Ovary and Gametophyte Development Are Coordinately Regulated by Auxin and Ethylene following Pollination

The differentiation and development of ovules in orchid flowers are pollination dependent. To define the developmental signals and timing of critical events associated with ovule differentiation, we have examined factors that regulate the initial events in megasporogenesis and female gametophyte development and characterized its progression toward maturity and fertilization. Two days after pollination, ovary wall epidermal cells begin to elongate and form hair cells; this is the earliest visible morphological change, and it occurs at least 3 days prior to pollen germination, indicating that signals associated with pollination itself trigger these early events. The effects of inhibitors of ethylene biosynthesis on early morphological changes indicated that ethylene, in the presence of auxin, is required to initiate ovary development and, indirectly, subsequent ovule differentiation. Surprisingly, pollen germination and growth were also strongly inhibited by inhibitors of ethylene biosynthesis, indicating that male gametophyte development is also regulated by ethylene. Detailed characterization of the development of both the female and male gametophyte in pollinated orchid flowers indicated that pollen tubes entered the ovary and grew along the ovary wall for 10 to 35 days, at which time growth was arrested. Approximately 40 days after pollination, coincident with ovule differentiation as indicated by the presence of a single archesporial cell, the direction of pollen tube growth became redirected toward the ovule, suggesting a chemical signaling between the developing ovule and male gametophyte. Taken together, these results indicate that both auxin and ethylene contribute to the regulation of both ovary and ovule development and to the coordination of development of male and female gametophytes.     

Interspecific incompatibility due to stylar barriers in tuber-bearing and closely related non-tuber-bearing Solanums

Summary The phenomenon of interspecific incompatibility between various wild tuber-bearing and closely related non-tuber-bearing Solanum species was studied. One area of investigation included an examination of possible protein interactions in the incompatibility reaction using SDS electrophoresis. Pollen tube inhibition and morphology were examined in conjunction with biochemical analysis. Two sets of crosses were examined: interspecific tuber-bearing species crosses and interspecific tuber-bearing × non-tuber-bearing species crosses. These crosses had consistent pollen tube inhibition in the upper one-third of the style. The upper third of the styles of incompatibly pollinated, compatibly pollinated, and unpollinated styles was studied under fluorescence microscopy to observe pollen tube growth and morphology. Interspecific tuber-bearing × non-tuber-bearing species crosses demonstrated consistent pollen tube inhibition just below the stigma with frequent pollen tube swelling and bursting and extensive callose deposition along the pollen tube wall. Interspecific tuber-bearing species crosses had pollen tube inhibition further down the style with pollen tube tip tapering and extensive callose deposition. Stylar proteins of the lower two-thirds of the styles were analyzed with SDS electrophoresis. No unique protein differences were found to be specifically associated with the interspecific incompatibility reaction in this portion of the style.Cooperative investigation of the U.S. Department of Agriculture, Agricultural Research Service, and the Wisconsin Experiment Station. Supported in part by the USDA, Cooperative States Research Service Competitive grant no. 83-CRCR-1-1253     

The progamic phase, embryo and endosperm development in an intraspecific Tulipa gesneriana L. cross and in the incongruent interspecific cross T. gesneriana × T. agenensis DC.

