Dimorphism in inflorescence scent of dioecious wild grapevine

Microstructure of Croatian Wild Grapevine (Vitis vinifera subsp. sylvestris Gmel Hegi) Pollen Grains Revealed by Scanning Electron Microscopy

Wild grapevine (Vitis vinifera subsp. sylvestris Gmel Hegi) is dioecious with male and female plants, whereas domesticated grapevine is mostly hermaphrodite with self-fertile hermaphrodite flowers. The pollen morphology of wild grapevine has been poorly studied. There is no detailed palynological study of V. sylvestris in Croatia and neighboring countries. Here, scanning electron microscopy (SEM) was used to analyze the pollen of V. sylvestris from male and female individuals growing at two natural sites in Croatia. The selective APT3 marker was used to confirm the flower phenotype with the genetic background. SEM analysis showed that the pollen grains of V. sylvestris were isopolar and radially symmetrical, with foveolate perforated ornamentation, regardless of the flower type of the individuals. All male flowers were 3-colporate and prolate in shape, whereas female individuals varied from subprolate to spheroidal and had inaperturate pollen grains. Pollen shape, dimensions and exine ornamentation proved very informative, and here we address the most polymorphic traits in the analyzed V. sylvestris individuals. Principal component analysis (PCA) and clustering based on pollen morphology variables clearly differentiated individuals by their flower type, and no grouping specific to population was observed, pointing to the conserved pollen structure of V. sylvestris. The results indicate the need to continue the palynological study of V. sylvestris and serve as a good phenotypic basis for functional genetic studies on genes involved in pollen morphology and function.

Osmophores of Caryocar brasiliense (Caryocaraceae): a particular structure of the androecium that releases an unusual scent

Caryocar brasiliense is a flagship species of the Brazilian Cerrado. It produces flowers with a strong peculiar scent, which are pollinated by bats and occasionally moths with short mouthparts. However, the cues responsible for attracting these nocturnal pollinators remain unknown. We aimed to identify osmophores of C. brasiliense, describe the ultrastructure of the cells involved in the synthesis and release of floral odour, and identify the constituents of the floral bouquet. We performed field observations and histochemical and ultrastructural analyses of flowers focusing on the androecium. Gas chromatography-mass spectrometry was used to analyse the scents emitted. Filament epidermal cells were found to possess an unusual shape and be responsible for the main production and release of odour. These cells, called foraminous cells, are elongate and possess pores where their cell walls are abruptly thin. The cuticle is practically absent over the pores, which facilitates odour emission. The foraminous cells have conspicuous nuclei and organelle-rich cytoplasm where oil droplets can be seen prior to anthesis. The features of these cells remain similar during anthesis, but many vesicles fuse with the plasma membrane and the number of oil droplets in the cytosol decreases. Twenty-nine components were found in the scent, especially fatty acid derivatives and N- and S-bearing compounds. Our analyses revealed that the androecium of C. brasiliense has a particular structure that acts as an osmophore. The scent from the androecium resembles that of the entire flower, which is an unprecedented finding for a plant with single flowers as the pollination unit.

Understanding intraspecific variation of floral scent in light of evolutionary ecology

Background and Aims

Among the various floral traits involved in pollinator attraction and potentially under selection mediated by pollinators, floral scent/fragrance has been less investigated than other components of floral phenotype. Whether or not pollinator-mediated selection impacts floral scents depends on the heritability of scent/fragrance and the occurrence of some variation within species. Although most studies have investigated how scent varies among species, growing amounts of data are available on variation at the intraspecific level.

Methods

The results of 81 studies investigating intraspecific variation of floral scents in 132 taxa were reviewed. For each study, whether variation was found in either identity, proportion or absolute quantities of volatile organic compounds (VOCs) was recorded, as well as information with the potential to explain variation, such as methodology, plant origin or pollination biology.

Key Results

Variation was found for almost all investigated species, both among individuals (among and sometimes within populations) and within individuals across different temporal scales. Cases in which such variation is a possible result of pollinator-mediated selection were analysed, by discussing separately selection related to variation in pollinator identity/behaviour among populations or across time, deceit pollination and sex-specific selection. Not surprisingly, in many cases, pollinator-mediated selection alone does not explain the observed variation in floral scent. This led us to review current knowledge on less investigated factors, such as selection mediated by natural enemies, genetic drift and gene flow, environmental constraints, phylogenetic inertia, or biochemical constraints that could be invoked to explain scent variation.

Conclusions

This review highlights the great potential of analysing floral scent variation and including it in integrated studies of floral phenotypes. We also have identified the current gaps in our understanding of this complex signal and we propose several methodological and conceptual future directions in this research area.