Cyclic pollen production in Cedrus deodara

Based on a seven-year study of pollen production and release in two different natural populations of Cedrus deodara from Garhwal Himalaya, India, we determined that pollen output follows a two-year cycle. Pollen productivity determinations considered various sources of variability, including the number of pollen strobili per branch, strobili per tree, microsporophylls per tree and pollen grains per tree. Each of these parameters revealed significant year-to-year and population effects. Microsporangium dehiscence took from 2.5 to 3.5 days. Maximum dehiscence was observed between 12:00 and 14:00 h, which coincides with diurnal highest temperature and lowest relative humidity. Annual production of pollen per tree varied from averages of 4.7 × 10⁹, 7.2 × 10⁹ and 5.1 × 10⁹ in years of low production, with alternate high years, producing 12.6 × 10⁹, 14.1 × 10⁹, 13.3 × 10⁹ and 14.0 × 10⁹ pollen grains per tree. Annual pollen production in individual trees of C. deodara ranged from 1.4 × 10⁹ to 22.3 × 10⁹.     

POLLEN-OVULE RATIOS IN THE CRUCIFERAE

Several recent studies have revealed sources of variation in pollen-ovule ratios that might limit their usefulness as breeding system indicators. In this survey, pollen-ovule ratios were determined for 66 crucifer taxa (Cruciferae) with known breeding systems to establish whether a clear distinction could be made between the pollen-ovule ratios of allogamous and autogamous crucifers and whether this was sufficient to allow a standard to be set for the Cruciferae. Both the mean number of pollen grains per flower and the pollen-ovule ratio were significantly larger in the allogamous taxa than in the autogamous taxa, whereas the mean number of ovules per flower was not significantly different. However, the pollen-ovule ratio was found to be a much better indicator of the breeding system than the mean number of pollen grains by itself, based on the degree of overlap between the values for the allogamous and autogamous taxa. Variation in the amount of pollen produced by different sets of stamens and a correlation between the pollen-ovule ratio and seed size were identified as potential sources of error in calculating and interpreting the pollen-ovule ratios. This survey suggests that the pollen-ovule ratios of allogamous crucifers are typically greater than 3,500, that those of autogamous crucifers are typically less than 1,000, and that these standards can be used for indicating the breeding systems of other species in the Cruciferae.     

POLLEN-OVULE RATIOS AND BREEDING SYSTEM EVOLUTION IN SOLANUM SECTION BASARTHRUM (SOLANACEAE)

Pollen-ovule ratio, pollen grains per flower, pollen grain volume, pollen biomass, ovules per ovary, seeds per fruit, seed volume, seed biomass, and corolla size were studied to reveal patterns of variation within a monophyletic group. Solanum sect. Basarthrum includes 22 neotropical species of two clades, one having small, few-seeded red fruits, and the other having larger, many-seeded green fruits. The former includes self-incompatible species and a dioecious species, while the latter includes self-incompatible species, self-compatible (and autogamous) species, and a domesticate (the “pepino”). Although the pollen quantity and ovule quantity of the self-incompatible species are significantly higher in the green-fruited subgroup than in the red-fruited subgroup, the pollen-ovule ratios of the self-incompatible species are not significantly different between the two subgroups, suggesting parallel evolution of the pollen-ovule ratio. Furthermore, the pollen-ovule ratio tracks the breeding system: self-incompatible species have significantly higher pollen-ovule ratios than self-compatible species, resulting both from fewer pollen grains and more ovules of the latter. The pollen-ovule ratio of the dioecious species is among the highest of all, resulting only from fewer ovules. The pepino is self-compatible but has a pollen-ovule ratio like its wild self-incompatible progenitors and shows wide variation in seed production and pollen quality, but not in pollen quantity. Among all species, pollen size and quantity are inversely proportional, as are seed size and quantity.     

