**10. Conclusions**

*Pollen stages Processes affected by abiotic stress*

presentation

viability **Pollen rehydration** ⋅ Low air relative humidity prevents pollen rehydration and

(\*): This phase is absent when pollen leaves the anther when it opens (e.g. Poaceae) or is launched from the anther (e.g.

The pollen:ovule ratio (P/O) has traditionally been used as a rough estimator of plant breeding systems (Cruden 2000), but little is known about the effect of environmental stresses on this ratio. When pollen is dispersed in aggregates of hundreds of grains (e.g. massulate orchids), the locular space is restricted and limited locular fluid limits nutrition and volume increase [34]. Changes in pollen volume can be measured under optimal or stressed conditions [28, 286]. Pollen dispersed as aggregates provides greater pollination success when the ovary contains multiple ovules [168, 287] and water loss during presentation and dispersal under heat and drought conditions affects only the externally exposed pollen grains and not the internal ones. To improve pollination success, some plants produce different types of pollen (different size, shape, colour, carbohydrate and water content) in one flower. One type, fecundative pollen, is fertile and able to emit the pollen tube and fertilise, while the other type is sterile nutritive pollen that serves as a reward for pollinators who – at the same time – get dusted with fecundative pollen [288]. The flower morphology can affect accessibility of pollen by different pollinators. Self-incompatible dimorphic *Primula* species have two different flower types with reciprocal anther and style length, producing pollen with different water content depending on the position and exposure of the anthers with respect to the corolla tube [289, 290]. Three flower types, producing three types of pollen grains, occur in trimorphic species (e.g. *Lythrum salicaria*) [289, 291]. The differences in flower morphology result in non-random mating patterns in plant populations and may play an important role in pollinator selection

**Table 7.** Stages of male gametophyte development in angiosperms and processes affected by abiotic stresses.

**Pollen dispersal**

castor bean)

**9.4. Breeding systems and pollen:ovule ratio**

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

and adapatation to different environments [292, 293].

⋅ High air relative humidity induces precocious rehydration of

⋅ Low or high temperature extends or reduces pollen

⋅ Low air relative humidity affects pollen viability ⋅ High air relative humidity induces precocious pollen rehydration and can prevent anther dehiscence

⋅ Some volatile compounds emitted by bees affect pollen

pollen grains and pollen tube emission

⋅ Drought reduces flower longevity

affects water availability from the stigma

The diversity in adaptation mechanisms available in nature to secure reproductive success in angiosperms is considerable (Tables 6 and 7). This diversity can serve as a valuable resource to advance our insights into stress adaptation mechanisms that will benefit breeding strategies for crop species. Cytological and morphological studies, combined with other science disci‐ plines (physiology, genetics and genomics) will continue to improve our understanding of pollen development and its adaptation to the environment. The number of genes and mutants involved in male reproduction is steadily increasing [294], but several research areas require further attention:


throughout pollen development, this leads to decreased numbers of viable pollen at anthesis. Larger pollen numbers could be obtained in plants with larger anthers. Anther size is a trait that has been used for selection of cold tolerance in rice [297] and the growth hormone gibberellic acid plays an important role in controlling stamen development [298]. Elucidating the mechanism of interrupting or pausing pollen development under unfav‐ ourable conditions may also provide useful information about avoiding stress damage. Understanding these mechanisms will require a better understanding of the signals driving gametophyte development *per se*. The haploid genome of the male gametophyte is derived from the sporophyte, but very little is known about its functionality in regulating pollenspecific development and metabolism. Achieving this challenge is now within reach, thanks to sensitive new-generation transciptome analysis techniques [29, 71, 299].

