Airborne pollen characteristics and the influence of temperature and precipitation in Raleigh,North Carolina,USA (1999–2012)

The incidence of allergic diseases has been increasing in recent decades, in part due to increased exposure to aeroallergens, particularly pollen. Allergic diseases have a major burden on the health care system, with annual costs in the USA alone exceeding $30 billion. There is evidence that the production of aeroallergens, including pollen, is increasing in response to environmental and climatic change, which has important implications for the treatment of allergy sufferers. In this study, pollen data from a Rotorod sampler in Raleigh, North Carolina, was used to characterize and examine trends in the atmospheric pollen seasons for trees, grasses, and weeds over the period 1999–2012. The influence of mean monthly antecedent and concurrent temperature and precipitation on the timing, duration, and severity of the pollen seasons was assessed using Pearson’s product-moment correlation coefficients and multiple linear regression models. An increasing trend was noted in seasonal tree pollen concentrations, while seasonal and peak weed pollen concentrations declined over time. The atmospheric pollen seasons for grasses and weeds trended toward earlier start dates and longer durations, while the tree pollen season trended toward an earlier end date. Peak daily tree pollen concentrations were strongly associated with antecedent temperature and precipitation, while peak daily grass pollen concentrations were strongly associated with concurrent precipitation. The strongest relationships between climate and weed pollen were associated with the timing and duration of the pollen season, with drier antecedent and warmer concurrent conditions tied to longer weed pollen seasons.     

Do pollen carryover and pollinator constancy mitigate effects of competition for pollination?

Pollinator constancy and pollen carryover are both thought to mitigate competitive effects that result when shared pollinators cause loss of pollen to heterospecific flowers. I present analytical and simulation models to investigate how pollinator constancy and pollen carryover interact with each other and with the relationship between pollen receipt and seed set to determine pollination success in competitive environments. With inconstant pollinators, increased pollen carryover reduces variance in pollen receipt without affecting average pollen receipt. Consequently, for flowers requiring at least a threshold quantity of pollen for success, rare flowers with inconstant pollinators benefit from reduced carryover, especially for high pollen receipt thresholds, whereas common flowers benefit from increased carryover, especially for low receipt thresholds. Pollinator constancy is predicted to increase pollen receipt, especially if pollen carryover rates are low. As a result, increased pollinator constancy reduces the range of pollen receipt thresholds for which carryover is beneficial. Similarly, for flowers whose pollination success is a convex function of pollen receipt, carryover is expected to increase fecundity if pollinators are inconstant, but with even a low degree of pollinator constancy, carryover reduces fecundity. These results predict that rare plants with many ovules per flower benefit from dispersing aggregations of pollen, especially if their pollinators exhibit constancy, whereas plants with inconstant pollinators and low thresholds of pollen receipt benefit from pollen grains dispersing individually to increase the number of flowers reached by the pollen.     

Sunny-side up: flower heliotropism as a source of parental environmental effects on pollen quality and performance in the snow buttercup, Ranunculus adoneus (Ranunculaceae)

Floral traits affect mating success via their influence on the microenvironment in which sexual reproduction occurs as well as their impact on pollinator attraction. Here we investigate the importance of flower heliotropism as a source of parental environmental effects on pollen quality and performance. Flowers of the snow buttercup, Ranunculus adoneus, closely track the sun's rays. We experimentally restrained flowers to test for effects of heliotropism on pollen quality and performance after pollination. When equivalent amounts of pollen were transferred to recipient pistils, pollen from solar-tracking donor flowers exhibited a 32% advantage in germination compared to pollen from stationary (tethered) donor flowers. By the end of anthesis, pistils of tracking flowers contained 40% more germinating pollen grains and 44% more pollen tubes midway down the style than pistils of stationary ones. Solar tracking had no direct effect on pollen tube growth. The greater amount of germinating pollen in tracking flowers accounted for the treatment effect on pollen tube density. A survey of pollen receipt and pollen germination in naturally tracking flowers indicated that solar tracking primarily affects pollen tube density by promoting pollen germination rather than pollen deposition. We conclude that flower heliotropism, by enhancing the paternal environment for pollen development and the maternal environment for pollen germination, represents a source of positive parental environmental effects on pollen performance in snow buttercups.     

