Pollen and Pollination
Liberty Hyde Bailey quoting Focke:
p. 231: In a hybrid Sinningia, the pollen of the second year of flowering was better than that of the first.
p. 234: In longer-lived plants, often all the flowers of the first years are sterile, while later, when the plant has reached a certain age, a few fruits are formed; this has been noticed, for example, in Rubus Idaeus x caesius, R. Bellardii x caesius, Calceolaria integrifolia x plantaginea, Crinum Capense x scabrum.
Cook: Rose Hybridization (1905)
As the pollen sometimes is very scarce, it is better to put it only on two, or three pistils, repeating the operation the following day.
Cook: Some Interesting Results in Rose Hybridization (1906)
It took twenty flowers of the Marechal Niel to produce enough pollen to fertilize one flower.
Cook: Hybridizing (1907)
I have several [rose] seedlings, where the pollen was taken from three and four different varieties mixed together, and they are the richest color in red of any I have ever raised.
|14 'Beitrage zur Kenntniss,' &c., 1844, s. 345.
15 'Nouvelles Archives du Museum,' tom. i. p. 27. 1865
Variation of Animals and Plants vol. 2
With respect to plants, nearly the same results were obtained by Kölreuter and Gärtner. This last careful observer, after making successive trials on a Malva with more and more pollen-grains, found,14 that even thirty grains did not fertilise a single seed; but when forty grains were applied to the stigma, a few seeds of small size were formed. In the case of Mirabilis the pollen grains are extraordinarily large, and the ovarium contains only a single ovule; and these circumstances led Naudin15 to make the following experiments: a flower was fertilised by three grains and succeeded perfectly; twelve flowers were fertilised by two grains, and seventeen flowers by a single grain, and of these one flower alone in each lot perfected its seed: and it deserves especial notice that the plants produced by these two seeds never attained their proper dimensions, and bore flowers of remarkably small size. From these facts we clearly see that the quantity of the peculiar formative matter which is contained within the spermatozoa and pollen-grains is an all-important element in the act of fertilisation, not only for the full development of the seed, but for the vigour of the plant produced from such seed.
The Principles of Botany, and of Vegetable Physiology (1805)
Karl Ludwig Willdenow
Koelreuter examined, in a very laborious manner, how many globules of pollen might be required to complete an impregnation. His chief discoveries on this point are as follow:
All the anthers of Hibiscus syriacus contained 4863 globules of pollen, 50 or 60 of which were necessary to complete impregnation. But whenever he took less than 50 globules, then not all the seeds ripened, but those, which were formed, were perfect. Ten globules were the least he could take in this flower, as less would not suffice for it. The Mirabilis Jalappa had 293 globules of pollen in one flower, Mirabilis longiflora 321. But in each of the two plants 2 or 3 globules were sufficient for impregnation. The seed did not appear more perfect, though many more globules were put upon the stigma.
To ascertain whether, in flowers with more than one style, each ought to become impregnated separately, Koelreuter in several of them cut all off but one, and the fecundation was as successful as ever. Even in flowers, in which the style was entirely separated, fecundation took place through one of them. These experiments shew, that the hollow tubes of one style communicate with all the rest, and that more styles and more pollen are formed, merely to ensure their final determination. From this circumstance philosophers have concluded, that the cellular texture of all germens fixed in the receptacle, must cohere amongst each other.
Michurin: Breeding with Rosa lutea (foetida) ()
As for the cases when requested for purposes of cross pollen species R. lutea and in particular the Persian Yellow, then I managed to find an easy way to get her to give anthers. For this is only to remove half-opened flower buds early in the morning and then put them on the dry paper in the shade, preferably in a room where they were the next morning to give a sufficient amount of pollen, which collect the most convenient for use with walls of glass jars or cups, pre-filled with broken flower stamens.
