Pollen has higher water content when dispersed in a tricellular state than in a bicellular state

ABSTRACT

Pollen is generally dispersed in a sexually immature and somewhat dehydrated, metabolically quiescent state. Yet, in some species, pollen at anthesis is well-hydrated and metabolically active, and in 30 % of angiosperms pollen is dispersed after having formed its sperm cells. Pollen water content and sexual maturity may be correlated, either because both are subject to trade-offs between dispersal viability and post-pollination performance, or because the traits display developmental linkages. We inferred relative water content of sexually immature (“bicellular”) and sexually mature (“tricellular”) pollen of 30 species of angiosperms using a hydration index (HI) that ranges from zero to one, based on how near fresh pollen volume is to its minimal (dehydrated) or maximal (hydrated) volume. Tricellular pollen had 30 % higher HI than bicellular pollen (P < 0.005), after controlling for initial pollen size (larger pollen had higher HI; P < 0.05). A literature survey of 344 species indicated that the tricellular and hydrated states were strongly associated, although all four trait state combinations were present (P < 0.0001). Our results suggest that a common mechanism for the repeated origins of tricellular pollen has been via the loss of controlled pollen dehydration, which enables either accelerated or extended pollen development.

Keywords:
desiccation stress; evolution of development; pollen dispersal; pollen germination; pollen hydration; spore dormancy; trade-off