Pollen-pistil size correlation and pollen size-number trade-off in species of Argentinian Nyctaginaceae with different pollen reserves

Data on pollen and pistil traits from 14 Argentinean Nyctaginaceae species with starch or lipids as pollen reserves are presented. We expect differences in the traits between these two groups of species, but the same pattern within each group for (a) the relationship between pollen size and pistil length assuming that pollen tube length is predetermined by provisions in the pollen independently of the pollen reserve type, and for (b) a trade-off between size and number because available resources for male function are not unlimited. In particular we expect that (a) species with longer pistils will have larger pollen grains, (b) pollen grain size and the number of pollen grains per flower will be negatively correlated. Significant differences in the mean pollen number per flower and mean pollen size were observed between species with different pollen reserve type but not for pistil length. Then, correlation analyses were performed for species with starchy pollen or with pollen with lipids separately. Pollen size - pistil length correlation was positive and significant for species with starchy pollen but not for species with pollen with lipids. On the other hand, pollen size - number correlation was not significant for both starchy and oil-rich species. Results suggest that pollen reserve type would be a relevant factor that constraint pollen size in species of Nyctaginaceae, and that this pollen trait should be considered when studying pollen-pistil relationships, mainly between species of those families with mixed pollen reserves.     

Development of a symptom load index: enabling temporal and regional pollen season comparisons and pointing out the need for personalized pollen information

This study describes the development of an index, the symptom load index, to compare the severity of pollen seasons for pollen allergy sufferers during different seasons in different geographical regions. This is done by comparing symptom data with pollen data. This study is based on symptom data of the Patients’ Hayfever Diary (PHD) and on pollen data from the European Aeroallergen Network (EAN). The PHD allows objective assessment of the season severity. Season length calculated on the basis of pollen data from the EAN and all records of users in a specific region are included in calculation of the symptom load index. It is thus not influenced by pollen counts during the season. Its application to pollen seasons of the three important pollen types birch, grass and ragweed in Austria and Germany from 2009 to 2013 proved that the impact of a pollen season on allergy sufferers is not correlated with total pollen loads. The possibility to calculate the pollen season in the allergy sufferers’ point of view is a novelty that inaugurates new pathways for the future of pollen information. Furthermore, such an index will be of value for the establishment of personalized pollen information and for calculations of pollen thresholds.     

POLLEN-POLLEN AND POLLEN-STYLE INTERACTIONS DURING POLLEN TUBE GROWTH IN ERYTHRONIUM GRANDIFLORUM (LILIACEAE)

Using pairs of pollen donors, I analyzed the growth of pollen tubes growing in different channels of the same style in Erythronium grandiflorum. After 24 hr the length of pollen tubes of randomly selected pollen donors was affected by the presence of other donors. The mean pollen tube lengths of donors did not differ when taken across all of the donor pairings, but in individual pairs, pollen from one donor was often significantly longer than pollen from the other donor when tested across several recipients. Pollen tube lengths were also consistently longer for pollen paired with pollen from the same donor than when paired with pollen from other donors, apparently because of mutual stimulation between the pollen populations from the same plant. In a second experiment, the amount of pollen tube attrition after five days of growth of pollen from a donor growing near (within 3 m) the recipient depended upon the source of other pollen growing in the same style. Local pollen experienced more attrition paired with self pollen than when paired with outcross pollen. Pollen from different outcross distances also modified the attrition of local pollen, but local pollen usually outcompeted pollen from greater distances. Since the growth of local pollen was modified depending upon the source of other pollen growing in the same style, it is probable that recipient styles are selectively inhibiting pollen tubes to produce the patterns of pollen tube interaction observed. The results from these two experiments indicate that the amount of attrition for pollen can be dependent on the composition of the pollen pool. Both direct pollen tube interactions and mediation by the stylar tissue appear to affect the growth rate and attrition of pollen in Erythronium.     

