THE MOTION OF WINDBORNE POLLEN GRAINS AROUND CONIFER OVULATE CONES: IMPLICATIONS ON WIND POLLINATION

Comparisons are presented between the three-dimensional airflow patterns created around and by a scale model of a conifer ovulate cone and the trajectories of windborne pollen grains around Picea, Larix, and Pinus ovulate cones. Three general components of the airflow pattern around an ovulate cone model are 1) doldrum-like eddies, rotating over the adaxial surfaces of cone scales and directed toward attached ovules, 2) airflow spiralling around the cone axis along cone scale orthostichies and parastichies, and 3) a complex pattern of vortices (“umbilicus”) directed toward the leeward surface of the ovulate cone. The observed trajectories of pollen grains around cones of Picea, Larix, and Pinus conform to two of these three airflow components: 1) pollen grains are seen to roll along cone scales toward the distal scale margin and to become reentrained in airflow directed backward toward attached ovules, and 2) pollen grains passing around the cone are deflected into the “umbilicus” airflow pattern, where they either settle on or impact with cone scales (approach trajectories), or where they approach the leeward cone surface but are deflected away by airflow passing under the cone (Z-shaped trajectories). Vectoral analyses of pollen grain motion reveal a complex pattern of trajectories influenced by boundary layer conditions defined by ovulate cone geometry and ambient airflow speed. Wind tunnel studies of ovulate cones subtended by leaves and stem indicate that leaves circumscribing the cone act as a snowfence, deflecting windborne pollen toward the cone. Vectoral analyses of airflow patterns and pollen grain trajectories close to ovulate cones indicate that wind pollination in conifers is a non-stochastic aerodynamic process influenced by cone-leaf morphology and the behavior of pollen grains as windborne particles.     

WIND POLLINATION OF TAXUS CUSPIDATA

The air disturbance patterns created by and around the ovules of Taxus cuspidata are quantified for various orientations to the direction of ambient airflow, and are shown to largely dictate the motion (vectoral trajectories) and mode of deposition of windborne pollen on ovule surfaces. Perpendicular orientation to the direction of airflow results in two regions characterized by high densities of adhering pollen — one on the windward surface of the ovule, resulting from direct inertial collision, and another on the leeward surface resulting from non-inertial sedimentation. Parallel and inclined orientations of the ovule to the direction of airflow produce quantitative and qualitative variations in the pattern of adhering pollen resulting from inertial and non-inertial deposition. Direct collision of windborne pollen grains with the micropylar ends of ovules occurs for all orientations to wind direction. The aerodynamics of the ovulate shoot complex of Taxus cuspidata is related to that previously described for conifer ovulate cones, cycad megastrobili, and simulated wind tunnel analyses of archaic Paleozoic ovules based on scale models. Water transport of pollen (adhering to integument and bract surfaces) to micropyles quantitatively alters the distribution of adhering pollen grains on ovule surfaces. Although there is no evidence that pollen grains of this species are osmotically ruptured, observations do not preclude the possibility that water transport of pollen may reduce the number of viable pollen grains reaching the micropyle.     

Aerodynamics of Ephedra trifurca

Computer simulations are used to predict the behavior of pollen grains with different physical properties within the acceleration field created around the ovules of the gymnosperm Ephedra trifurca. A modelling procedure is given that (1) calculates the number of pollen grains captured by an ovule's pollination-droplet and (2) gives a correlation between pollination efficiency and the physical properties (= mass, size) of different types of pollen. Based on this procedure, the number of Ephedra pollen grains captured by micropyles can be less than the number captured from other species. However, the mass and size of Ephedra pollen grains appear to coincide with those predicted to yield a local maximum of pollination efficiency, i.e. slightly larger or smaller values of either mass or size would decrease the probability of capture. In addition, the properties of Ephedra pollen grains operate synergistically in the aerodynamic environment around ovules and are focused to collide with pollination-droplets. By analogy, the properties of Ephedra pollen coincide with those predicted for a localized adaptive peak. The physical properties of pollen grain types other than E. trifurca that can maximize pollen capture are not generally represented in the aerobiology of Ephedra during the pollination season. Therefore, the phenology of pollen release, community taxonomic-composition, and the physics of particle capture play collectively important roles in the reproductive success of Ephedra trifurca.     