Pipino, et al. Pollen morphology as fertility predictor in hybrid tea roses (2011)
Fertility of hybrid tea roses is often reduced due to their interspecific origin but also to intensive inbreeding. New genotypes used as pollen donors represent an economic risk for a breeding programme, as their influence on seed production is unknown. In this study 11 cut rose genotypes were selected from a company database as high fertile or low fertile male parents, according to the number of seeds per hybridisation. Pollen morphology and in vitro germination of the selected genotypes were characterised. Pollen was either small (mean diameter <30 μm), shrunken, and irregular (abnormal), or large (mean diameter >30 μm), elliptical and crossed by furrows (normal). High correlations were found between the number of seeds produced per hybridisation and the pollen diameter (r = 0.94) or the percentage of normal pollen (r = 0.96). In order to evaluate the predictive power of the models, we conducted regression analyses and performed a validation experiment on genotypes not present in the database and without background information on fertility. Pollen diameter and percentage of normal pollen were characterised and fitted in the regression models for seed set predictions. Validation with an independent dataset gave a good prediction for 83.3% of the data. This indicates that using either the mean pollen diameter or the percentage of normal pollen resulted in effective fertility prediction. This tool could enhance the genetic variability in crossings between hybrid tea roses, thus creating possibilities for less economically risky exploitation of new tetraploid genotypes as male parents.
W. H. Morse: Pollen Selection (1902)
Pollen grains vary in size and vitality, though they may have been grown in the same stamen. In fact, I am selecting my pollen grains. Method used: A piece of unglazed paper is used, shaking the ripe pollen onto it and curving the paper, and at the same time elevating one end so that the pollen runs down onto a plate. On looking at the pollen with a lens we find that a certain amount of inferior grains are left on the paper, and by repeating the operation only the heaviest grains reach the plate. From experience I have found that these selected grains carry with them the general make-up of the plant bearing them, unless the vigor of the stigma overpowers the pollen life. If such is the case the progeny is intermediate between the two in all points. But if the vigor of the pollen predominates it carries with it all the characteristic traits of its parent except color, which invariably leans to the seed-bearing parent. Mixed pollens from differently colored flowers produced all flaked flowers in Gladiolus. But one grain of pollen from a red-flowered Gladiolus on a light salmon-colored one produced a most beautiful pink. The few remaining seed gave results inferior to both parents.
Rose Annual 48: 188-193 (1963)
A Self-Pollination Mechanism And Other Items In Rose Species
Eileen Whitehead Erlanson
It was found in one of my earliest studies (1) that the pollen cells mature before the embryo sac and egg cells. Most of the pollen is shed when the petals first open and insects will carry some of it to flowers opening for the second day, which may then be ready for cross-fertilization. However, a few pollen grains remain on the shriveled anthers on the third day and the stigmas should still be receptive. The rising and incurving of the stamens could very well provide self-fertilization for any remaining eggs not already cross-fertilized. My experiments showed that the roses are self-fertile.
Proc. of the Nat'l Acad. of Sci. of the U.S.A.
Increased Mutation Rate from Aged Datura Pollen
Cartledge, Murray and Blakeslee
The use of pollen abortion as an index of the mutation rate has been discussed in an earlier paper2 where the technique employed is described. Two types of abortion, due respectively to gene and to grosser chromosomal mutations, can be distinguished with a fair degree of accuracy by differences in appearance of the aborted grains. The classification of mutations in table 2 is based on this appearance of the grains and not on cytological study or breeding behavior. The highest rate of pollen abortion mutations found in our aged seed experiments was 8.7% for 21 mutations in 242 plants grown from seven- to eight-year old seed.2 Experiments with heat and other factors (excepting strong radiation) have given lower rates from treated seeds. It seems clear, however, that higher rates have been induced by the use of aged pollen. A total of 29 pollen abortion mutations have been recorded in 193 plants from pollen aged for four to thirteen days, giving a mutation rate of 15.0 %. The 191 control plants, grown from the fresh seeds of a self (made with fresh pollen) on a sib of the plants used for the tests, showed no mutations. The data are shown in table 2. The mutation rates from progenies of fewer than five plants average very high, but the number of plants is extremely small. Percentages for these cases have been put in parentheses in table 2. From these small progenies there are 18 plants recorded with nine, or 50.0%, showing pollen abortion. The conditions of the storage of the pollen, especially in respect to moisture content, were not accurately controlled. It would appear that the conditions which adversely affected the size of the progenies tended to raise the mutation rate. Among the larger progenies, those from eight- to ten-day old pollen show higher rates of mutation than do those from five- to six-day old pollen. The rates are, respectively, 13.9% from 122 plants with 17 mutations, and 5.7% from 53 plants with three mutations.