Pollenkitt – its composition, forms and functions

Two types of sticky pollen coat material exist in angiosperms, both produced by the anther tapetum. Pollenkitt is the most common adhesive material present around pollen grains of almost all angiosperms pollinated by animals, whereas tryphine seems to be restricted only to Brassicaceae. Tapetal cell protoplasts have different patterns of development according to the products formed during their development and degeneration. If tryphine is formed, the tapetal cell protoplasts lose their individuality at the microspore stage. If pollenkitt is formed, their contents degenerate at later stages. Cell content is totally reabsorbed, when ripe pollen is not surrounded by any gluing material. Current knowledge of pollenkitt formation, deposition on pollen grains and chemical composition are reviewed and discussed. Methods for detecting this viscous fluid are also presented. The many functions of pollenkitt, deduced from personal observations and the literature, act in the period between anther opening and pollen hydration on the stigma; they are: (1) to hold pollen in the anther until dispersal; (2) to enable secondary pollen presentation; (3) to facilitate pollen dispersal; (4) to protect pollen from water loss; (5) to protect pollen from ultra-violet radiation; (6) to maintain sporophytic proteins responsible for pollen–stigma recognition inside exine cavities; (7) to protect pollen protoplasts from fungi and bacteria; (8) to keep together pollen grains during transport; (9) to protect pollen from hydrolysis and exocellular enzymes; (10) to render pollen attractive to animals; (11) to render pollen visible to animal eyes; (12) to hide pollen from animal eyes; (13) to avoid predation of pollen through smell; (14) to enable adhesion to insect bodies; (15) to enable pollen packaging by bees and to form corbicules; (16) to provide a digestible reward for pollinators; (17) to enable pollen clumps to reach the stigma; (18) to allow self-pollination; (19) to facilitate adhesion to the stigma; (20) to facilitate pollen rehydration. Depending on the developmental program of the species, these functions may act during pollen presentation, in relation to pollinators, during pollen dispersal and when pollen reaches the stigma.     

DIFFERENTIAL SUCCESS OF POLLEN DONORS IN A SELF-COMPATIBLE LILY

If pollen donors are equally effective at siring seeds, the presence of equal proportions of pollen from two pollen donors on a stigma will lead to equal proportions of seeds sired by each pollen donor. Variation in germination rates, pollen-tube growth, and embryo viability may cause one donor to sire more seed than another. We looked for differential donor success in the field by simultaneously applying equal amounts of pollen from two pollen donors. We simultaneously applied equal amounts of self and outcross pollen to receptive stigmas and simultaneously applied pollen from two donors at different physical distances from the recipient. Following simultaneous application of self and outcross pollen, significantly more of the seeds were sired by outcross pollen donors. Seed set following simultaneous application of two outcross donors was also nonrandom. Pollen donors from 100 m were more likely to sire seeds when competing with pollen from plants nearby (1 m). To determine whether pollen-tube growth rates were responsible for these patterns of paternity, we varied the timing of deposition of outcross pollen allowing self pollen tubes a head start on the stigma. Outcross pollen was applied 3 or 24 h after self pollen. In spite of this time delay, the majority of the seeds were again sired by outcross pollen. There was no significant difference in the amount of seeds sired by self pollen between the two delay treatments. This result suggests that mechanisms operating after ovule fertilization may contribute to the discordance between the proportions of the pollen present and the proportions of seeds sired.     

Automated pollen monitoring system using laser optics for observing seasonal changes in the concentration of total airborne pollen

The development of a simple and automatic pollen measurement methodology is required to manage allergic problems caused by airborne pollen. We developed a device and algorithm to automatically monitor airborne pollen by using basic laser optics technology. The device measures the sideward and forward scattering intensities of laser light from each particle. Because this device provides detailed temporal variation in the pollen concentration, the dispersal dynamics of airborne pollen can be effectively analyzed. We compared the pollen counts obtained with the automated method and standard Hirst-type method. Scatter-plot graphs were constructed of the forward and sideward scattering intensities of the observed particles. An extract window methodology was used to estimate the concentrations of the major allergenic pollens. The extract window parameters were obtained for major types of allergenic pollen. The results suggest the possibility of developing a device for monitoring several types of airborne pollen simultaneously. We determined the standard extract window to be used for estimating the concentration of all types of airborne pollen together. A field experiment was performed to evaluate the automated monitoring system with the standard extract window. The estimated temporal variation pattern of the total airborne pollen concentration agreed well with the observed temporal variation pattern for the whole pollen season. The pollen monitor was able to estimate the overall features of seasonal changes in the total airborne pollen concentration.     

First volumetric airborne pollen sampling in Montevideo City,Uruguay

A volumetric aeropalynological sampling was carried out for the first time in MontevideoUruguay, from October 2000 to September 2001, using a Rotorod sampler Model 40. During the year 76 pollen types were identified. Airborne pollen was recorded over the year but a maximum pollen period was observed from August to April. For the rest of the months, pollen concentration was below 1% of the total annual pollen (TP). The pollen spectrum was characterized by the dominance of herbaceous pollen (NAP), which represented 68% of the TP and dominates the spectrum from November to March. Poaceae was the most frequent and abundant pollen type accounting for 45% of TP. The pollen spectrum reflected the floristic diversity of the city and most of the sources of airborne pollen are present in local and regional flora. Fourteen pollen types reach more than 1% of the TP and most of them are cited as allergenic pollen in other regions. These results may prove important for future medical research.     