AERODYNAMICS OF WIND POLLINATION IN SIMMONDSIA CHINENSIS (LINK) SCHNEIDER

Wind tunnel analyses of Simmondsia chinensis (Link) Schneider or “jojoba” were conducted to quantify the behavior of airborne pollen grains around individual branches and leaves and near individual carpellate flowers. Field data (wind velocity) were used to ensure a correspondence between wind tunnel and natural conditions. Based upon the visualization of individual pollen grain trajectories, it is concluded that pollen deposition on stigmatic surfaces is influenced by large-scale aerodynamic patterns, generated by foliage leaves, and small-scale airflow patterns, formed around and by floral parts and stigmas. Leaves are seen to deflect airborne pollen grains into trajectories that can intersect ambient airflow at 90° angles, showering decumbent carpellate flowers with pollen. Similarly, flowers can deflect pollen upward and downwind, toward other flowers. The extent of floral bract and sepal recurvature is shown to influence the extent of pollen deposition by determining the characteristic airflow pattern around stigmas. Available evidence concerning the relatively recent evolutionary transition to anemophily in Simmondsia is interpreted within the context of morphological adaptations and exaptations favoring wind pollination.     

THE AERODYNAMICS OF POLLEN CAPTURE IN TWO SYMPATRIC EPHEDRA SPECIES

Wind-tunnel analyses of the behavior of airborne pollen around ovules of two Ephedra species (E. trifurca and E. nevadensis) indicate that at certain airflow speeds (0.5 m/sec and 1.0 m/sec) each species is capable of biasing pollination in favor of conspecific pollen. A computer procedure was designed to evaluate the physical basis for this aerodynamic discrimination. This procedure indicates that differences in size and density confer significantly different inertial properties to the two pollen species. Operating within the specific aerodynamic environments generated around ovules from each species, these differences are sufficient to account for the biases observed in the probability of pollination. Within natural populations, there exists significant variation in pollen size (and possibly in density). Accordingly, it is possible that, under certain ambient wind conditions, ovules from each species can select subsets of the entire airborne population of Ephedra pollen.     

AIRFLOW PATTERNS AROUND SOME EARLY SEED PLANT OVULES AND CUPULES: IMPLICATIONS CONCERNING EFFICIENCY IN WIND POLLINATION

Aerodynamic analyses showing characteristic airflow patterns and the potential for wind-mediated pollination are presented for models of Paleozoic (Carboniferous) ovules and ovulate cupules (i.e., Genomosperma kidstoni, G. latens, Salpingostoma dasu, Physostoma elegans, Eurystoma angulare, and Stamnostoma huttonense). Lobes on ovules and cupules are shown to produce localized regions of turbulent flow with a concomitant reduction in airflow velocity. Data based upon models that mimic the characteristics of windborne pollen (= pseudopollen) show that these regions of turbulent flow correspond to those in which suspended pseudopollen impact with ovule and/or cupule surfaces. These data have bearing on a sequence of ovule morphologies purported to show the evolution of the integument by the progressive reduction in length of “preintegumentary” lobes and their acropetal fusion. As the preintegumentary lobes of the models studied consolidate around the megasporangium, regions of turbulent flow and high pseudopollen impact become localized around the pollen chamber or salpinx. The general morphologic trend envisioned for the evolution of the ovule is seen to be associated with an aerodynamic streamlining and an increased potential for wind-mediated pollination. Data for hair-bearing ovules and for ovulate cupules are discussed within the context of possible selective pressures favouring streamlining.     

AEROBIOLOGY AND POLLEN CAPTURE OF ORCHARD-GROWN PISTACIA VERA (ANACARDIACEAE)

The phenology of pollen release and pollen capture by Pistacia vera was studied in the field and laboratory respectively. Inflorescences of Pistacia vera were examined in a wind tunnel to determine whether the behavior of airborne conspecific pollen around receptive flowers differed as a result of changes in the shape and size of the inflorescence. In addition, the behavior of unclumped (single) and clumped pollen grains was studied to determine differences in the probability of their capture. Wind speeds within a commercial orchard during pollen shedding averaged 0.9–2.2 m/sec and atmospheric pollen concentrations were highest between 0900–1100 hr MST. Each of three stages in inflorescence development (defined on the basis of the number of exserted stigmas) was examined under identical ambient airflow conditions with equal concentrations of airborne pollen (1,000 grains/m³). The general pattern of pollen grain motion involves direct inertial collision by windward surfaces and by sedimentation of pollen onto leeward surfaces; clumped pollen rarely sedimented onto leeward surfaces. Small changes in ambient wind speed (0.5 m/sec to 1.0 m/sec) produced significant changes in the pattern of pollen motion around inflorescences and altered the number of pollen grains captured by leeward surfaces. Thus, wind pollination in P. vera is affected both by windspeed and by the shape or size of flower clusters. Differences in the behavior of clumped and unclumped pollen result from their inertial properties and responsiveness to local changes in the direction and speed of airflow. Unclumped pollen has a higher probability of being captured by leeward surfaces. The apparent insensitivity of pollen motion to differences in inflorescence size may ensure equitable pollination during the acropetal development of flowers.     

