Table 1.

Pollen and Stigma Structural Features and Their Roles

Structural FeatureProposed Functional Significance
Pollen grain sizeBiotic and abiotic pollinator preference and fluid dynamics
Number of pollen grains per pollination unitMay increase delivery efficiency
Pollen coatProtects 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 patternInteracts 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 porosityMicrochannels are sites of water egress and ingress during desiccation and hydration; progressive desiccation limits pollen viability and life expectancy
Aperture size, number, and complexityAffect 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
IntineThickness 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 coatDefines 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