A systematic review of the effects of temperature and precipitation on pollen concentrations and season timing,and implications for human health

Climate and weather directly impact plant phenology, affecting airborne pollen. The objective of this systematic review is to examine the impacts of meteorological variables on airborne pollen concentrations and pollen season timing. Using PRISMA methodology, we reviewed literature that assessed whether there was a relationship between local temperature and precipitation and measured airborne pollen. The search strategy included terms related to pollen, trends or measurements, and season timing. For inclusion, studies must have conducted a correlation analysis of at least 5 years of airborne pollen data to local meteorological data and report quantitative results. Data from peer-reviewed articles were extracted on the correlations between seven pollen indicators (main pollen season start date, end date, peak date, and length, annual pollen integral, average daily pollen concentration, and peak pollen concentration), and two meteorological variables (temperature and precipitation). Ninety-three articles were included in the analysis out of 9,679 articles screened. Overall, warmer temperatures correlated with earlier and longer pollen seasons and higher pollen concentrations. Precipitation had varying effects on pollen concentration and pollen season timing indicators. Increased precipitation may have a short-term effect causing low pollen concentrations potentially due to “wash out” effect. Long-term effects of precipitation varied for trees and weeds and had a positive correlation with grass pollen levels. With increases in temperature due to climate change, pollen seasons for some taxa in some regions may start earlier, last longer, and be more intense, which may be associated with adverse health impacts, as pollen exposure has well-known health effects in sensitized individuals.

     

GAMETOPHYTIC SELECTION IN RAPHANUS RAPHANISTRUM: A TEST FOR HERITABLE VARIATION IN POLLEN COMPETITIVE ABILITY

Competition among many microgametophytes for a limited number of ovules can lead to both nonrandom fertilization by pollen genotypes and selection for greater sporophytic vigor. The evolutionary implications of this process depend on the extent of heritable genetic variation for pollen competitive ability. Using flower color in wild radish as a genetic marker, we demonstrate differences among pollen donors in competitive ability. Significant differences were found in four out of five pairs of donors. For three pairs of donors, competitive differences were observed in certain maternal plants but not others. To test for heritability of pollen performance, we conducted a selection experiment. We manipulated the intensity of pollen competition for two generations and then tested for differences in the performance of pollen from two selected lines. Competitive ability of pollen derived from each line was assessed relative to a standard unrelated pollen donor, using pollen mixtures on six wild maternal plants. The intensity of previous pollen competition had no overall effect on the proportion of seeds sired by each selected line. In two maternal plants, pollen from intense previous competition was actually inferior, contrary to expectation. Thus, we found no evidence for heritable variation in this trait. Other factors, such as male-female interactions, may influence the outcome of pollen competition. Prevailing theory on the genetic basis of effects of pollen competition on subsequent generations is not supported by our results. Improved protocols for future experiments are discussed.     

Feeding friend and foe: ample pollen mitigates the effects of pollen theft for a gynodioecious plant,Polemonium foliosissimum (Polemoniaceae)

Background and AimsMost angiosperms rely on pollinators to transport pollen and effect fertilization. While some floral visitors are effective pollinators, others act as thieves, consuming pollen but effecting little pollination in return. The importance of pollen theft in male and female reproductive success has received little attention. Here, we examined if pollen consumption by flies altered pollen receipt and exacerbated pollen limitation for a bumblebee-pollinated plant, Polemonium foliosissimum (Polemoniaceae).MethodsTo examine the effect of pollen-thieving flies, we took a three-pronged approach. First, we used single-visit observations to quantify pollen removal and pollen deposition by flies and bumblebees. Second, we manipulated pollen in the neighbourhood around focal plants in two years to test whether pollen reduction reduced pollen receipt. Third, we combined pollen reduction with hand-pollination to test whether pollen thieving exacerbated pollen limitation. Polemonium foliosissimum is gynodioecious in most populations in the Elk Mountains of central Colorado, USA. Thus, we also tested whether pollen theft affected hermaphrodites and females differently.ResultsFlies removed significantly more pollen and deposited less pollen per visit than did bumblebees. Reduction of pollen in the neighbourhood around focal plants reduced pollen receipt in both years but only nearly significantly so in 2015. In 2016, plants were significantly pollen-limited; hand-pollination significantly increased seeds per fruit for both hermaphrodites and females. However, the reduction of pollen around focal plants did not exacerbate pollen limitation for either hermaphrodites or females.ConclusionsOur results suggest that plants tolerate significant consumption of pollen by thieves and pollinators by producing ample pollen to feed both and fertilize available ovules. Our results demonstrate that pollen limitation in P. foliosissimum is driven by lack of effective pollinators rather than lack of pollen. Teasing out these effects elucidates the relative importance of drivers of reproductive success and thus the expected response to selection by different floral visitors.     

