Measurement of pollen clump release and breakup in the vicinity of ragweed (A. confertiflora) staminate flowers

Abstract

Pollen dispersal is fundamental in a plant's reproductive ecology and as a result of fragmentation of the landscape, questions have been raised on how isolated plant populations can still exchange genetic information. A common mechanism for gene exchange is wind pollination (anemophily). A successful anemophilous species native to North America is ragweed (Ambrosia), a weed that is hard to eradicate after it takes root on crop fields and its pollen causes very strong allergenic effects (hay fever). Despite the extremely clumpy nature of ragweed pollen that should limit its long distance dispersal, ragweed has spread rapidly across the world during the last century while maintaining a high genetic diversity. One of the reasons for its success may be prolific pollen production as well as an efficient pollen clump release and breakup mechanism. Efficient pollen entrainment and dispersal into the atmosphere is highly influenced by flow field characteristics as pollen released into the atmosphere initially encounters flow structures created by the plant's morphology. Here, the flow field in the wake of a mature ragweed spike having multiple staminate flowers as well as pollen release was studied using Particle Image Velocimetry and high‐speed, inline holographic cinematography. The latter enabled to track the pollen in a 3D volume. The spike was set up in a wind tunnel and exposed to wind speeds ranging from 1 to 2 m/s. Results indicated that ragweed pollen was released in multiple sized clumps containing tens to several thousand single grains. Clumps were both “pulled apart” at the point of entrainment while exiting the flowers as well as broke up in smaller sized clumps in high shear rate flow regions not far from the spike. In addition, the largest clumps that did not travel far, deposited in the vicinity of the spike, breaking apart into many small clumps upon deposition. Based on our results, initial release and dispersal of ragweed pollen clumps is characterized by three concomitant mechanisms of clump breakup occurring at entrainment, in mid‐air and on deposition, which make it perfectly suited to disperse pollen over a wide range of spatial scales.

Anatomy solves the puzzle of explosive pollen release in wind‐pollinated urticalean rosids

“…Other functions for mucilage are plausible and can even co‐occur, such as (1) to prevent the tissue rupture in the organs that move very quickly in this complex mechanism of explosive release of the pollen; (2) to prevent the rapid desiccation of the released pollen (see Fahn and Cuttler, ) and, consequently, the loss of pollen longevity since in some species of Urticaceae the pollen grain is considered partially hydrated (Franchi et al., ), and (3) to agglutinate the released pollen, guaranteeing that large amounts of pollen are deposited on the stigma of a pistillate flower (Sabban et al., ; Timerman et al., ), thus improving pollen competition. The last two propositions are supported by the fact that the process of anther dehiscence begins before filament distension, which would allow the contact of the pollen grains with the various floral organs that produce mucilage.…”

Turbulence-induced resonance vibrations cause pollen release in wind-pollinated Plantago lanceolata L. (Plantaginaceae)

“…Alternatively, pollen may be removed passively from the anther by aerodynamic forces (e.g. lift and drag) acting directly on pollen grains within a dehisced anther [9,13,14]. A third possibility, to be explored here, is that agitation of stamens or their supporting scapes or catkins by turbulent winds leads to accelerations sufficient to expel grains into the air column [7,9,[15][16][17].…”

Pollen Aggregation in Relation to Pollination Vector

“…Conversely, a larger clump would be more susceptible to the force of drag (proportional to frontal area), would project farther into the boundary layer, and thus would be more likely to abscise. Interestingly, only two empirical studies of release in relation to PDU size have been undertaken, and both found that pollen release was enhanced by aggregation in the anemophilous Ambrosia (Martin et al 2009;Sabban et al 2012). In contrast to anemophily, pollen aggregation is advantageous in biotic pollination (Pacini 2000;Harder and Johnson 2008), as it can, for example, improve siring success by increasing the adhesion of pollen to floral visitors (via adhesive substances), reducing pollen loss during grooming (via threaded pollen or pollinia), and delivering multiple pollen grains to stigmas (e.g., zoophilous ovaries are often multiovulate, whereas anemophilous ovaries are invariably uniovular).…”