**7. Consequences of synchrony and asynchrony in pollen development: Pollen competition**

The synchrony of the first meiotic division is likely due to the presence of cytomictic channels that unite the cytoplasm of all the meiocytes present at meiosis within a loculus [203, 204]. These channels close during the meiotic inter-phase and synchrony can be lost from the second meiotic division onwards; the two nuclei within one meiocyte can divide independently, but a certain proportion (30–40% in *Lycopersicum peruvianum*) can still divide synchronously [205]. Nevertheless, the dissolution of the callose wall that keeps the tetrad cells together is synchro‐ nous and is controlled by callase, which is produced and released by the surrounding tapetal cells [95]. Meiotic asynchrony can cause the second haploid mitosis and other cellular processes (vacuolisation, starch hydrolysis storage in plastids, intine formation) to be asynchronous [206]. In orchids, the process of microspore development is synchronous because of the persistence of cytomictic channels throughout meiosis, uniting all the microspores of a loculus until pollen mitosis [33]. Pollen maturation is not a synchronous event from the first mitotic division onwards. Because a large amount of ovules needs pollinating in the ovary, the staggered pollen maturation in orchids may offer an advantage in that overcrowding and competition of germinating pollen on the stigma can be avoided [33].

At anthesis, the release of microspores is controlled by the sporophyte; all pollen grains from a loculus are dehydrated and released irrespective of their developmental stage. In addition, pollen desiccation at the end of pollen development affects all pollen grains of the anther at the same time. The mix of asynchronic and synchronic events during pollen development results in a mixture of pollen grains at slightly different stages of maturity; the difference in physiological stage means that different pollen grains may contain different amount of reserves when they are released together during anthesis. Asynchrony in pollen development is obvious from differences in starch engorgement, vacuolation and pollen size at different stages of development (Figure 3 B–D). Asynchrony can also explain why in vitro pollen germination tests show variable efficiency, particularly for some plant species and for plants grown under stressful circumstances. In vitro pollen germination issues may reflect the in vivo situation; the higher the asynchrony of microspore development, the higher is the percentage of unviable and immature pollen grains at maturity. Environmental stresses such as drought, frost, heat, high humidity (rain and mist) exacerbate the degree of developmental asynchrony [114, 207], causing a further reduction in viable pollen count. Application of heat stress is a common technique used for improving yield of haploid embryos during microspore embryo‐ genesis [208, 209]. Through induction of asynchrony in pollen development, abiotic stresses can affect the production of viable pollen at the gametophytic level. Very little is known about this process and its molecular and physiological basis.

*Stage of development Metabolic activity Physiological effect*

⋅ Water redistributed to other flower

⋅ Desiccation leads to higher pollen osmotic

⋅ Size of pollen grains affects desiccation

⋅ High relative humidity causes precocious rehydration and extra-stigmatic pollen tube emission (especially in recalcitrant pollen)

⋅ The physiological state of the stigmatic surface plays an important role in pollen

pressure

rehydration

⋅ Evaporation through anther cuticle ⋅ Synthesis of protective molecules, proteins (LEA, dehydrins)

humidity, temperature, content in osmotic molecules and their biosynthetic enzymes, levels of protective molecules

⋅ Time for rehydration/pollen tube emission depends on water content, osmotic molecules, biosynthetic enzymes, stigma adhesion

(\*): The ripening and desiccation stages can – at least in some species – be totally or partially overlapping.

**Table 4.** Synthesis of osmotically active components in pollen, and their effect at different stages of development. Orthodox and recalcitrant grains could have a similar physiological behaviour until the onset of desiccation, but the amount and quality of the osmotic molecules and the activity of their biosynthetic enzymes distinguish the two

**7. Consequences of synchrony and asynchrony in pollen development:**

The synchrony of the first meiotic division is likely due to the presence of cytomictic channels that unite the cytoplasm of all the meiocytes present at meiosis within a loculus [203, 204]. These channels close during the meiotic inter-phase and synchrony can be lost from the second meiotic division onwards; the two nuclei within one meiocyte can divide independently, but a certain proportion (30–40% in *Lycopersicum peruvianum*) can still divide synchronously [205]. Nevertheless, the dissolution of the callose wall that keeps the tetrad cells together is synchro‐ nous and is controlled by callase, which is produced and released by the surrounding tapetal cells [95]. Meiotic asynchrony can cause the second haploid mitosis and other cellular processes (vacuolisation, starch hydrolysis storage in plastids, intine formation) to be asynchronous [206]. In orchids, the process of microspore development is synchronous because of the persistence of cytomictic channels throughout meiosis, uniting all the microspores of a loculus until pollen mitosis [33]. Pollen maturation is not a synchronous event from the first mitotic division onwards. Because a large amount of ovules needs pollinating in the ovary, the staggered pollen maturation in orchids may offer an advantage in that overcrowding and

competition of germinating pollen on the stigma can be avoided [33].

Desiccation\* ⋅ Resorption of water by phloem of filament

parts

720 Abiotic and Biotic Stress in Plants - Recent Advances and Future Perspectives

Presentation and dispersal ⋅ Pollen water content is affected by

Rehydration and pollen tube emission

**Pollen competition**

categories in the later stages of pollen development.

Asynchronous development is responsible for pollen competition. Competition between grains occurs at different stages: during development, after rehydration on the stigma and during pollen tube growth. Asynchronous development, combined with the fact that the haploid pollen grains have a different genetic composition due to recombination of the sporophyte genome during meiosis, leads to differences in ability to compete during pollen development and this presents a continuous selective force throughout male gametophyte development. The tapetum cells secrete nutritive substances synchronously, but the asyn‐ chronous pollen grains have a different capacity to use these substances for development, causing competition. Asynchrony in development and differences in genetic composition then lead to competition during rehydration and pollen tube growth and the speed of pollen rehydration depends on the orientation of the pore(s) with respect to the stigma surface. The competition to be the first to fertilise the ovule(s) is an important selective force in plant sexual reproduction and played an important role in both plant and animal evolution [210].