Modelling pollen sedimentation in Danish lakes at c.ad 1800: an attempt to validate the POLLSCAPE model

Aims To validate the POLLSCAPE simulation model of pollen dispersal and deposition, and evaluate the effect of factors such as pollen productivity, wind speed and regional plant abundance, using a data set of ad 1800 pollen assemblages and historical land cover data. Location Denmark. Methods ad 1800 land cover from historical maps is digitized for 2000 m radii around 30 Danish lakes (3.5–33 ha). The simulation model POLLSCAPE is used to predict sedimentary pollen assemblages in the lakes from the plant abundance data inferred from these maps, with different model parameter settings for wind speed, pollen productivity, regional pollen loading, etc. The model predictions are compared with observed ad 1800 pollen assemblages from the lake sediment records. Furthermore, pollen productivity is estimated from the ad 1800 pollen and vegetation data using the Extended R‐value model. Results Generally the model reproduces the patterns in the observed pollen assemblages, and for most pollen types there are significant correlations between observed and predicted pollen proportions. The pollen proportions predicted by the POLLSCAPE model are sensitive to the pollen productivity estimates used, the regional background pollen loading and average wind speed. There is a difference in background pollen loading between eastern and western Denmark, especially of Calluna pollen. The fit between predicted and observed pollen assemblages is best at wind speeds around 2.5 m s^?1, and decreases rapidly at lower wind speeds. The pollen productivity estimates from the ad 1800 data set are comparable with estimates from moss polsters in modern analogues of traditional cultural landscapes in Sweden and Norway. Main conclusions The POLLSCAPE model reproduces the patterns in the observed pollen assemblages from the lakes well, considering the uncertainty in the historical plant abundance data. This study indicates that the simulation model can be a useful tool for investigating relationships between vegetation and pollen composition, but also that the simulated pollen proportions are sensitive to the pollen productivity estimates, the regional background and to wind speed.     

Post-pollination mechanisms in Nicotiana longiflora and N. plumbaginifolia: pollen tube growth rate, offspring paternity and hybridization

In natural populations where interfertile species coexist, conspecific and heterospecific pollen can be delivered to the stigmas. Post-pollination mechanisms might determine the seed siring success of different pollen donors within species as well as the chances for hybridization between species. Nicotiana longiflora and N. plumbaginifolia occur in sympatry in Northwest Argentina, where they have overlapping flowering seasons and share floral visitors. We explored (1) pollen tube growth rates for outcross versus self pollen in single-donor pollinations; (2) siring success of self versus outcross pollen donors in competitive pollinations, and (3) possibilities for hybridization by performing two- (outcross conspecific vs. heterospecific) and three-pollen donor (self vs. outcross vs. heterospecific) crosses. In N. longiflora, both pollen tube growth rate and siring success favored outcross pollen over self pollen and strong rejection of heterospecific pollen. In N. plumbaginifolia, pollen tube growth rate was similar for self and outcross pollen, self pollen sired similar numbers of offspring than outcross pollen and heterospecific pollen sired roughly the same number of progeny than self pollen. Results suggest that in natural sympatric populations, interspecific crosses would likely lead to unidirectional hybridization with N. plumbaginifolia as the seed parent.     

Seasonal distribution of pollen in the atmosphere of melbourne: an airborne pollen calendar

Environmental monitoring of pollen grains in the atmosphere of Melbourne has been achieved using Burkard volumetric traps. Twenty-two families of flowering plants and confiers were identified in the pollen counts. About 62% of these pollen grains belonged to trees, 20% to grasses and 9% to herbs and weedy plants. During spring and summer, the atmosphere contained about 70% of the total annual pollen count. Tree pollen, predominantly elm and cypress, occurred abundantly in late winter and spring, with grass pollen predominantly in spring and early summer. These three types of pollen grains occurred in significant amounts, together accounting for more than 60% of the total annual catch. A seasonal incidence chart (pollen calendar) for Melbourne based on 2 years observation has been constructed. This pollen calendar is useful in identifying sources of allergies against particular seasonal airborne pollen types. Comparison of the time of occurrence of a particular pollen type using the pollen calendar and the time of allergic symptoms, can lead to accurate diagnosis and preventive measures being taken. This study has confirmed that grass pollen is the major source of allergenic pollen in the external environment triggering hay fever and allergic asthma in spring and early summer in Melbourne, Australia.     

