β-Glucan synthetase II activity upon callose formation in the flower ofArabidopsis thaliana

Callose formation was observed in the pollens during flower development and pollen tube grown in the pistil ofA. thaliana. The accumulation of callose occurred in the tetrad in the flower bud and pollen tube. Therefore, the activity of β-glucan synthetase II (GS II), which is responsible for synthesizing the callose, was measured in the flowers on the same developmental stages. The enzyme activity was increased by about 10% while the level of callose contents was increased by about 70% in tetrads. Then, callose accumulation was increased during pollen tube growth by about 30% higher than the other stages and enzyme activity was detected, 30% more too. These results suggest that callose plays an important role in the growth of pollen and pollen tube by increasing GS II activity.     

冬季低温条件下香石竹小孢子败育的细胞学研究

利用压片法及石蜡切片法观察冬季低温下香石竹小孢子发育过程,以明确低温导致香石竹小孢子败育的因素,为杂交育种奠定基础。结果表明:(1)冬季低温下香石竹只有部分小孢子发育正常,经过小孢子母细胞、减数分裂和四分体等时期,最后发育成花粉。(2)石蜡切片法观察到冬季低温下香石竹1.5~1.6cm长花蕾中有61%的花粉母细胞发生败育,1.7~1.8cm长花蕾中有71%的花粉母细胞发生败育。(3)部分已经进入四分体时期的小孢子胼胝质未能及时溶解,妨碍了小孢子释放而导致败育。研究认为,花粉母细胞和四分体的发育异常是冬季低温下香石竹小孢子败育的主要原因。     

Tetrad pollen formation in quartet mutants of Arabidopsis thaliana is associated with persistence of pectic polysaccharides of the pollen mother cell wall

The quartet (qrt) mutants of Arabidopsis thaliana produce tetrad pollen in which microspores fail to separate during pollen development. Because the amount of callose deposition between microspores is correlated with tetrad pollen formation in other species, and because pectin is implicated as playing a role in cell adhesion, these cell-wall components in wild-type and mutant anthers were visualized by immunofluorescence microscopy at different stages of microsporogenesis. In wild-type, callose was detected around the pollen mother cell at the onset of meiosis and around the microspores during the tetrad stage. Microspores were released into the anther locule at the stage where callose was no longer detected. Deposition and degradation of callose during tetrad pollen formation in qrt1 and qrt2 mutants were indistinguishable from those in wild-type. Enzymatic removal of callose from wild-type microspores at the tetrad stage did not release the microspores, suggesting that callose removal is not sufficient to disperse the microspores in wild-type. Pectic components were detected in the primary wall of the pollen mother cell. This wall surrounded the callosic wall around the pollen mother cell and the microspores during the tetrad stage. In wild-type, pectic components of this wall were no longer detectable at the time of microspore release. However, in qrt1 and qrt2 mutants, pectic components of this wall persisted after callose degradation. This result suggests that failure of pectin degradation in the pollen mother cell wall is associated with tetrad pollen formation in qrt mutants, and indicates that QRT1 and QRT2 may be required for cell type-specific pectin degradation to separate microspores.     

Octad pollen formation in Cymbopetalum (Annonaceae): the binding mechanism

Our recent study of tetrad pollen formation in Annona (Annonaceae) revealed that after meiosis the callose-cellulose envelope forms a special conjugation with individual microspores and the forthcoming callose digestion is incomplete. The undigested part forms a central binder holding the four microspores of the tetrad together. This process causes the microspores to rotate 180 degrees. In this paper we describe pollen formation in another annonaceous genus, Cymbopetalum, in which the pollen is shed in octads, through use of light microscopy, epifluorescence microscopy, and TEM. In Cymbopetalum, two meiocytes, connected by abundant cytomictic channels, are produced in each sporangium. Octad pollen formation in Cymbopetalum is shown to be comparable to the synchronized formation of two connected Annona tetrads, which then integrate into a single octad. Unique features of Annona polyad formation, e.g. special binding between the callose-cellulose envelopes and microspores, incomplete callose digestion, and microspore rotation, also occur in Cymbopetalum. In addition, formation of the Cymbopetalum octad involves development of a cushion-like structure that binds the distal pronexine of all eight microspores, and there is the production of intine protrusions. The evolutionary origin of the callose-cellulose binding mechanism within the family is discussed.     

