arabidopsis CLV2 have nonidentical roles, whereas distantly related grass KN1‐like and RCN1/2 proteins show functional convergence and conservation, respectively. Duplications near the base of grasses in BA1 / LAX, FUL, IDS1 and LHS1 have led to diverse roles in grass inflorescence development. We conclude that developmental gene duplication followed by functional diversification appears to have played a major role in the evolution of novel morphological structures within grass inflorescences.

Developmental Gene Evolution and the Origin of Grass Inflorescence Diversity

The shoot apical meristem of grasses produces the primary branches of the inflorescence, controlling inflorescence architecture and hence seed production. Whereas leaves are produced in a distichous pattern, with the primordia separated from each other by an angle of 180°, inflorescence branches are produced in a spiral in most species. The morphology and developmental genetics of the shift in phyllotaxis have been studied extensively in maize and rice. However, in wheat, Brachypodium, and oats, all in the grass subfamily Pooideae, the change in phyllotaxis does not occur; primary inflorescence branches are produced distichously. It is unknown whether the distichous inflorescence originated at the base of Pooideae, or whether it appeared several times independently. In this study, we show that Brachyelytrum, the genus sister to all other Pooideae has spiral phyllotaxis in the inflorescence, but that in the remaining 3000+ species of Pooideae, the phyllotaxis is two-ranked. These two-ranked inflorescences are not perfectly symmetrical, and have a clear “front” and “back;” this developmental axis has never been described in the literature and it is unclear what establishes its polarity. Strictly distichous inflorescences appear somewhat later in the evolution of the subfamily. Two-ranked inflorescences also appear in a few grass outgroups and sporadically elsewhere in the family, but unlike in Pooideae do not generally correlate with a major radiation of species. After production of branches, the inflorescence meristem may be converted to a spikelet meristem or may simply abort; this developmental decision appears to be independent of the branching pattern.

/pdf/early-inflorescence-development-in-the-grasses-poaceae-3okvh1j1yy.pdf

Early inflorescence development in the grasses (Poaceae).

Pseudoviviparous Reproduction of Poa alpina var. vivipara L. (Poaceae) during Long‐term Exposure to Elevated Atmospheric CO2

Inflorescence structures of 81 species and two subspecies (representing 61 genera) of the Chloridoideae have been investigated using stereo microscopy (SM). Inflorescences of the Chloridoi- deae are true polytelic inflorescences. Thirteen inflorescence subtypes are distinguished based on modes of the truncation and the homogenization. These are categorized into four basic inflorescence types. A key for the identification of these inflo- rescence types is presented. Generally, inflorescence characters have definite systematic value in recog- nizing supra-generic taxa in the Chloridoideae. Twelve processes have been identified as responsi- ble for inflorescence diversification that awaits verification by further systematic study.

Inflorescence structures and evolution in subfamily Chloridoideae (Gramineae)

Terms are proposed to designate different subzones and regions within the paracladial zone of the inflorescence, in the Poaceae. In some taxa the inflorescence is composed of short paracladia only. In others the main axis bears short paracladia in the distal region and long paracladia in the proximal region, so that two subzones can be distinguished. In general the inflorescences of grasses lack bracts other than those belonging to the spikelets. In the Andropogoneae, however, some inflorescences have long paracladia that lack bracts and prophylls but others have long paracladia with well developed bracts and prophylls. The former are designated as "long paracladia without trophotagma", and the latter as "long paracladia with trophotagma".

Abelardo C. Vegetti

Papers

The structure of the paracladial zone in Poaceae