Within-crown distribution of male and female cones on clonal Sitka spruce trees

The within-crown distribution of cones (strobili) was mapped on 48 mature grafts of Sitka spruce [Picea schensis (Bong.) Carr.]. The total number of cones per tree was increased by a mainstem injection of 20 mg GA_4/7 and 8 mm wide bark ring. The cones and buds were classified as either lateral or terminal on each individual branch. The distribution of lateral female, terminal female, lateral male and terminal male cones, showed a general progression from the upper distal to lower proximal regions of the tree crown in all 12 clones. Evidence for preferential allocation of cones to particular branch types is presented. The region of the tree with the greates number of cones varied with treatment and reflected differences in the sex ratio of the individual trees. There is evidence for a relationship between branch length, position within the crown and the type of cone produced.     

13. Lake Saloio (Nazaré, western Portugal)

The possible impact of altitude and the related microclimatic conditions on the total production of fruiting branches, inflorescences, flowers and pollen grains of olive trees Olea europaea was analysed. A total of 90 Picual cultivar trees, the most extensive olive cultivar in the Iberian Peninsula, were studied for a three-year period (2007–2009). The study shows that production of flowers and pollen grains in a cultivar of the olive tree varies according to the microclimate. Our study also indicates that the olive trees frequently can have up to half a million flowers per tree. Moreover, the total flower production differs between years and study areas. In the Picual cultivar, the average production of pollen grains per anther is usually more than 60?000 grains. The total production of pollen per tree is around 72?000 million on average. The most favourable microclimatic conditions for reproduction in olive trees are found in years and olive growing areas with low temperature and high precipitation records during the months prior to flowering of the olive trees. We hypothesise that olive trees tend to increase their pollen production rate as altitude increases, which can be interpreted as a reproductive strategy to ensure fertilisation.     

Reproductive ecology of dioecious Siparuna (Monimiaceae) in Ecuador–a case of gall midge pollination

The reproductive ecology of the dioecious genus Siparuna , belonging to the primitive family Monimiaceae, was investigated in Ecuador. Thirteen species were studied, and species growing at high altitudes (above 500 m) were compared with species growing at lower altitudes. The genus has specialized flowers pollinated by Cecidomyiidae (Asynapta sp. and other genera) that lay eggs chiefly in male flowers since these are more easily accessible than female flowers due to their morphology. Several traits ensure that pollen transfer is accomplished: 1. long flowering of individuals and single flowers; 2. specific floral morphology (unisexuality, landing platform, restricted pore) allowing the flowers to be visited by Cecidomyiidae seeking egg-laying sites, and 3. a low number of ovules that have to be pollinated in each flower. The low number of ovules per flower results in a high pollen-ovule (P/O) ratio even though pollen production in male flowers is only moderate.
High altitude species had larger flowers, higher pollen-ovule ratios, fewer ovules per female flower, and longer periods of anthesis. Gender differences were seen in the longer functionality of female flowers; the larger numbers of male flowers produced; and the average diameter at breast height (DBH) biased for one of the two sexes depending on the species investigated, with no clear trend in the size distributions of individuals of both sexes. Sex ratios departed significantly from unity both to male and female bias but usually were in favour of males. There were no general differences in the ratio of functional male to female flowers at the time of observation nor in the sizes of male and female flowers. The evolution of unisexual flowers in Siparuna can be explained as a result of the differential predation by larvae: unimportant in male flowers, destructive if occurring in female flowers.     

A lagged, density-dependent relationship between jack pine budworm Choristoneura pinus pinus and its host tree Pinus banksiana