CONIFER OVULATE CONE MORPHOLOGY: IMPLICATIONS ON POLLEN IMPACTION PATTERNS

Empirically determined patterns of pollen impaction on the surfaces of pine ovulate cones are correlated with regions of nonlaminar flow created by the spatial arrangement and morphology (aspect ratios) of scale-bract complexes. Results from the serial discharge of pollen, upwind of ovulate cones, indicate that ovules on ovulate cones are preferentially impacted by pollen from their own species. Analyses indicate that while aerodynamic factors dominate the entrapment of pollen by ovulate cones, other factors such as pollen impaction-rebound and rebound-reentrainment are significant. Surface characteristics in addition to the settling velocities of pollen may play important roles in determining pollination efficiency. Wind tunnel analyses of the aerodynamic effects of scale-bract arrangement and aspect ratios indicate that each complex behaves as an aerofoil, deflecting air eddies toward the micropylar ends of ovules. The ovulate cone geometry, as a whole, deflects unidirectional wind into cyclonic vortices around the cone axis, each scale-bract deflecting nonimpacted pollen along orthostichies and parastichies. The morphology of the typical conifer ovulate cone is interpreted as a structure that optimizes anemophilous reproduction.     

攀枝花苏铁传粉生物学研究

在攀枝花苏铁(Cycas panzhihuaensis L.Zhou et S.Y.Yang)自然群体中,雌雄株在数量上基本相等,但雄株的无性系产量是雌株的1.31倍,小孢子叶球的数量是大孢子叶球的2.21倍,呈现偏雄现象。小孢子叶球散粉次序是从轴基部向顶部和从小孢子叶的基部向顶部进行的,散粉高峰出现在午后,风传花粉浓度在2.55m内随着距离增加而迅速下降,而在2.55m以外维持在一个较低的水平上。在大孢子叶球内发现有两种蚂蚁和一种蜚蠊类昆虫在活动,没有发现这些昆虫对大孢子叶球的破坏,小孢子叶球则几乎没有昆虫探访。清晨在大孢子叶叶片上出现许多水样液滴,可能起着将大孢子叶叶片上沉积的花粉传递至胚珠的作用。研究表明,在攀枝花苏铁的传粉过程中,风是将花粉从小孢子叶球传至大孢子叶球的唯一媒介,大孢子叶球内活动的昆虫和大孢子叶叶片上的液滴同样起着传粉媒介作用,但不排除风传花粉一步到位的可能性。     

Pollination Biology of Cycas panzhihuaensis L. Zhou et S.Y.Yang

The population of cone-bearing cycad, Cycas panzhihuaensis L. Zhou et S. Y. Yang, was male-biased. Although the number of male individuals was almost equal to that of female individuals, the clonal and cone production of male individuals was 1.31 and 2.21 times as much as that of female individuals respectively. The sequence of pollen shedding was from the base to the top of the cone and microsporophyll. The peak of daily pollen shedding occurred at noon and in the afternoon. The airborne pollen concentration decreased quickly within 2.55 m and finally maintained at a low level along with the distance from emitting male cone. The megastrobilus was transformed into a receptive state at the beginning of pollination due to the regular morphological changes of megasporophylls. Two kinds of ants and one kind of cockroaches were found to be active within the megas- trobili during pollination, albeit causing no tissue damage. The microstrobili gave off a strong odor of fennel that could dispel all the insects nearby. The sterile foliar structure of the megasporophyll was able to secret aqueous droplets at dawn which might function as transporting adhered pollen grains by dislodging and accumulating them on or near micropyles during the process of droplets falling. Resuits from field observation showed that pollination of C. panzhihuaensis might be accomplished by different pollinators. Pollen grains were firstly wind-transported from microstrobili to megastrobili and then insects and secreted droplets on the megasporophyll either directly or indirectly carried the pollen grains to ovules within a megastrobilus. However, insects might play as a subsidiary pollinator due to the preferential concentration of airborne pollen grains transported to the megasporophylls.     