 The development of the embryo and endosperm has been investigated in an intraspecific Tulipa gesneriana cross and in the incongruent cross T. gesneriana ×T. agenensis at intervals of 10 days, from 12 to 82 days after pollination (DAP). In both tulip crosses, the zygote gives rise to an apparently undifferentiated cell mass, the proembryonal cell mass, on which a suspensor then develops. Subsequently, a globular embryo is formed on top of the suspensor. This embryo finally elongates, giving rise to a spindle-shaped embryo. The cellular endosperm fills the whole embryo sac in mature seeds, except for a region immediately around the embryo. In both crosses, aberrant developments were found. In the intraspecific T. gesneriana cross, the pollen tubes did not open in a number of ovules. In other ovules, the pollen tubes seemed to have opened, but an embryo or endosperm was not found or only endosperm was observed. In the cross T. gesneriana ×T. agenensis, fewer pollen tubes entered the ovules than in the intraspecific T. gesneriana cross. The ovules with embryo and endosperm formation of the incongruent interspecific cross showed, in general, retarded development in comparison with the intraspecific T. gesneriana cross. The first globular embryos and spindle-shaped embryos were found at the later fixation dates and the relatively lower number of spindle-shaped embryos at 82 DAP had a shorter average length. The number of ovules with deformations in embryo and/or endosperm development was also higher in the cross T. gesneriana × T. agenensis in comparison with the intraspecific T. gesneriana cross. Between 87% and 100% of the ovules with embryo and endosperm development showed normal development in the intraspecific T. gesneriana cross, while in the incongruent interspecific cross, from 22 DAP, between 17% and 56% of the ovules showed normal development. Of those ovules with aberrations in embryo and/or endosperm formation, about 80% had a deformed endosperm, of which more than 50% also contained a deformed embryo. Embryos of the incongruent cross might be saved by the application of embryo rescue techniques. Received: 10 December 1996 / Revision accepted: 23 April 1997     

The signals to trigger the initiation of ovule enlargement are from the pollen tubes: The direct evidence

In angiosperms, initiation of ovule enlargement represents the start of seed development, the molecular mechanism of which is not yet elucidated.It was previously reported that pollen tube contents,rather than double fertilization, can trigger ovule enlargement. However, it remains unclear whether the signal(s) to trigger the initiation of ovule enlargement are from the sperm cells or from the pollen tubes.Recently, we identified a mutant drop1- drop2-, which produces pollen tubes with no sperm cells. Taking advantage of this special genetic material, we conducted pollination assays, and found that the ovules pollinated with drop1- drop2- pollen could initiate the enlargement and exhibited significant enlarged sizes at 36 h after pollination in comparison with those unpollinated ovules. However, the sizes of the ovules pollinated with drop1- drop2- pollen are significantly smaller than those of the ovules pollinated with wildtype pollen. These results demonstrate that the pollen tube, rather than the sperm cells, release the signal to trigger the initiation of ovule enlargement, and that double fertilization is required for further enlargement of the seeds.     

RECIPROCAL DIFFERENCES IN INTRASPECIFIC CROSSES OF TOBACCO RESULT FROM EMBRYO DEATH

Tetraploid tobacco plants (Nicotiana tabacum) derived from cultured cell lines (TC) are partially cross-incompatible with their diploid progenitors (C). C × TC crosses (TC-derived tetraploids as the pollen parent) yield only 2% viable seed. The remaining seeds are normal size but lack an embryo (apoembryonic seeds). Apoembryonic seeds do not occur in the reciprocal TC × C crosses. Sections of ovules from C × TC crosses revealed that an embryo formed but that embryo growth slowed at 5 days postpollination and that by the 12th day following pollination, the embryo had disappeared although the ovule continued to develop. Endosperm degeneration occurred concurrently with embryo death. Culturing ovules from C × TC crosses has increased the yield of F₁ plants from 2% to as much as 25% indicating that embryo rescue is possible. Surviving F₁ plants from TC × C crosses have close to triploid chromosome numbers and are fertile.     

Abscisic Acid Levels during Early Seed Development in Sechium edule Sw

The time-course growth of single tissues in pollinated and unpollinated ovules of Sechium edule Sw. is described in relation to the endogenous levels of abscisic acid. Quantitation of abscisic acid (ABA) in the minute amounts of material obtained after ovule dissection has been performed by using a highly specific and sensitive solid-phase radioimmunoassay based on a monoclonal antibody raised against free (S)-ABA. While the absolute amount of ABA rises in both types of ovules, only in unpollinated ones does this leads to an increase in the hormone concentration. Infact in pollinated ovules the rapid growth following pollination prevents, through a dilution effect, the increase in ABA concentration. Growth patterns and endogenous ABA levels are similar for integuments and nucellus tissues either in pollinated or unpollinated ovules. It is suggested that the growth inhibition induced by the increase in ABA concentration after anthesis could be counteracted by the pollination triggered fast ovule growth.     