Abstract. 1. Survival of newly emerged jack pine budworm Choristoneura pinus pinus is related to the density of available pollen cones (microsporangiate strobili) produced by its host tree, jack pine Pinus banksiana. 2. A 7‐year time series of observations from a plot network in Ontario, Canada, compared the propensity of jack pine to produce pollen cones, τ, on trees that were either defoliated or undisturbed by the jack pine budworm. 3. Non‐defoliated jack pine trees have a high propensity to produce pollen cones. More than one‐third of these trees produced pollen cones in every year of the series. Propensity varied significantly among plots and trees. Temporal patterns in propensity were also highly variable but within a plot propensity was often autocorrelated in time. 4. Defoliation by the jack pine budworm was associated with forest plots composed of the oldest and the largest trees and with the fewest trees per hectare. Within a plot, outbreaks lasted 3 or 4 years although individual trees were only defoliated in 1 or 2 years. 5. The propensity to produce pollen cones in jack pine was reduced in the years after defoliation. The most pronounced reductions in propensity occurred where defoliation was most severe. 6. The reduction in propensity to produce pollen cones resulting from previous defoliation, coupled with the dependence of jack pine budworm survival on the availability of pollen cones, induces a lagged, negative feedback between the density of the consumer and that of its resource. 7. The lagged, density‐dependent relationship between jack pine budworm and its jack pine host contributes to oscillatory dynamics of the jack pine budworm. Comparison of the outbreak behaviour of jack pine budworm with that of the closely related eastern spruce budworm C. fumiferana suggests that differences in the strength of the host‐plant interaction may account for differences in the relative frequency of outbreaks in the respective systems.     

Male and female reproductive phenology and annual production of male cones in two natural populations of Cedrus deodara

Phenology of reproductive cones, microsporangium dehiscence and pollen production were studied in natural populations of Cedrus deodara at two different locations, the warmer Khirsu (1800  m a.s.l.) and the cooler Teka (1900  m a.s.l.) in the Pauri forest division of Garhwal Himalaya, India. The reproductive phenophases (cone bud initiation to pollination) goes up to four months with a blooming and pollination period of one month. The prolonged period of male cones during the immature phase was due to two months dormancy, and the dormancy, in turn, could be due to high precipitation during this phase. The pattern of opening of male cones was basipetal succession. Microsporangium dehiscence was dependent on air temperature and relative air humidity with a peak between noon and 2  pm. Flowering shows a high level of asynchrony among individuals within the population, leading to a possibility for geitonogamous selfing. Annual production of male cones varies greatly from one year to the next. A reduced number of pollen cones per tree in 2001 resulted in a decline in overall annual pollen production. In conclusion, the results revealed that the production of male cones represent only one factor that substantially influences the mating system, which will interact with the flowering phenology to influence the genetic composition of the population.     

Seasonal Variation in Pollen Catch in a Black Spruce Orchard

Pollen monitoring was conducted over three years (1987–1989) in a 12.4 ha Picea mariana (Mill.) B.S.P. (black spruce) seedling seed orchard established from 1979 to 1984. Pollen traps were installed around and within the orchard and collected daily. Pollen-cone production was highest in 1988 with an average of 0, 11, 20, and 38 cones per tree for 5-, 8-, 9-, and 10-year-old trees. Trees of the oldest section of the orchard bore about 60% of the total number of orchard pollen cones in 1987 and 1988 but only 26% in 1989. Pollen contamination during seed-cone receptivity was important in all three years assessed. The increase in pollen contamination from 1988 to 1989 resulted, in part, from 1) the roguing of 63% of trees in the oldest section of the orchard and 2) a 17-fold decrease in orchard pollen production from 1988 to 1989 compared to a 5-fold decrease from contamination sources.     

Peculiarities of sexual reproduction inFagus crenata forests in relation to annual production of reproductive organs

Annual production rates of reproductive organs inFagus crenata forests in the lower area of the species' range were studied using 10 litter traps in 1980–1986. The production rates of dispersed pollen were estimated by multiplying the number of fallen male inflorescences per ha per year by the mean amount of pollen per inflorescence before anthesis. Large annual fluctuations in the production rates of male and female inflorescences were recognized, whereas their annual trends were synchronized with each other. Pollen production rates were within the range 1.0–6900 (mean: 1630)×10⁹ha⁻¹ yr⁻¹, the maximum/minimum ratio attaining 7000.F. crenata was the lowest producer of pollen among seven tree species studied: the number of pollen grains equivalent to a single ovule was in the range 6.0–14×10⁴. Furthermore, the mean dry weight of a single pollen grain (3.77×10⁻⁵mg) was higher than for wind-pollinated species. Three factors seemed to cause the low seed fertility ofF. crenata. The dry-matter production rate in the best seed year reached 3252 kg ha⁻¹ yr⁻¹, of which pollen accounted for 259 kg ha⁻¹ yr⁻¹. Unproductive years with less than 10% of the maximum production occurred four times in a 7-yr period. In such years there were fewer male and female inflorescences, and more fruit dropped as a result of insect damage. Lower nut dissemination would play an important role in suppressing any increase in nut predators, and fewer flowers would be produced to avoid wastage of photosynthates in a cool-temperate climate.     

