It is a matter of timing: asynchrony during pollen development and its consequences on pollen performance in angiosperms-a review.

Functional pollen is needed to successfully complete fertilization. Pollen is formed inside the anthers following a specific sequence of developmental stages, from microsporocyte meiosis to pollen release, that concerns microsporocytes/microspores and anther wall tissues. The processes involved may not be synchronous within a flower, an anther, and even a microsporangium. Asynchrony has been barely analyzed, and its biological consequences have not been yet assessed. In this review, different processes of pollen development and lifetime, stressing on the possible consequences of their differential timing on pollen performance, are summarized. Development is usually synchronized until microsporocyte meiosis I (occasionally until meiosis II). Afterwards, a period of mostly asynchronous events extends up to anther opening as regards: (1) meiosis II (sometimes); (2) microspore vacuolization and later reduction of vacuoles; (3) amylogenesis, amylolysis, and carbohydrate inter-conversion; (4) the first haploid mitosis; and (5) intine formation. Asynchrony would promote metabolic differences among developing microspores and therefore physiologically heterogeneous pollen grains within a single microsporangium. Asynchrony would increase the effect of competition for resources during development and pollen tube growth and also for water during (re)hydration on the stigma. The differences generated by developmental asynchronies may have an adaptive role since more efficient pollen grains would be selected with regard to homeostasis, desiccation tolerance, resilience, speed of (re)hydration, and germination. The performance of each pollen grain which landed onto the stigma will be the result of a series of selective steps determined by its development, physiological state at maturity, and successive environmental constrains.

Structural aspects and ecophysiology of anther opening in Allium triquetrum.

BACKGROUND AND AIMS: Tissue desiccation is considered to be involved in anther opening, and it is agreed that environmental humidity affects its timing. Different sources of evidence suggest that the later steps of the process (i.e. stomium opening and outward wall bending) are regulated in different ways. Anther opening was studied in Allium triquetrum under four regimes of relative humidity (RH) to analyse the effect of this parameter and to speculate about its possible regulation. METHODS: Anther histology was studied in cross-sections under a microscope. The times of visible anther opening and complete outward wall bending were recorded separately for each level of RH. Frequency distributions were plotted to express anther behaviour. KEY RESULTS: When a longitudinal stomium breaks the anther remains closed due to adherence of walls on each side of the stomium. Anther opening occurs when the adhering walls subsequently separate. Later, the walls shrink laterally and bend outward. The anthers of the inner whorl opened during the morning of the first day of anthesis, while those of the outer whorl opened during the afternoon. Low RH (20 %) did not cause any evident acceleration of anther opening, but it did cause delay and inhibition of the opening of some anthers in the outer whorl. High RH (55 and 98 %) caused different degrees of delay and also inhibition of anther opening, but most anthers opened within the expected range of time. The time taken for outward wall bending was shortened at 20 % RH. Anther wall outward bending was inhibited at 55 % and 98 % RH. CONCLUSIONS: Anther opening occurred at a specific moment of anther development, separated in time from stomium breakage, and seemed related to dehydration caused by reabsorption of water by contiguous tissues. Outward bending of the wall was facilitated by evaporation. Anther opening and anther wall outward bending seemed to be regulated differently in relation to water control.