In vitro pollination of angiosperm ovules with gymnosperm pollen grains

Summary Pollen grains of the gymnosperm species Ephedra distachya and Pinus wallichiana germinated abundantly on the in vitro cultured placentae of the angiosperm species Nicotiana tabacum, Melandrium album and Allium moly. Some P. wallichiana pollen tubes entered the micropyles of M. album. Embryological observations of cross-pollinated M. album ovules 2 or 3 d after cross-pollination revealed the presence of pollen tube remnants within the embryo sacs. Karyological disturbances in the two- and three-celled proembryos and endosperm nuclei indicated their probable hybrid origin. In some crosses, gynogenetic haploid proembryos were also noticed.     

Investigation of Sexual Process in Interspecific Crosses Between Gossipium hirsutum and G. arboreum

The present investigation is the first of a series of cytological researches on inter-specific hybridization of cottons. The present report deals with the cytological behavior of sex and related cells revealed in crossing Gossipium hirsutum (female parent) with G. arboreum. Self-pollenated G. hirsutum served as control. The germination of pollen grains on the stigma and the growth of pollen tubes in the style and the entry of pollen tubes into the micropyles in crosses are in a fairly normal' manner when compared with controls. The percentages of pollen tubes entering ovules of the crossed and controls have no significant difference, being 48.2% in crosses and 54.7% in controls. Fertilization proceeds also normal in cross pollinated ovules. About 92.7% of pollinated ovules are found to have completed the fusion of sexual elements. It is thus: shown that the growth of foreign pollen tubes and fusion of female elements with foreign sperms are not the factors which cause failure in hybridization. The development of endosperm in crosses is abnormal. The number of free nuclei of hybrid endosperm is much smaller than that of controls; the formation of hybrid cellular endosperm is 6 or 7 days earlier than that of controls. The hybrid endosperm cells start to abort soon after their formation. About 14 days after pollination they become completely disintegrated. The hybrid embryo which thus far developed normally becomes arrested in its development and aborts also after that time. It may be concluded that the sterility of crosses between G. hirsutum and G. arboreum is primarily attributed to the abortion of hybrid endosperm. The sound development of the hybrids should be sought in the improvement nutrition of the hybrid embryo from resource other than their own endosperm.     

Overproduction and selective abortion of ovules based on the order of fertilization revisited

Given that seeds fertilized by slowly growing pollen are of low quality genetically, we theoretically reanalyzed the hypothesis that plants selectively abort ovules fertilized later to enhance the mean quality of resulting seeds. We assumed that both superior and inferior pollen exist, the superior pollen growing faster to fertilize ovules, resulting in seeds of higher quality than those of ovules fertilized by inferior pollen. We developed two models to determine the conditions under which selective abortion is favored. In the first model, ovules in one flower are fertilized by pollen grains that arrive at different times, with each visit bringing both fast- and slow-growing pollen. In the second model, ovules in two flowers are fertilized by all pollen grains that arrive at the same time. In the first model, we found that selective abortion based on the order of fertilization is never advantageous irrespective of the duration of the time lag between the two visits. Rather, random abortion is possibly favored. In the second model, although selective abortion based on the order of fertilization can be advantageous, the parameter region favoring it is rather restricted. This is because overproduction can be advantageous only if the quantity of the superior pollen is not limited in one flower but is limited in the other flower. In addition, the degree of overproduction was very low, implying that the merit of overproduction (increase in the number of superior seeds) is low compared to the cost of overproducing ovules. These results suggest that selective abortion of ovules based on the order of fertilization is not as advantageous as previously considered.     

Different regulatory processes control pollen hydration and germination in Arabidopsis

To elucidate the functional differences in how Arabidopsis stigmas regulate pollen hydration and germination, we analyzed receptivity of stigmas, epidermal surfaces (leaves, stems of inflorescence bolts, and floral organs), and an abiotic surface (cover glass) for pollen hydration and germination. Using 65% relative humidity (RH), we found that mature pollen grains were able to hydrate and germinate on stigmas at flower developmental stages 9–13, but not on the distal end of pistils at stage 8, epidermal surfaces, or glass. Furthermore, under 100% RH, pollen grains could hydrate on all tested surfaces, but pollen germination was observed only on the young floral organs (stages 9–12) and the stigmas at stages 9–13. The distal ends of pistils at stage 8, the epidermal surfaces, and the cover glass did not support pollen germination even under 100% RH. Our results indicate that pistil factors regulating pollen hydration and germination are synthesized at stage 9 when stigmatic papillar cells begin to develop. Although pistil factors involved in pollen hydration may only be present on the stigma, the factors involved in pollen germination may localize on both the stigma and surfaces of unopened floral organs.     

