Embryo sac development has been investigated in unpollinated, cross pollinated and gibberellic acid (GA2) treated flowers of Pyrus communis L. While pollination and GA3 treatments do not alter embryo sac development, they prolong embryo sac viability. In untreated unpollinated flowers, ovules degenerate between 12 and 21 d after anthesis, while in cross pollinated and GA3 treated flowers this degeneration is postponed by about 10 d. Thus, in a cross pollinated flower this extends the period over
which a successful fertilization can take place. This increased period of viability is accompanied by an elongation of the embryo sac itself. Elongation takes place two weeks prior to fertilization in cross pollinated flowers.The extension of life span of embryo sacs following pollination and treatment with gibberellic acid indicates that a stimulus induced by ‘pollination’ could be mediated by GA3 Whatever its mechanism of operation, the prolongation of embryo sac viability by pollination represents a selective advantage, in that the period at which the ovules are receptive to fertilization must be significantly extended.
Background. In most flowering plants, pollen is dispersed as monads. However, aggregated pollen shedding in groups of four or more pollen grains has arisen independently several times during angiosperm evolution. The reasons behind this phenomenon are largely unknown. In this study, we followed pollen development in Annona cherimola, a basal angiosperm species that releases pollen in groups of four, to investigate how pollen ontogeny may explain the rise and establishment of this character. We f
ollowed pollen development using immunolocalization and cytochemical characterization of changes occurring from anther differentiation to pollen dehiscence.
Results. Our results show that, following tetrad formation, a delay in the dissolution of the pollen mother cell wall and tapetal chamber is a key event that holds the four microspores together in a confined tapetal chamber, allowing them to rotate and then bind through the aperture sites through small pectin bridges, followed by joint sporopollenin deposition.
Conclusion. Pollen grouping could be the result of relatively minor ontogenetic changes beneficial for pollen transfer or/and protection from desiccation. Comparison of these events with those recorded in the recent pollen developmental mutants in Arabidopsis indicates that several failures during tetrad dissolution may convert to a common recurring phenotype that has evolved independently several times, whenever this grouping conferred advantages for pollen transfer.
Background and AimsAn intense pollen–pistil interaction precedes fertilization. This interaction is of particular relevance in agronomically important species where seeds or fruits are the edible part. Over time some agronomically species have been selected for the ability to produce fruit without seeds. While this phenomenon is critical for commercial production in some species, very little is known about the events behind the production of seedless fruit. In this work, the relationship between
pollen–pistil interaction and the onset of fruiting was investigated in citrus mandarin.MethodsPistils were sequentially examined in hand-pollinated flowers paying attention to pollen-tube behaviour, and to cytochemical changes along the pollen-tube pathway. To evaluate which of these changes were induced by pollination/fertilization and which were developmentally regulated, pollinated and unpollinated pistils were compared. Also the onset of fruiting was timed and changes in the ovary examined.Key ResultsConspicuous changes occurred in the pistil along the pollen-tube pathway, which took place in a basipetal way encompassing the timing of pollen-tube growth. However, these changes appear to be developmentally regulated as they happened in the same way and at the same time in unpollinated flowers. Moreover, the onset of fruiting occurred prior to fertilization and the very same changes could be observed in unpollinated flowers.ConclusionsPollen–pistil interaction in citrus showed similarities with unrelated species and families belonging to other taxa. The uncoupling of the reproductive and fruiting processes accounts for the parthenocarpic ability of unpollinated flowers to produce fruit in citrus. However, the maintenance of a functional reproductive process reflects the potential to produce seeded fruits, providing a basis for the understanding of the production of seeded or unseeded fruits and further understanding of the process of parthenocarpy in other species.
• Premise of the study: A variety of mechanisms to prevent inbreeding have arisen in different angiosperm taxa during plant evolution. In early‐divergent angiosperms, a widespread system is dichogamy, in which female and male structures do not mature simultaneously, thus encouraging cross pollination. While this system is common in early‐divergent angiosperms, it is less widespread in more recently evolved clades. An evaluation of the consequences of this system on outbreeding may provide clues
on this change, but this subject has been little explored.
• Methods: In this work, we characterized the cycle and anatomy of the flower and studied the influence of temperature and humidity on stigmatic receptivity in Annona cherimola, a member of an early‐divergent angiosperm clade with protogynous dichogamy.
• Key results: Paternity analysis reveals a high proportion of seeds resulting from self‐fertilization, indicating that self‐pollination can occur in spite of the dichogamous system. Stigmatic receptivity is environmentally modulated—shortened by high temperatures and prolonged by high humidity.
• Conclusions: Although spatial and temporal sexual separation in this system seems to effectively decrease selfing, the system is modulated by environmental conditions and may allow high levels of selfing that can guarantee reproductive assurance.