Homogalacturonan deesterification during pollen–ovule interaction in Larix decidua Mill.: an immunocytochemical study

Studies on angiosperm plants have shown that homogalacturonan present in the extracellular matrix of pistils plays an important role in the interaction with the male gametophyte. However, in gymnosperms, knowledge on the participation of HG in the pollen–ovule interaction is limited, and only a few studies on male gametophytes have been reported. Thus, the aim of this study was to determine the distribution of HG in male gametophytes and ovules during their interaction in Larix decidua Mill. The distribution of HG in pollen grains and unpollinated and pollinated ovules was investigated by immunofluorescence techniques using monoclonal antibodies that recognise high methyl-esterified HG (JIM7), low methyl-esterified HG (JIM5) and calcium cross-linked HG (2F4). All studied categories of HG were detected in the ovule. Highly methyl-esterified HG was present in the cell walls of all cells throughout the interaction; however, the distribution of low methyl-esterified and calcium cross-linked HG changed during the course of interaction. Both of these categories of HG appeared only in the apoplast and the extracellular matrix of the ovule tissues, which interact with the male gametophyte. This finding suggests that in L. decidua, low methyl-esterified and calcium cross-linked HG play an important role in pollen–ovule interaction. The last category of HG is most likely involved in adhesion between the pollen and the ovule and might provide an optimal calcium environment for pollen grain germination and pollen tube growth.     

Influence of intraovular reserves on ovule fate in apricot (Prunus armeniaca L.)

 In many plant species with multiovulate ovaries, a considerable reduction in the number of ovules takes place. However, the underlying physiological causes are not clear. In Prunus spp., although flowers present two ovules, usually only one seed is produced. We have followed the development and degeneration of the two ovules in apricot (Prunus armeniaca L.) and examined the extent to which carbohydrates within the ovule might be involved in determining the fate of the ovule. While the primary ovule grows in the days following anthesis, growth of the secondary ovule is arrested. Starch distribution along the different ovular tissues exhibits several changes that are different in the two ovules. Primary ovule growth is inversely related to starch content and this growth takes place independently of pollination since it occurs in the same way in pollinated and unpollinated flowers. In the secondary ovule, starch disappears simultaneously from all ovular structures and callose is layered at the chalazal end of the nucellus. The size of the secondary ovule does not change significantly from anthesis to degeneration, and callose starts to accumulate 5 days after anthesis. Likewise, this process occurs independently of pollination. These results are discussed in terms of the implications of the starch content of ovules in fertilization success and ovule fate. Received: 26 August 1997 / Revision accepted: 17 December 1997     

Breeding systems in Clivia (Amaryllidaceae): late‐acting self‐incompatibility and its functional consequences

Late‐acting (ovarian) self‐incompatibility, characterized by minimal or zero seed production following self‐pollen tube growth to the ovules, is expected to show phylogenetic clustering, but can otherwise be difficult to distinguish from early‐acting inbreeding depression. In Amaryllidaceae, late‐acting self‐incompatibility has been proposed for Narcissus (Narcisseae) and Cyrtanthus (Cyrtantheae). Here, we investigate whether it occurs in the horticulturally important genus Clivia (Haemantheae) and test whether species in this genus experience ovule discounting in wild populations. Seed‐set results following controlled hand pollinations revealed that Clivia miniata and C. gardenii are largely self‐sterile. Self‐ and cross‐pollinated flowers of both species had similar proportions of pollen tubes entering the ovary, and those of C. gardenii also did not differ in the proportions of pollen tubes that penetrated ovules, thus ruling out classical gametophytic self‐incompatibility acting in the style, but not early inbreeding depression. Flowers that received equal mixtures of self‐ and cross‐pollen set fewer seeds than those that received cross‐pollen only, but it was unclear whether this effect was a result of ovule discounting or interactions on the stigma. The prevention of self‐pollination by the emasculation of either single flowers or whole inflorescences in wild populations did not affect seed set, suggesting that ovule discounting is not a major natural limitation on seed production. Flowers typically produce one to three large fleshy seeds from approximately 16 available ovules, even when supplementally hand pollinated, suggesting that fecundity is mostly resource limited. The results of this study suggest that Clivia spp. are largely self‐sterile as a result of either a late‐acting self‐incompatibility system or severe early inbreeding depression, but ovule discounting caused by self‐pollination is not a major constraint on fecundity. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2014, 175 , 155–168.     