The collection of pollen by bees

Bees require pollen for their reproduction and pollen comprises the basic larval food for bees. Most bees acquire pollen passively during flower visitation, but female bees may also collect pollen actively with the aid of various structural and behavioral adaptations. Most bees have evolved adaptations to concentrate pollen into discrete loads and transport pollen back to their nests. The various structural and behavioral adaptations of female bees for acquiring and transporting pollen are the basis of this review.     

Marking live conifer pollen for long-distance dispersal experiments

Long-distance dispersal (LDD) theory requires a method for marking live LDD pollen. Such a method must complement more intensive sampling methods inclusive of molecular cytogenetics, proteomics and genomics. We developed a new method for marking live Pinus taeda pollen using two dyes, rhodamine 123 and aniline blue, dissolved in a sucrose solution. Marked and unmarked pollen were compared with respect to in vitro germination, storage, terminal velocity, and in vivo pollen tube penetration of ovules. We found that: (1) both types of marked pollen retained their capacity for germination, (2) both types of marked pollen had similar aerodynamic properties when compared to unmarked pollen controls, (3) marked pollen retained its germination capacity for 48 h, and (4) of the marked pollen, only the aniline-marked pollen penetrated ovules during pollination. Germination declined rapidly for both types of marked pollen after 48 h and before 37 days at -20°C storage, while unmarked pollen lots retained 93% germination at all stages. This method for marking live P. taeda pollen is feasible for tracing LDD pollen only if released and deposited within 48 h of dye treatment.     

Variations in mugwort (Artemisia spp.) airborne pollen concentrations at three sites in central Croatia, in period from 2002 to 2003

In spite of the low atmospheric pollen levels, Artemisia sensitisation and allergy has been reported widely. The aim of the study was to determine the length of pollen season, intradiurnal, daily and monthly pollen variation, and the effect of some meteorological parameters on atmospheric pollen concentrations in Central Croatia. Seven-day Hirst volumetric pollen and spore traps were used for pollen sampling. The Artemisia pollen season lasted from the end of July until the end of September with the highest concentrations in August. The percentage of the total pollen count ranged from 0.52% to 0.92%. The intradiurnal peak occurred between 10 a.m. and 12 a.m. Statistical analysis showed a significant correlations between higher air temperature and high pollen concentration as well as high precipitation and low pollen concentration. Results of this study are expected to help in preventing the symptoms of allergic reaction in individuals with Artemisia pollen hypersensitivity.     

Latrunculin B has different effects on pollen germination and tube growth

The actin cytoskeleton is absolutely required for pollen germination and tube growth, but little is known about the regulation of actin polymer concentrations or dynamics in pollen. Here, we report that latrunculin B (LATB), a potent inhibitor of actin polymerization, had effects on pollen that were distinct from those of cytochalasin D. The equilibrium dissociation constant measured for LATB binding to maize pollen actin was determined to be 74 nM. This high affinity for pollen actin suggested that treatment of pollen with LATB would have marked effects on actin function. Indeed, LATB inhibited maize pollen germination half-maximally at 50 nM, yet it blocked pollen tube growth at one-tenth of that concentration. Low concentrations of LATB also caused partial disruption of the actin cytoskeleton in germinated maize pollen, as visualized by light microscopy and fluorescent-phalloidin staining. The amounts of filamentous actin (F-actin) in pollen were quantified by measuring phalloidin binding sites, a sensitive assay that had not been used previously for plant cells. The amount of F-actin in maize pollen increased slightly upon germination, whereas the total actin protein level did not change. LATB treatment caused a dose-dependent depolymerization of F-actin in populations of maize pollen grains and tubes. Moreover, the same concentrations of LATB caused similar depolymerization in pollen grains before germination and in pollen tubes. These data indicate that the increased sensitivity of pollen tube growth to LATB was not due to general destabilization of the actin cytoskeleton or to decreases in F-actin amounts after germination. We postulate that germination is less sensitive to LATB than tube extension because the presence of a small population of LATB-sensitive actin filaments is critical for maintenance of tip growth but not for germination of pollen, or because germination is less sensitive to partial depolymerization of the actin cytoskeleton.