Pollination biology and seed production of dioeciousCaryodendron orinocense (Euphorbiaceae) in a plantation in coastal Ecuador

Caryodendron orinocense Karst. (Euphorbiaceae) is an important neotropical, dioecious crop tree providing oil—and protein-rich seeds. Increases in size and number of leaves, flowers, and fruits were estimated for 100 trees in a plantation in costal Ecuador. Height increase was one meter per year—female trees grew less than non-flowering and male individuals. Trees matured at about 4 m height, but flowering and production of fruits remained low until tree height exceeded 8 m. Flowering and leaf production started at the beginning of the rainy season, and flowers were pollinated by large flies. Foliage damage did not seriously reduce leaf number nor fruit production. The plantation studied yielded 150 kg seeds per hectare per year. A plantation with 10 m high trees and a sex ratio of nine females per male will produce 500 kg seeds per hectare per year after about 12 years. Production will increase much more in older plantations.     

Plant reproductive biology of herbaceous monocots in a Venezuelan tropical cloud forest

The reproductive biology of 29 herbaceous understory monocot species was studied in a tropical cloud forest of northern coastal Venezuela (Henri Pittier National Park). Of the 23 plant species, 6 (26.1%) were self-incompatible and 17 (73.9%) self-compatible, the latter consisting of 12 (52.2%) fully self-compatible and 5 (21.7%) partially self-compatible species. An analysis of sexual systems and temporal variation of sex expression showed that of 29 monocot species, 14 (48.3%) were hermaphrodite and adichogamous, 14 (48.3%) monoecious and dichogamous, and one (3.4%) dioecious. The combination of monoecy and dichogamy prevented autogamy in 11 (91.7%) of the 12 self-compatible and monoecious species; one (8.3%) was autogamous. Of all species, 24 (82.8%) were obligate outbreeders, 2 (6.9%) spontaneously autogamous, and 3 (10.3%) partially autogamous. Ovule and flower abortion were not significantly different between the different groups. Pollen-ovule ratio was significantly different between monoecious and hermaphrodite species. The number of pollen grains and ovules per flower and the pollen-ovule ratio were variable among various pollination types. The average pollen-ovule ratio was higher in beetle-, dipteran-, and wind-pollinated species than in bee-, bird-, and bat-pollinated species. The first three pollination types were restricted to monoecious species, and the second three groups were restricted to hermaphrodite species. Of 33 plant species investigated, 12 (36.4%) displayed vegetative reproduction. The reproductive features of the understory monocot species are discussed in the context of life form, devices of cross-pollination, and geological history of the tropical cloud forest.     

Size-number trade-offs and pollen production by Papilionaceous legumes

The intra- and interspecific relations between the size and number of pollen grains were examined for 21 species of papilionaceous legumes to test for the trade-off expected from the subdivision of limited resources. We observed a tradeoff between pollen grain diameter and the number of grains produced per flower within 17 of the 21 species examined. For 12 of these species, the observed partial regression coefficient for In (pollen grain diameter) equaled -3, as expected. The remaining five species exhibited more negative partial regression coefficients than expected. Relations between pollen grain size and number that are more negative than expected may result from a decelerating relation between pollen grain size and the resource investment per grain. Flower production significantly influenced pollen production for only eight species so that pollen production seems to be determined primarily on a per flower basis. Interspecifically, pollen size also varied inversely with pollen number for the 21 species even after the effect of phylogenetic relatedness was removed. Pollen size varies relatively little compared to the number of pollen grains produced per flower and therefore may be the primary target of natural selection. The presence of a trade-off in pollen production probably constrains the evolutionary options by which flowering plants can maximize male success.     