75-96 (Jan 1931)
Sterility in wild roses and in some species hybrids
Eileen Whitehead Erlanson
In partially sterile pollen many of the grains though apparently perfect morphologically are not able to function in fertilization, as has been found in grapes (DORSEY 1914), in wheat (SAX 1922, WATKINS 1924) and in Nicotiana (EAST 1921).
Student's Text-book of Botany, Volume 2, p. 453
Sydney Howard Vines
It is also known that in a great number of ambisporangiate flowers, pollination is effected by the transfer of pollen from one flower to another: in some of these cases it has been demonstrated that it is only the pollen of another flower which can effect fertilisation; in other cases, that the pollen of the same flower, though not absolutely useless, has less fertilising power than that of another flower; and in yet other cases, that though the pollen of the flower itself has sufficient fertilising effect, yet the progeny is less vigorous than when pollen is supplied from another flower.
Sperm Competition and Sexual Selection (1998) p 162
Flowering Competition n Flowering Plants
Lynda F. Delph and Kayri Havens
… in Raphanus sativus (wild radish) maternal plants preferentially abort stylar ovules (those closest to the style), thereby selecting against those pollen donors that preferentially fertilize stylar ovules (Marshall and Ellstrand 1988). Instead, they favour maturation of ovules towards the basal end of the fruit, especially when stressed. In addition, multiply-sired fruits of wild radish are selectively matured over singly-sired fruits (Marshall and Ellstrand 1986; Marshall 1988, 1991).
American Journal of Botany 76(7): 1081-1088 (July 1989)
Effects of pollen donor identity on offspring quality in wild radish, Raphanus sativus
Marshall D. L., Whittaker K. L.
There were significant effects of paternity on two measures of early growth: leaf number and plant height. Paternal effects on three measures more closely related to fitness; final plant weight, day of first flower production, and total flower number were also significant. Under the conditions of this experiment, final plant weight was probably the best predictor of fitness. The pollen donor that sired the largest seeds in the previous experiment sired offspring that were largest after 8 weeks of growth. Half of the plants were grown under low-water conditions. Paternal effects on growth were not masked by the environmental effects. In fact, some paternal effects became stronger under stress.
Evolution, 47(4): 1080-1093 (Aug., 1993)
Pollen Aperture Polymorphism and Gametophyte Performance in Viola diversifolia
Isabelle Dajoz, Irene Till-Bottraud, Pierre-Henri Gouyon
Environmental factors (and/or stress) also influence pollen vigor. In wild radish, pollen from control plants sire more seeds than pollen from stressed ones, even though stressed plants and controls show no significant differences in pollen size (Young and Stanton 1990).
Science 248: 1631-1633. 1990.
Influence of environmental quality on pollen competitive ability in wild radish.
Young, H. J., and M. L. Stanton
Pollen of Raphanus raphanistrum produced under low nutrient conditions sired fewer seeds than pollen produced under better conditions when the two types were applied on a stigma together. No difference was seen in single-donor crosses.
American Journal of Botany 77(2): 178–187. 1990
Effects of pollination intensity in Campsis radicans.
Bertin, R. I.
Effects of pollination intensity on fruit, seed and seedling characteristics and joint effects of pollen donor and pollination intensity on fruit production were examined. Large pollen loads were more likely to initiate fruit production than small pollen loads, and the former fruits contained more seeds and a greater total seed mass. No further increases in seed number or mass occurred for pollen loads above 4000 grains. The weight of individual seeds was unaffected by pollen load. Effects of pollen donor were generally larger than effects of pollen load, and fruit production from small loads of pollen from one donor were sometimes equal to fruit production from larger pollen loads from another donor. The ratio of pollen grains deposited to resultant seeds increased with pollen load, and several explanations are proposed. Seeds from heavy pollinations emerged better than seeds from light pollinations, but did not differ in speed of germination or in the performance of seedlings up to 126 days. The emergence differences are probably due to differing intensities of pollen tube competition.
Tomato Growers Co-operative 16: 11–12. 1966.
Pollen germination as affected by variety and number of pollen grains.
Hornby, C. A., and W. B. Charles.
During the work on developing cool-temperature tolerant tomatoes, there was reason to question the effects of amounts of pollen on a given stigma, as well as of genetic differences between varieties. Pollen grains were counted and applied to stigmas, and after 48 hours, stigmas were removed. After staining with water soluble aniline blue, the pollen tubes could be counted under ultraviolet light. Several experiments yielded similar results, and can be illustrated with the following data which is expressed as means of percentages of pollen germination from three replications.
|Number of pollen
grains per stigma
|Percentages pollen germination / Variety|
There were highly significant differences between varieties and among the numbers of pollen grains used per stigma.
