Does climate affect pollen morphology? Optimal size and shape of pollen grains under various desiccation intensity

Abstract

Abstract. Seed production is likely constrained by pollen limitation and the viability of pollen grains decreases rapidly in time due to water evaporation. Any decrease in the surface-to-volume ratio, through increase in size or change in shape of a grain, reduces the rate of water loss. However, grain size trade-offs with the number of grains that can be produced by a plant. Here, we tested the hypothesis that under higher desiccation stress pollen grains become larger and more spherical. We analyzed data on the pollen morphology of eight Rosaceae species and the desiccation intensity based on temperature, potential evapotranspiration and altitude. To explain the mechanisms underlying our results, we present a model that optimizes the size and shape of pollen grains under different conditions. We report that pollen grains under more intense desiccation stress during flowering periods tend to be larger but do not change shape. This conclusion is consistent with the results of a theoretical model presented here. Our report fills a gap in our knowledge about a fundamental process in plant reproduction. We also discuss the significance of our results in light of current palynological and ecological problems (e.g., global climate change).

Cedrus atlantica pollen morphology and investigation of grain size variability using laser diffraction granulometry

“…Consequently, we propose that due to the size variability in Cedrus atlantica pollen, and lack of evidence for climatic influence, it would not be possible to use this as a proxy for climate or environmental reconstruction, as differences in the size of fossil pollen may simply result from the observed natural variation in size between pollen grains. Our findings contrast the suggested climatic influence on grain size observed in other species (Ejsmond et al 2011(Ejsmond et al 2015Griener & Warny 2015), and underscore the need for further investigation of the complex controls on pollen grain size .…”

“…It has also been hypothesised that the size and shape of the pollen grain itself may be influenced by climate (Ejsmond et al 2011). Temperature has been previously linked to pollen grain size (Kurtz &Liverman 1958), andSchoch-Bodmer (1936) proposed that grain size variability in pollen is a result of fluctuations in moisture availability in ambient air during pollen development.…”

“…Temperature has been previously linked to pollen grain size (Kurtz &Liverman 1958), andSchoch-Bodmer (1936) proposed that grain size variability in pollen is a result of fluctuations in moisture availability in ambient air during pollen development. Ejsmond et al (2011) analysed eight Rosaceae species and found grain size increased under desiccation stress (determined by temperature, potential evapotranspiration and altitude). A positive relationship was also found between temperature and pollen size in 232 plant species from 11 taxonomic groups, suggesting possible climatic influence on size during the flowering period (Ejsmond et al 2015) and offering potential for pollen grain size to aid in reconstructions of past environments.…”

Large pollen at high temperature: an adaptation to increased competition on the stigma?

“…The surface-to-volume ratio is one of the major traits that determine the rate of water loss, which in turn translates to viability and chances of successful pollination (Aylor 2003;Ejsmond et al 2011). Thus, larger grains should have an advantage over smaller ones when desiccation intensity increases.…”

“…Thus, larger grains should have an advantage over smaller ones when desiccation intensity increases. Indeed, as long as we assume that desiccation is the major force determining the viability of pollen grains, we can expect that the optimal strategy for a plant flowering under high desiccation intensity is to produce small numbers of relatively large pollen grains (Ejsmond et al 2011). Such a tendency has been confirmed by the intraspecific analysis of pollen size of eight species of Rosaceae according to which plants flowering in conditions with high temperatures and high potential evapotranspiration (PET) produced significantly larger pollen grains than conspecifics growing at sites with lower temperatures and lower PET (Ejsmond et al 2011).…”