Abstract. Seed production is likely constrained by pollen limitation and the viability of pollen grains decreases rapidly in time due to water evaporation. Any decrease in the surface-to-volume ratio, through increase in size or change in shape of a grain, reduces the rate of water loss. However, grain size trade-offs with the number of grains that can be produced by a plant. Here, we tested the hypothesis that under higher desiccation stress pollen grains become larger and more spherical. We analyzed data on the pollen morphology of eight Rosaceae species and the desiccation intensity based on temperature, potential evapotranspiration and altitude. To explain the mechanisms underlying our results, we present a model that optimizes the size and shape of pollen grains under different conditions. We report that pollen grains under more intense desiccation stress during flowering periods tend to be larger but do not change shape. This conclusion is consistent with the results of a theoretical model presented here. Our report fills a gap in our knowledge about a fundamental process in plant reproduction. We also discuss the significance of our results in light of current palynological and ecological problems (e.g., global climate change).
The pollen of six taxa of the genus Rubus endemic to Poland (R. capitulatus, R. chaerophylloides, R. ostroviensis, R. posnaniensis, R. seebergensis and R. spribillei)was studied using light and scanning electron microscopy. The study objective was to extend the knowledge of the species pollen morphology and to investigate whether pollen morphology may help to taxonomically distinguish a very rare bramble species. Study material was derived from six natural localities where the species occur in the Wielkopolska region (Poland). Ten quantitative pollen grain features were analysed as well as the following qualitative ones: outline, shape, type of bridge and exine sculpture. Only mature, correctly formed pollen grains (30 per specimen) were measured and 180 pollen grains were examined in total. It was found that R. capitulatus and R. seebergensis had the highest mean values of P, E and Le features. In addition, pollen grains of R. capitulatus were most flattened (mean P/E ratio equals 1.14), while those of R. seebergensis were most elongated (mean P/E ratio equals 1.27). R. spribillei was the only species with striae of similar width as muri, whereas striae in the remaining species were wider. R. chaerophylloides and R. posnaniensis were characterised by specific, similar exine sculpture. Nevertheless, it should be stated that differences between pollen features of the species studied are so small and the variability of these features are so large that it is difficult to clearly identify the species studied. Therefore, pollen grain morphology can serve only as an auxiliary feature for the determination of these species.
The genus Rubus L. (Rosaceae) not been investigated satisfactorily in terms of palynology. This genus is taxonomically very difficult due to the large number of species and problems with their delimitation, as well as very different distribution areas of particular species. The aim of this study was to investigate pollen morphology and for the first time the ranges of intrageneric and interspecific variability of Rubus species, as well as verify the taxonomic usefulness of these traits in distinguishing studied taxa from this genus. The selected species of the genus Rubus were analysed for 11 quantitative pollen characteristics and the following qualitative ones: exine ornamentation, pollen outline and shape, as well as bridge structure. Analyses were conducted on a total of 1740 pollen grains, which represent 58 blackberry species belonging to a majority of subgenera and all the sections and series found in Poland. The most important characters included exine ornamentation (exine ornamentation type, width and direction of grooves and striae, number and diameter of perforations) and length of the polar axis (P). The arrangement of the examined species on the dendrogram does not corroborate division of the genus Rubus into subgenera, sections and series currently adopted in taxonomy. This fact is not surprising because the taxonomy of the genus was not based on pollen characters. Pollen features should be treated in taxonomy as auxiliary, because they fail to differentiate several (10) individual species, while the other ones create groups with similar pollen traits.
This paper presents the results of the study on pollen morphology of five Polish species of the genus Sorbus L. (S. aria (L.) Crantz, S. aucuparia L. Emend. Hedl., S. chamaemespilus (L.) Crantz, S. intermedia (Ehrh.) Pers. and S. torminalis (L.) Crantz). Observations were carried out with light and scanning microscopes. Investigated pollen grains were small or medium size, most often subprolate or prolate spheroidal in outline. Exine of medium thickness, consisted of two layers. Mainly 3-zonocolporate or sometimes 4-zonocolporate (S. chamaemespilus and S. torminalis) grains were noted. Usually striatae type of sculpture was observed, except the grains of S. intermedia, which were characterized by intermediate type of sculpture, with the features both striate and regulate ornamentation.Pollen morphology of the investigated Sorbus species corresponds with their systematic classification. It is relatively easy to separate pollen grains of the following subgenera: Sorbus, Chamaemespilus and Torminaria. Only subgenus Aria includes grains of different morphology and therefore it is impossible to determine the diagnostic features of this subgenus. Pollen morphology does not confirm a close relationship between S. aria and S. intermedia.It is possible to distinguished pollen grains of all the analysed native species. However, there are no important differences in pollen morphology between the subspecies of Sorbus aucuparia. The key for Polish Sorbus species, based on pollen grain characters is given too.
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