The timetable for development of maternal tissues sets the stage for male genomic selection in Betula pendula (Betilaceae)

The influence of the sequence of maternal tissue development in Betula pendula upon the potential for male gamete selection was investigated, and the timing of the fixed abortion of one of the two ovules was determined. We used scanning electronic microscopy, confocal laser scanning microscopy, and blue light microscopy. The stigmas remain fresh throughout male anthesis, and may also last after its end, depending on ambient temperatures. The presence of germinated pollen does not induce stigmatic necrotization, and grains may arrive at different times. The pollen tube tips remain within the stigma base until the end of female anthesis. The ovules will not develop until after necrotization of the stigmas. The pollen tubes thus have a fair start to the ovules, regardless of their different arrival times and of the original positions of the pollen grains at the stigma surface. Therefore, competition among different microgametophytes is possible, in spite of low pollination intensity. Our results indicate that when the first pollen tube penetrates an ovule, this ovule starts to outgrow the other one, and even if the other is also penetrated, its vascular support soon atrophies and the megagametophyte will shrivel. Fertilization of both ovules was never seen in this study.     

EQUATIONS FOR THE MOTION OF AIRBORNE POLLEN GRAINS NEAR THE OVULATE ORGANS OF WIND-POLLINATED PLANTS

A technique is presented that is capable of predicting the motion of airborne pollen grains and the probability of pollen capture by wind-pollinated plants. Equations for the motion of rigid-walled particles (= pollen grains, spores, or Sephadex beads) in a supporting, compressible fluid (= air) are derived from the first principles of fluid dynamics. These equations are incorporated into a computer program (MODEL) which can be used with a desktop computer. The operation of MODEL requires empirical data on the pattern of airflow or the motion of a pollen species around the surfaces of the taxonomically relevant ovulate plant organ. With this information, MODEL can predict the behavior of any pollen species for which physical properties (size and density) are specified or empirically known. The significance of this procedure lies in the quantification of physical phenomena that influence the mechanics and fluid dynamics of pollen capture in wind pollination. The technique is illustrated and tested by its application to two grass species (Setaria geniculata and Agrostis hiemalis) for which velocity fields of pollen motion have been previously reported.     

Pollen tube pathway through the gynoecium of Monotropsis odorata (Monotropaceae)

Monotropsis odorata Elliott. is one of the most cryptic and relatively rare North American endemic species of the Monotropaceae; therefore, information concerning the biology of this achlorophyllous, mycotrophic angiosperm is limited. Bright field and fluorescence microscopic investigations of postpollination gynoecia reveal that pollen tubes follow a continuous pathway from the receptive surface of the stigma to micropyles of the ovules. The pollen tube pathway is characterized by discrete strands of transmitting tissue connecting an exudate-coated stigma to an exudate-filled, stylar canal that leads directly to exudate-coated placentae. Pollen tube growth appears restricted to those areas associated with an exudate. The gynoecial pathway for pollen tube growth in this taxon conforms to those described from other monotropàceous species.     

POLLINATION,FERTILIZATION AND OVULE ABORTION IN OXALIS MAGNIFICA

Pollination, fertilization and ovule abortion were studied in Oxalis magnifica (Rose) Knuth, a strongly self-incompatible herb that regularly matures only a fraction of its ovules. Examination of cleared ovules indicated that among 9 individuals the average number of ovules fertilized ranged from 48–92%. The remaining ovules either failed to produce female gametophytes, or more commonly contained unfertilized female gametophytes, despite large numbers of compatible pollen grains that were placed on stigmas. Abortion of fertlized ovules could be detected first by the flattened and enlarged appearance of the endosperm nuclei, followed by visible deterioration of the embryo. Among individuals the rate of embryo abortion varied from 3.4–47.9%. At lower levels of pollination an almost one-to-one relationship existed between the number of pollen grains placed on stigmas and the number of seeds matured in the capsule. No threshold number of pollen grains necessary for successful pollen tube growth and fertilization could be demonstrated. Reduction in seed number through embryo abortion provides an opportunity for selection among developing seeds. The potential for this form of selection varies widely among individuals of Oxalis magnifica, which showed a 14-fold variation in the average percentage of aborted ovules.     

Efficiency of random pollination and optimal sex ratio

Among all ovules produced by a plant population in an area of arbitrary size, the expectation and the variance is derived for the fraction of pollinated ovules. Particular emphasis is put on the use of parameters which are accessible to direct measurement. The results are applied to the problem of finding the sex ratio in a dioecious plant population which guarantees maximum population fitness, i.e. the maximum number of pollinated ovules per plant. The optimal sex ratio always shows a female excess. The excess increases as either the average number of pollen grains per male plant or the population density increases.