Pollen and Stigma Structural Features and Their Roles
| Structural Feature | Proposed Functional Significance |
|---|---|
| Pollen grain size | Biotic and abiotic pollinator preference and fluid dynamics |
| Number of pollen grains per pollination unit | May increase delivery efficiency |
| Pollen coat | Protects pollen cells from excess desiccation after dehiscence; protects from UV radiation and pathogen attack; stickiness, color, and aroma may affect interaction with pollination vectors; protein components involved in adhesion, signaling, and compatibility; lipids and proteins are necessary for hydration |
| Exine pattern | Interacts with biotic and abiotic pollination vectors; affects the surface area of the stigma interface; mediates stigma adhesion; retains pollen coat; affects wall strength and elasticity |
| Exine porosity | Microchannels are sites of water egress and ingress during desiccation and hydration; progressive desiccation limits pollen viability and life expectancy |
| Aperture size, number, and complexity | Affect environmental vulnerability to desiccation, fungal invasion, and mechanical stress; accelerated desiccation limits pollen viability and life expectancy; sites of focused water ingress during hydration; allow extreme volume changes accompanying desiccation and hydration; serve as portals for pollen tube exit during germination |
| Intine | Thickness and complexity are inversely coordinated with exine and pollen coat characteristics; specialized layers and inclusions at apertures are involved in pollen tube emergence and invasion of the stigma cell wall |
| Stigma coat | Defines stigmas as uniquely water-permeant sites on the plant; proteins and lipids are involved in adhesion, hydration, and germination; dry stigmas are pollen compatibility sites, with selective support of pollen hydration and germination, whereas wet stigmas often are covered in exudates from apoptotic cells and block inappropriate pollination only in later steps; at the point of contact between pollen and stigma, the two coatings may mix, a process that mediates coat conversion, hydration, germination, and stigma invasion |