Genotypic variation for reproductive characters,and the influence of pollen-ovule ratio on selfing rate in rape seed (Brassica napus)

The variance component for genotypic effects and covariable plant vigor were estimated for pollen production, number of ovules, the weight of the flower without anther sacs and pistil, and the gender of the whole plant, in the rape seed variety Topas. There was a significant effect of the genotype for all measured characters. We did not find any trade-off between the male and female functions. Small plants were relatively more male-biased than large plants. The weight of the flower without anther sacs or pistil was positively correlated with pollen production. The selfing rates of the lines were positively correlated with the pollen-ovule ratio, suggesting that the selfing rate of individual plants is positively correlated with pollen-ovule ratio, thus supporting the “mass-action” model. It is proposed that the observed increase in the population selfing rate during the flowering season for some species, including rape seed, can be caused by an overall decrease in male investment.     

Annual variation in floral phenology and pollen production in a 25‐year‐old plantation of Tectona grandis

Teak is a timber tree that is widely distributed in the tropics. Several studies on pollination and reproductive biology have been conducted, but generally information on flowering phenology and annual variation in total pollen production per tree is lacking. The reproductive phenology as well as flower‐ and pollen grain production of individuals in a population is important to theoreticians, field biologists and plant breeders, as they determine the distribution of genotypes within populations and influences the degree of differentiation among populations. This study reports flowering phenology and variation in total flower, fruit‐ and pollen production per tree in teak in a 25‐year‐old plantation across three consecutive years (2006–2008). The results show that the date of onset and end of flowering was highly variable across years. The longest flowering period of 93 days was observed in 2006. There was an asynchrony in the number of open flowers due to differences in time of anthesis among individuals (± 2 days) and inflorescences within individuals (± 6 h). The production of pollen grains per tree in 2007 was 33%, i.e. 16% more compared to 2006 and 2008. The fruit production per tree was 42% and 27% higher in 2007 compared to 2006 and 2008. Concentration of pollen grains (both on jelly‐coated microscopic slides and stigmas) were highest between noon and 2 pm. At this time, the stigmatic pollen load ranged between 4–8 pollen grains per stigma, which is sufficient for fruit development. The study concludes that the asynchronization of the flower opening might give rise to a high amount of self‐pollination in the stand, ultimately leading to poor fruit setting. Also, the large production of flowers and pollen per tree induced geitonogamy and decreased female fitness, as T. grandis is preferentially an out‐crossing species.     

Pollen production and release in <Emphasis Type="Italic">Mesua ferrea</Emphasis> Linn. (Guttiferae): a spatio-temporal pattern

Based on a five-year study of pollen production and release in two different natural populations of Mesua ferrea from Indo-Burma region of Northeast India, we determined that pollen output follows a spatio-temporal pattern. Pollen productivity determinations considered various sources of variability, including the number of flowers per branch, flowers per tree, anthers per tree and pollen grains per tree. Each of these parameters revealed significant year-to-year and population effects. Anthesis follows a forenoon pattern, whereas anther dehiscence pursues the diurnal pattern. The former was significantly correlated with the timing of floral visitation and pollen deposition on stigmas. The latter, however, had significant relationship with the deposition of pollen grains on microscopic slides. The Apis and Xylocopa bees are the efficient pollinators to achieve the reproductive success in M. ferrea. Annual production of pollen per tree varied from averages of 1.07 ± 0.10 × 10¹⁰ and 3.24 ± 0.16 × 10¹⁰ in years of low production, with alternate high years, producing 3.85 ± 0.34 × 10¹⁰ and 8.22 ± 0.76 × 10¹⁰ pollen grains per tree.     