Variation in ovule and seed size and associated size-number trade-offs in angiosperms

Unlike pollen and seed size, the extent and causes of variation in ovule size remain unexplored. Based on 45 angiosperm species, we assessed whether intra- and interspecific variation in ovule size is consistent with cost minimization during ovule production or allows maternal plants to dominate conflict with their seeds concerning resource investment. Despite considerable intraspecific variation in ovule volume (mean CV = 0.356), ovule production by few species was subject to a size-number trade-off. Among the sampled species, ovule volume varied two orders of magnitude, whereas seed volume varied four orders of magnitude. Ovule volume varied positively among species with flower mass and negatively with ovule number. Tenuinucellate ovules were generally larger that crassinucellate ovules, and species with apical placentation (which mostly have uniovulate ovaries) had smaller ovules than those with other placentation types. Seed volume varied positively among species with fruit mass and seed development time, but negatively with seed number. Seeds grew a median 93-fold larger than the ovules from which they originated. Our results provide equivocal evidence that selection minimizes ovule size to allow efficient resource allocation after fertilization, but stronger evidence that ovule size affords maternal plants an advantage in parent-offspring conflict.     

Pollen tube reallocation in two preanthesis cleistogamous species, Ranalisma rostratum and Sagittaria guyanensis ssp. lappula (Alismataceae)

Pollen deposition on stigmas and pollen tube growth in two apocarpous species, Ranalisma rostratum and Sagittaria guyanensis ssp. lappula (Alismataceae), were examined with fluorescence microscopy. The reallocation of pollen tubes among pistils was observed in both species. The percentage of pollinated stigmas per flower was only 22.0% in R. rostratum and 51.0% in S. guyanensis, though the seed/ovule ratios are higher than 65% in both species. The number of pollen grains on each single stigma ranged from 0 to 96 in R. rostratum, and from 0 to 125 in S. guyanensis. When more than one pollen grain deposited on a stigma, all pollen tubes grew to the ovary, but only one of them turned towards the ovule and finally entered the nucleus. The other tubes grew through the receptacle tissue into ovules of adjacent carpels whose stigmas were unpollinated or pollinated later. The intercarpellary growth of pollen tubes could be a mechanism to increase the efficiency of sexual reproduction in an apocarpous gynoecium with low pollination on the pistils.     

Pollen tubes enter neighbouring ovules by way of receptacle tissue,resulting in increased fruit-set in Sagittaria potamogetifolia Merr

Using fluorescence microscopy, deposition of pollen on stigmas and pollen tube growth in the gynoecium of Sagittaria potamogetifolia Merr., a monoecious species with an apocarpous gynoecium, were observed. The maximum rate of pollination averaged 83.9 +/- 4.7 %, and the number of pollen grains per stigma ranged from zero to 30. Pollen tubes grew through one stigma to the base of the ovary at almost the same speed, but generally only one of the pollen tubes then turned towards the ovule and finally entered the nucellus through the micropyle. The other pollen tubes grew through the ovary base and the receptacle tissue into ovules of adjacent carpels whose stigmas were not pollinated or which had been pollinated later. This phenomenon is termed pollen tube 'reallocation' by the authors. To verify the direct effect of the phenomenon on fruit set, artificial pollination experiments were conducted in which two or more pollen grains were placed onto only one stigma in each gynoecium; frequently more than one fruitlet was obtained from each flower treated. The reallocation of pollen tubes among pistils in the gynoecium could effect fertilization of ovules of unpollinated pistils and lead to an increase in sexual reproduction efficiency. It would, to some extent, also increase pollen tube competition among pistils of the whole gynoecium.     