Some apparent genetic differences in pollen germination may be a result of "density" of pollen on a given stigma.
Journal of Horticulture and Practical
Gardening, 1: 312-313 (July 23, 1861)
INFLUENCE OF THE POLLEN IN THE SAME FLOWER
In the great bulk of the Scarlet or Horseshoe Geraniums there are but seven stamens, four long ones, one of medium length, but which is often wanting, and two almost sessile like the anthers of Wheat—that is, very short indeed, and opening at the bottom face to face. These two are they which reduce a whole family to beggary; first to dwarfs or Tom Thumbs, or better still, to minimums, or the smallest of that kind consistent with vigour sufficient to become a useful plant in cultivation, and, lastly, to the brink of ruin, and drive that race out of existence altogether, if there were not other means provided to arrest the decline, or keep it from manifesting itself at all in a state of Nature.
Journal of Horticulture and Cottage
Gardener, 2: 41-43 (October 15, 1861)
VARIEGATION, CROSS-BREEDING, AND MULING OF PLANTS.
Isaac Anderson Henry
Mr. Beaton as the first, perhaps, to find out, and certainly the first so far as I know, to announce this strange discovery, is entitled to its full merit. Its full value has not yet been sufficiently tested. For although I have produced the tiny things in the Rhododendron family which he has done with Pelargonium, inquiry should not stop here. And for my part I did not limit my aim merely to produce by them more dwarfish plants than the parents. Regarding as I did, the pollen of these small anthers as of finer particles than the pollen of the longer and larger ones, I used it as a provision of Nature's own suggesting, in preference to the latter in crossing the smaller species whose pollen-tubes I feared might not admit the grosser globules of these larger anthers. And when the two dwarf stamens failed, I used the smallest and shortest of the remaining stamens. I still cling to the belief that in this way I effected crosses in which with larger anthers I should have failed. I look on them as affording the chance of effecting unions with remote species or genera—as the links, in short, by which large and family groups might he united.
Sexual Plant Reproduction, 3(1): 7-17 (February, 1990)
Relationships of pollen size, pistil length and pollen tube growth rates in Rhododendron and their influence on hybridization.
E. G. Williams and J. L. Rouse
Pollen size and pistil length data have been collected for 93 species of Rhododendron (Ericaceae) belonging to a number of different subgeneric taxa. For a sample of eight species in section Vireya, pollen tube growth in the style after self- or interspecific pollination has been quantified. Pollen volume and the time taken for pollen tubes to reach the ovary were both related to pistil length. Pollen-tube growth rates were generally greater for species with longer pistils and larger pollen. Increasing temperature increased the rate of pollen-tube growth. There was no detectable effect of pollen tube density on tube growth rate in the style. After interspecific pollinations tube growth rates in foreign styles could be faster or slower than in self styles. A semisterile individual with two viable pollen grains per tetrad and a plant grafted as scion to a longer-styled stock both showed more rapid pollen-tube growth than expected on the basis of pistil size. Data collected for 26 species in section Vireya showed that where extreme disparity of pollen/pistil size causes failure of interspecific crosses, one or more bridging species with intermediate pollen/pistil size can generally be selected.
Effects Of Cross And Self Fertilisation In The Vegetable Kingdom
*It is, however, possible that the stamens which differ in length or construction in the same flower may produce pollen differing in nature, and in this manner a cross might be made effective between the several flowers on the same plant. Mr. Macnab states in a communication to M. Verlot 'La Production des Varietes' 1865 page 42, that seedlings raised from the shorter and longer stamens of rhododendron differ in character; but the shorter stamens apparently are becoming rudimentary, and the seedlings are dwarfs, so that the result may be simply due to a want of fertilising power in the pollen, as in the case of the dwarfed plants of Mirabilis raised by Naudin by the use of too few pollen-grains. Analogous statements have been made with respect to the stamens of Pelargonium. With some of the Melastomaceae, seedlings raised by me from flowers fertilised by pollen from the shorter stamens, certainly differed in appearance from those raised from the longer stamens, with differently coloured anthers; but here, again, there is some reason for believing that the shorter stamens are tending towards abortion.
