Water loss from flower heads predicts seed release in two invasive thistles

Abstract

Background: Non-random seed release caused by plant responses to weather conditions is important for seed dispersal. Much is known about the effects of wind speed and turbulence, but our understanding of the effects of water loss on seed release is either qualitative, or indirect and phenomenological. Aims: To quantify the empirical relationship between water loss and seed release. Methods: Capitula of the invasive thistles Carduus acanthoides and C. nutans were collected from the field and treated for either 0, 1, or 2 days in the laboratory at three different vapour pressure deficit levels (3.4, 9.5, and 17.0 hPa) to cause a range of water loss values. Total seed release was quantified before and during wind tunnel trials. Results: Water loss was the only significant predictor of whether or not capitula released any seeds. The number of seeds released was predicted by water loss, capitulum diameter, and herbivore damage, with the same amount of water loss having less effect on larger capitula. Conclusions: These results represent an important step towards using weather data to predict seed release for many xerochastic species. Incorporating the effects of water loss on seed release into mechanistic seed dispersal models will greatly improve predictions of when and how far seeds disperse. While initiation of dispersal in some species is more likely during precipitation events (Pufal and Garnock-Jones 2010), a majority of wind dispersed angiosperms and gymnosperms are xerochastic, meaning that drying enhances seed abscission and release (Greene et al. 2008). For many Asteraceae species, including the invasive thistles Carduus nutans L. and Carduus acanthoides L. (Figure 1), drying causes cohesion tissues located on the outer side of the involucral bracts to lose turgidity and buckle, causing the bracts to be lowered away from the seeds and thus exposing seeds to the wind (Fahn 1990). Drying may also cause contraction of the receptacle away from seeds (Smith and Kok 1984).The ubiquity of xerochastic plant species suggests that seed dispersal models could be made more realistic and accurate for a wide range of species if weather data describing potential evaporation could be used to mechanistically predict seed release. Two important pieces of information need to be re-examined to accomplish this goal. First, we must predict water loss from inflorescences using weather data. A number of models already exist that use weather data to predict water loss from entire plants (de Bruin and Holtslag 1982;Sumner and Jacobs 2005), and even from capitula separately from vegetative structures (Guilioni and Lhomme 2006). The second necessary piece of information, the relationship between water loss and seed release, must still be documented and is the focus of this paper.Currently, our understanding of the effects of water loss and weather on seed release is either qualitative or phenomenological. Physiologically, we understand what effects dry conditions have on capitula (Smith and Kok 1984;Fahn 1990) and seed attachm...

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