Production of intraspecific F1 hybrids between wild and cultivated accessions of cowpea (Vigna unguiculata (L.) walp.) using conventional methods

Cowpea (Vigna unguiculata (L.) Walp.) is an important food legume in the tropics. It belongs to the Phaseoleae (L.) tribe (Fabaceae family), it is diploid and its chromosome number is 22. Its gene pool includes the cultivated cowpea and its wild relatives, which are connected with Vigna subgenus, Catiang section. Cowpea has a great potential in increasing food legume production. The cowpea varieties, however, are susceptible to a number of insect pests, especially the pod borer Maruca testulalis and a pod sucking-bug complex (e.g.: Clavigralla tomentosicollis, Anoplocnemis curvipes and Riptortus dentipes), which cause severe damage. The crossing programme presented here exploits the variability existing in the wild African germplasm of V. unguiculata and cultivated cowpea. To incorporate the insect pest resistance into the cultivated cowpea economically, reciprocal crosses between wild forms and cowpea varieties were performed, using the stigmatic pollination methods at anthesis. Some barriers were found in these intraspecific crosses. In the majority of reciprocal crosses, the growth of the pollen tubes was arrested in the stigmatic tissue. Only 16.01% of the ovules were fertilised. In these ovules, embryo development was normal at about 20-25 days after pollination. The failure of the intraspecific crosses in about 80.7% of the cases is thus the result of the lack of fertilisation and the unfertilised ovules. There seems to exist considerable incompatibility within the primary cowpea gene pool. The breeding programme carried out under controlled conditions has proved to be less successful in developed cowpea intraspecific F1 hybrids. Further studies should concentrate on germplasm from Africa with documented resistance to major insect pests. In addition, the application of techniques for bypassing barriers to hybridisation of parent genotypes should enable these embryos to grow to plants.     

COORDINATED TIMETABLES FOR MEGAGAMETOPHYTE DEVELOPMENT AND POLLEN TUBE GROWTH IN RHODODENDRON NUTTALLII FROM ANTHESIS TO EARLY POSTFERTILIZATION

Rhododendron nuttallii T. W. Booth (Ericaceae) was used to derive concurrent timetables for megagametophyte, pollen tube and early postfertilization development from anthesis through 3 wk after pollination, based on timed collections of self-pollinated pistils. Stages of development were determined for over 33,500 cleared ovules, including, for selected collection dates, stages on different portions of the placenta. Pollen tube information was obtained by fluorescence microscopy of pistil squashes stained with aniline blue. Because of the very large number of ovules observed, it was possible to recognize a much more closely graded series of stages in megagametophyte development than is usually the case. While a range of stages occurred on all days, development progressed steadily from a majority of functional spores and 2-nucleate gametophytes on the day of anthesis to mostly a late zygote-primary endosperm stage at 18 days, and some 2-celled endosperm stages at 21 days, after pollination. At all times the most advanced stages, including first pollen tube entries, occurred on the outer surface of the lower half of the placenta, and the youngest on the inner surface of the uppermost portion. Fertilizable ovules were not found in any frequency until 8 days after pollination (then in only about 34% of the ovules); a few fertilized ones were seen after 10 days but constituted less than 5% until 12 days after pollination, thereafter increasing to about 60%. Fertilization occurred in any one of three morphologically recognizable stages distinguished by position and state of fusion of polar nuclei. Pollen germinated on the stigma 1–2 hr after pollination, and pollen tubes grew at a rate of about 1–1.25 cm/day, reaching the top of the ovary in 8–9 days with the first ovule entries seen after 10 days. There was a close correlation between megagametophyte development and pollen tube growth, with large numbers of functionally mature ovules not being found until pollen tubes had reached the ovary. While nuclei within ovules could not be distinguished in the squashes, three gametophyte stages that could be recognized—unelongated, elongated either without or with a pollen tube—were tallied for almost 29,000 ovules. The progression in these general stages corresponded well with that documented in more detail from cleared ovules. Unpollinated pistils showed a similar progression of gametophyte stages until the time fertilization would start to occur, after which there was continued accumulation of functionally mature ovules. A variety of abnormally developed and/or collapsing(ed) ovules or gametophytes were seen; collectively, they averaged over 8.6% of all ovules.     