CybeRose note: MacNab learned of this from Anderson-Henry, who worked for him at the Royal Botanic Garden of Edinburg.
Hybridizing at the Antipodes, Int'l Conf. on Genetics (1906)
H. H. B. Bradley
Seeing that the top division of the perianth is always the largest and best coloured, I generally use the anther, the filament of which is adnate to this division; whether this be the reason or not I do not know, but the progeny generally have more equal divisions to the perianth, and the bottom division is greatly improved.
On the other hand, with a view to getting as white a bloom as possible, I use the bottom division (generally all white) from the white red-striped varieties; and in the seedlings the flowers have much less colour; but the shape of the bloom is spoilt, the divisions being narrow.
Sexual Plant Reproduction 7(4): 215-220 (July 1994)
Effects of soil phosphorus on pollen production, pollen size, pollen phosphorus content, and the ability to sire seeds in Cucurbita pepo (Cucurbitaceae)
Tak-Cheung Lau and Andrew G. Stephenson
This study showed that growing conditions such as soil phosphorus can influence the size of a pollen grain and its chemical composition, which, in turn, can affect its ability to sire mature seeds.
Evolutionary Ecology 19(3): 275-288 (May 2005)
Effects of Nutrient Level on Maternal Choice and Siring Success in Cucumis sativus (Cucurbitaceae)
Teklehaimanot Haileselassie, Margaret Mollel and Io Skogsmyr
Our results show that female effects on siring ability vary with nutrient level. Pollen with a high pollen tube growth rate was more successful when nutrient availability for the female was high.
Sexual Plant Reproduction 11(5): 265-271 (November 1998)
Variation in sporophytic and gametophytic vigor in wild and cultivated varieties of Cucurbita pepo and their F1 and F2 generations
M. H. Jóhannsson and A. G. Stephenson
We found that pollen tubes from the F1 plants had significantly greater growth than tubes from the parental lines or the F2 generation, indicating that hybrid vigor extends to the microgametophytic generation. By partitioning the variance of pollen tube growth into 'within' and 'among' plant components of variation, we were able to show that the genotype of the microgametophyte influences pollen performance in vitro, but that expression of hybrid vigor in the microgametophyte is likely to be due to an environmental effect related to provisioning of the pollen grains during development.
TAG Theoretical and Applied Genetics 92(7): 885-890 (1996)
Effects of pollen selection on progeny vigor in a Cucurbita pepo x C. texana hybrid
M. Quesada, J. A. Winsor, A. G. Stephenson
We examined the effects of pollen selection for rapid pollen-tube growth on progeny vigor. First, we crossed a wild gourd (Cucurbita texana) to a cultivated zucchini (Cucurbita pepo cv 'Black Beauty') to produce an F1 and then an F2 generation. Half of the F1 seeds were produced by depositing small loads of C. texana pollen onto the stigmas of C. pepo. These small pollen loads were insufficient to produce a full complement of seeds and, consequently, both the fast- and the slow-growing pollen tubes were permitted to achieve fertilization. An F2 generation was then produced by depositing small loads of F1 pollen onto stigmas of F1 plants. The F2 seeds resulting from two generations of small pollen loads are termed 'the non-selected line' because there was little or no selection for pollen-tube growth rate on these plants. The other half of the F1 and F2 seeds were produced by depositing large pollen loads (>10000 pollen grains) onto stigmas and then allowing only the first 1% or so of the pollen tubes that entered the ovary to fertilize the ovules. We did this by excising the styles at the ovary at 12-15 h after pollination. The resulting F2 seeds are termed 'the selected line' because they were produced by two generations of selection for only the fastest growing pollen tubes. Small pollen loads from the F2 plants, both the selected and the non-selected lines, were then deposited onto stigmas of different C. pepo flowers, and the vigor of the resulting seeds was compared under greenhouse and field conditions. The results showed that the seeds fertilized by pollen from the selected line had greater vegetative vigor as seedlings and greater flower and fruit production as mature plants than the seeds fertilized by pollen from the non-selected line. This study demonstrates that selection for fast pollen-tube growth (selection on the microgametophyte) leads to a correlated increase in sporophyte (progeny) vigor.