Pollen load size, reproductive success, and progeny kinship of naturally pollinated flowers of the tropical dry forest tree Pachira quinata (Bombacaceae)

Several studies have demonstrated, using controlled pollinations, that the number and identity of pollen grains deposited onto a flower's stigma affect the reproductive success of plants. However, few studies have shown this relationship under conditions of natural pollination. Using the tropical dry forest tree Pachira quinata, we evaluated the relationship between the number of microgametophytes per pistil and the number of sires with respect to the production of fruits and seeds in a natural population of Pachira quinata. Our study demonstrates that fruit and seed production are directly related to the number of microgametophytes per pistil in natural populations of P. quinata. Only 6% of the marked flowers developed into mature fruits and 10% of the marked flowers initiated fruits but later aborted them. A mean of 23 pollen grains were required to produce a seed. Flowers with >400 pollen grains on the stigma always developed into mature fruits, whereas flowers that received <200 grains never matured fruits. Half of the pollen grains transferred to a flower stigma germinated and developed pollen tubes to the base of the style. The number of pollen grains on a stigma explained 34% of the variation in seed number per fruit, and the number of seeds produced per fruit is positively correlated with the size of the seeds. The population of P. quinata studied is predominantly outcrossing, and seeds within fruits are sired by one or a few donors. The total seed crop within trees was sired by three to five donors. Our study examined the implications of the variation in size of microgametophyte loads per pistil with respect to the breeding system and the paternity of progeny under natural conditions. The competitive ability of pollen and pollen tube attrition are important factors regulating fruit production in P. quinata.     

Branching habit and the allocation of reproductive resources in conifers

Background and Aims Correlated relationships between branch thickness, branch density, and twig and leaf size have been used extensively to study the evolution of plant canopy architecture, but fewer studies have explored the impact of these relationships on the allocation of reproductive resources. This study quantifies pollen cone production in conifers, which have similar basic reproductive biology but vary dramatically in branching habit, in order to test how differences in branch diameter influence pollen cone size and the density with which they are deployed in the canopy. Methods Measurements of canopy branch density, the number of cones per branch and cone size were used to estimate the amount of pollen cone tissues produced by 16 species in three major conifer clades. The number of pollen grains produced was also estimated using direct counts from individual pollen cones. Key Results The total amount of pollen cone tissues in the conifer canopy varied little among species and clades, although vegetative traits such as branch thickness, branch density and pollen cone size varied over several orders of magnitude. However, branching habit controls the way these tissues are deployed: taxa with small branches produce small pollen cones at a high density, while taxa with large branches produce large cones relatively sparsely. Conclusions Conifers appear to invest similar amounts of energy in pollen production independent of branching habit. However, similar associations between branch thickness, branch density and pollen cone size are seen across conifers, including members of living and extinct groups not directly studied here. This suggests that reproductive features relating to pollen cone size are in large part a function of the evolution of vegetative morphology and branching habit.     

A histological comparison of the development of pollen and female gametophytes in fertile and sterile Cryptomeria japonica

To determine a possible mechanism causing male and female sterility in Cryptomeria japonica male and female cones were collected from a C. japonica, tree, ShinDai2, that lacks pollen release and fertile seeds and specimens were processed to examine the development of pollen and female gametophytes using light microscopy and field emission scanning electron microscopy. Pre-meiotic development proceeded normally, but the formation of aberrant meiotic products was observed in cones of both sexes. In sterile microsporangia, heterogeneous microspore populations ranging from monads to polyads gave rise to mature pollen grains of non-uniform size. These pollen grains were covered with an amorphous layer and adhered to each other. In addition, they remained in the microsporangia and were not released even after the onset of pollen dissemination from fertile trees. In the ovules of sterile female cones, megaspores with abnormal shapes, numbers, and sizes formed, and the development of female gametophytes was arrested at the free nuclear or archegonium formation stages. These gametophytes collapsed, and no fertile embryo was generated. Results indicate that meiotic defects are important in the sterility mechanism.     

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.