Breeding systems in Cyrtanthus (Amaryllidaceae): variation in self‐sterility and potential for ovule discounting

• Breeding systems of plants determine their reliance on pollinators and ability to produce seeds following self‐pollination. Self‐sterility, where ovules that are penetrated by self‐pollen tubes that do not develop into seeds, is usually considered to represent either a system of late‐acting self‐incompatibility or strong early inbreeding depression. Importantly, it can lead to impaired female function through ovule or seed discounting when stigmas receive mixtures of self and cross pollen, unless cross pollen is able to reach the ovary ahead of self pollen (‘prepotency’). Self‐sterility associated with ovule penetration by self‐pollen tubes appears to be widespread among the Amaryllidaceae.
• We tested for self‐sterility in three Cyrtanthus species – C. contractus, C. ventricosus and C. mackenii – by means of controlled hand‐pollination experiments. To determine the growth rates and frequency of ovule penetration by self‐ versus cross‐pollen tubes, we used fluorescence microscopy to examine flowers of C. contractus harvested 24, 48 and 72 h after pollination, in conjunction with a novel method of processing these images digitally. To test the potential for ovule discounting (loss of cross‐fertilisation opportunities when ovules are disabled by self‐pollination), we pollinated flowers of C. contractus and C. mackenii with mixtures of self‐ and cross pollen.
• We recorded full self‐sterility for C. contractus and C. ventricosus, and partial self‐sterility for C. mackenii. In C. contractus, we found no differences in the growth rates of self‐ and cross‐pollen tubes, nor in the proportions of ovules penetrated by self‐ and cross‐pollen tubes. In this species, seed set was depressed (relative to cross‐pollinated controls) when flowers received a mixture of self and cross pollen, but this was not the case for C. mackenii.
• These results reveal variation in breeding systems among Cyrtanthus species and highlight the potential for gender conflict in self‐sterile species in which ovules are penetrated and disabled by pollen tubes from self pollen.

     

Hybridization between Fagopyrum (Polygonaceae) species native to China

Thirty-eight crosses between seven Fagopyrum species native to China were studied. Following normal pollination, there were very low inter-group crossabilities (only 0 3%) between five species in the large-achene group and two species (E gracilipes and E pleioramosum) in the small-achene group, and high (15 50%) within the large-achene group, i.e. E esculentum, E tataricum, E zuogongense, E pilus and E megaspartanium. Crosses between various types of (lowers (homostyle, heterostyle etc.) suggested that the intraspecific incompatibility also occurs in interspecific crosses. The hybrid seeds obtained from interspecific crosses all withered or shrivelled and lacked activity. Only one hybrid plant was obtained, tetraploid E esculentum x tetraploid E zuogongense. The hybrid displayed anomalous behaviour at meiosis, pollen grain sterility and withering of achenes, confirming the genetic differences between E esculentum and E zuogongense. The results support the hypothesis that E megaspartanium and E pilus are the possible ancestors of cultivated buckwheat     

The evolution of ovule number and flower size in wind-pollinated plants

In angiosperms, ovules are "packaged" within individual flowers, and an optimal strategy should occur depending on pollination and resource conditions. In animal-pollinated species, wide variation in ovule number per flower occurs, and this contrasts with wind-pollinated plants, where most species possess uniovulate flowers. This pattern is usually explained as an adaptive response to low pollen receipt in wind-pollinated species. Here, we develop a phenotypic model for the evolution of ovule number per flower that incorporates the aerodynamics of pollen capture and a fixed resource pool for provisioning of flowers, ovules, and seeds. Our results challenge the prevailing explanation for the association between uniovulate flowers and wind pollination. We demonstrate that when flowers are small and inexpensive, as they are in wind-pollinated species, ovule number should be minimized and lower than the average number of pollen tubes per style, even under stochastic pollination and fertilization regimes. The model predicts that plants benefit from producing many small inexpensive flowers, even though some flowers capture too few pollen grains to fertilize their ovules. Wind-pollinated plants with numerous flowers distributed throughout the inflorescence, each with a single ovule or a few ovules, sample more of the airstream, and this should maximize pollen capture and seed production.     