American Journal of Botany. 89:1899-1906. (2002)
The influence of light on paternal plants in Campanula americana (Campanulaceae): pollen characteristics and offspring traits
Julie R. Etterson and Laura F. Galloway
Plants grown under high light produced more pollen grains per flower than those grown under low light. However, the response was genotype specific; some individuals responded little to changes in light availability while others substantially reduced pollen production. As a consequence, paternity ratios may vary between light environments if more pollen is associated with greater siring success. We crossed a subset of these plants to produce the offspring generation. The paternal and maternal light environments influenced offspring seed mass, percentage germination, and days to germination, while only maternal light levels influenced later life traits, such as leaf number and size. Maternal and paternal environmental effects had opposite influences on seed mass, percentage germination and days to germination. Finally, there was no direct relationship between light effects on pollen production and offspring trait expression.
Ecology 82: 2781-2789. (2001)
Parental environmental effects on life history in the herbaceous plant Campanula americana.
Galloway, L. F.
Germination season determines life history in C. americana: fall germinating individuals are annuals while spring germinating seed are biennials. Maternal plants grown under low and high light and low nutrient conditions produced more biennial offspring while the remaining maternal environments had an equal frequency of annual and biennial offspring.
Maternal light environment also did not affect percentage germination. However, the paternal light environment did influence germination percentage. Seeds whose fathers grew under low-light conditions germinated in larger numbers than seeds from fathers in which light was not as limited.
American Journal of Botany. 88(5): 832-840. (May 2001)
The effect of maternal and paternal environments on seed characters in the herbaceous plant Campanula Americana (Campanulaceae)
Laura F. Galloway
Both parental light and nutrient environments influenced seed characters in Campanula americana. Perhaps the most striking finding was that the maternal and the paternal environments frequently influenced seed characters jointly, such that the combination of parental environments determined the offspring phenotype. For example, paternal nutrient status only affected seed mass under low maternal nutrient conditions in the Mountain population. Similarly, the paternal light environment only influenced seed size when maternal plants were grown under medium- and high-light levels. With the exception of percentage germination, interactions between the parental environments had a greater influence on offspring characters than either parental environment alone. These interactions typically resulted in the expression of paternal environmental effects under some but not all maternal environments. However, they were equally common under limited and ample maternal resources. Interaction between parental environments is supported by a study with Plantago that found high paternal temperature increased germination of seeds produced on low-temperature maternal plants, but decreased the germination of seeds produced on high-temperature maternal plants (Lacey, 1996 ).
Effects of Cross and Self Fertilisation in the Vegetable Kingdom (1876) pp. 395-396
The stigmas on two lately expanded flowers on a variety of cabbage called Ragged Jack, were well covered with pollen from the same plant. After an interval of twenty-three hours pollen from the Early Barnes cabbage growing at a distance was placed on both stigmas, and as the plant was left uncovered pollen from other flowers on the Ragged Jack would certainly have been left by the bees during the next two or three days on the same two stigmas. Under these circumstances, it seemed very unlikely that the pollen of the Barnes cabbage would produce any effect but three out of the fifteen plants raised from the capsules thus produced were plainly mongrelized, and I have no doubt that the twelve other plants were affected, for they grew much more vigorously than the self-fertilized seedlings from the Ragged Jack planted at the same time and under the same conditions.
American Journal of Botany 86:261-268. (1999)
The effects of pollen load size and donor diversity on pollen performance, selective abortion, and progeny vigor in Mirabilis jalapa (Nyctaginaceae)
Richard A. Niesenbaum
The influence of pollen competitive environment on pollen performance (pollen germination, stigmatic penetration, and pollen tube growth rate), the maturation or abortion of initiated fruit, seed size, and seedling vigor was explored by manipulating the size and diversity of stigmatic pollen loads on Mirabilis jalapa. All aspects of pollen performance significantly increased with the number of pollen grains on a stigma or pollen tubes in a style, but was not influenced by the diversity of pollen donors. Plants tended to mature single-ovulate fruits that came from flowers where pollen load size and diversity were greatest and aborted those where these were lowest. No plants from seeds resulting from pollinations with a single pollen grain survived, but other fitness measures were mostly determined by maternal plant. The data suggest that pollen performance is influenced by pollen competitive environment, and both the genetic diversity of the pollen load and number of competing pollen tubes are important determinants of seed/fruit abortion.