Cryptic self‐incompatibility and distyly in Hedyotis acutangula Champ. (Rubiaceae)

Distyly, floral polymorphism frequently associated with reciprocal herkogamy, self‐ and intramorph incompatibility and secondary dimorphism, constitutes an important sexual system in the Rubiaceae. Here we report an unusual kind of distyly associated with self‐ and/or intramorph compatibility in a perennial herb, Hedyotis acutangula. Floral morphology, ancillary dimorphisms and compatibility of the two morphs were studied. H. acutangula did not exhibit precise reciprocal herkogamy, but this did not affect the equality of floral morphs in the population, as usually found in distylous plants. Both pin and thrum pollen retained relatively high viability for 8 h. The pollen to ovule ratio was 72.5 in pin flowers and 54.4 in thrum flowers. Pistils of pin flowers remained receptive for longer than those of thrum flowers. No apparent difference in the germination rate of pin and thrum pollen grains was observed when cultured in vitro, although growth of thrum pollen tubes was much faster than that of pin pollen tubes. Artificial pollination revealed that pollen tube growth in legitimate intermorph crosses was faster than in either intramorph crosses or self‐pollination, suggesting the occurrence of cryptic self‐incompatibility in this species. Cryptic self‐incompatibility functioned differently in the two morphs, with pollen tube growth rates after legitimate and illegitimate pollination much more highly differentiated in pin flowers than in thrum flowers. No fruit was produced in emasculated netted flowers, suggesting the absence of apomixis. Our results indicate that H. acutangula is distylous, with a cryptic self‐incompatibility breeding system.     

Pollen tube growth and early ovule development following self- and cross-pollination in<Emphasis Type="Italic"> Eucalyptus nitens</Emphasis>

Controlled self- and cross-pollinations were conducted on flowers of five mature Eucalyptus nitens trees. Levels of self-sterility of the trees ranged from 25.8 to 93.6%. Pollen tube numbers in styles and ovule penetration by pollen tubes was investigated 2 weeks after pollination by fluorescence microscopy. There were no significant differences between treatments in the number of pollen tubes present in styles or in the percentage of ovules penetrated by pollen tubes. Embryology of material harvested 2 and 4 weeks after pollination was investigated by bright-field microscopy. Fertilisation had taken place by 2 weeks after pollination with nearly every ovule showing evidence of fertilisation. Cross-pollination resulted in a greater proportion of healthy, developing ovules, at both 2 and 4 weeks after pollination, compared with self-pollination. The proportion of degenerating ovules increased from 2 to 4 weeks after pollination. The reduced ability of E. nitens to set self-pollinated seed compared with cross-pollinated seed appears to be controlled by a post-zygotic mechanism. Differences in ovule size may potentially assist in the identification of trees incapable of setting self-pollinated seed.     

焦核与大核龙眼雌配子体发育、授粉受精及早期胚胎发育的比较研究

利用人工授粉,采用压片法对大核龙眼‘九月乌’和焦核龙眼‘闽焦64-1’、‘闽焦64-2’、‘白核’等的自交与杂交后花粉管的生长特性进行研究,同时应用常规石蜡切片技术对大核与焦核龙眼的雌配子体以及合子胚早期发育进行观察。结果表明,龙眼胚珠在单核胚囊形成前就开始败育,且焦核品种(系)的败育率显著高于大核品种。不同亲本组合的授粉率存在差异,所有授粉组合在授粉36～48 h后均能观察到1个花粉管生长并进入胚囊受精。焦核品种(系)的胚胎在谢花后10 d开始败育,且败育率明显高于大核品种。受精是龙眼子房发育的首要条件,胚珠败育的雌蕊在谢花后10 d不膨大,不能发育形成焦核果实。谢花后10～30 d的早期胚胎败育是形成焦核龙眼的主要原因。焦核品种‘白核’胚乳具有成胚能力。约有24%的‘闽焦64-1’胚珠在胚胎发育过程中,其助细胞、合点端细胞及胚乳发生异常,这可能与早期胚胎败育有关。