Floral development of Lavatera trimestris and Malva hispanica reveals the nature of the epicalyx in the Malva generic alliance

The epicalyx is a structure below the calyx that is often integrated in floral display. In Malvales, the epicalyx is interpreted to be formed by bracts derived from inflorescence reduction. In this study, we compare the epicalyx and flower development of Lavatera trimestris and Malva hispanica, which are close relatives but show contrasting morphologies. Both species exhibit cymose branching, stipulate subtending leaves, a short plastochron between the appearance of the alternating epicalyx and calyx, a centrifugally developing androecium and a multicarpellar gynoecium. The predominantly trimerous structure and leafy morphology of the epicalyx suggest its origin from a former subtending leaf with leaf‐like stipules. The bilobed epicalyx in M. hispanica represents a loss of the adaxial epicalyx lobe rather than modified bracts. In Malvoideae, the bracts and bracteoles in the flowering branches can be completely absent and are variable in position and number when present. Individual bracts and bracteoles could correspond to further reductions of former subtending leaves instead of precursors of the epicalyx. Although the centrifugal androecium behaves as a branched‐like structure, it is a dynamic complex floral whorl with extended growth capacity. The umbrella in L. trimestris is a swollen part of the style without a well‐understood role in floral or fruit morphology.     

Release of developmental constraints on tetrad shape is confirmed in inaperturate pollen of Potamogeton

Background and Aims

Microsporogenesis in monocots is often characterized by successive cytokinesis with centrifugal cell plate formation. Pollen grains in monocots are predominantly monosulcate, but variation occurs, including the lack of apertures. The aperture pattern can be determined by microsporogenesis features such as the tetrad shape and the last sites of callose deposition among the microspores. Potamogeton belongs to the early divergent Potamogetonaceae and possesses inaperturate pollen, a type of pollen for which it has been suggested that there is a release of the constraint on tetrad shape. This study aimed to investigate the microsporogenesis and the ultrastructure of pollen wall in species of Potamogeton in order to better understand the relationship between microsporogenesis features and the inaperturate condition.

Methods

The microsporogenesis was investigated using both light and epifluorescence microscopy. The ultrastructure of the pollen grain was studied using transmission electron microscopy.

Key Results

The cytokinesis is successive and formation of the intersporal callose wall is achieved by centrifugal cell plates, as a one-step process. The microspore tetrads were tetragonal, decussate, T-shaped and linear, except in P. pusillus, which showed less variation. This species also showed a callose ring in the microsporocyte, and some rhomboidal tetrads. In the mature pollen, the thickening observed in a broad area of the intine was here interpreted as an artefact.

Conclusions

The data support the view that there is a correlation between the inaperturate pollen production and the release of constraint on tetrad shape. However, in P. pusillus the tetrad shape may be constrained by a callose ring. It is also suggested that the lack of apertures in the pollen of Potamogeton may be due to the lack of specific sites on which callose deposition is completed. Moreover, inaperturate pollen of Potamogeton would be better classified as omniaperturate.Key words: Alismatales, callose, microsporogenesis, pollen aperture, Potamogeton illinoensis, P. polygonus, P. pusillus, tetrad shape     

Two callose synthases,GSL1 and GSL5, play an essential and redundant role in plant and pollen development and in fertility

Callose, a β-1,3-glucan that is widespread in plants, is synthesized by callose synthase. Arabidopsis thaliana contains a family of 12 putative callose synthase genes (GSL1–12). The role of callose and of the individual genes in plant development is still largely uncertain. We have now used TILLING and T-DNA insertion mutants (gsl1-1, gsl5-2 and gsl5-3) to study the role of two closely related and linked genes, GSL1 and GSL5, in sporophytic development and in reproduction. Both genes are expressed in all parts of the plant. Sporophytic development was nearly normal in gsl1-1 homozygotes and only moderately defective in homozygotes for either of the two gsl5 alleles. On the other hand, plants that were gsl1-1/+ gsl5/gsl5 were severely defective, with smaller leaves, shorter roots and bolts and smaller flowers. Plants were fertile when the sporophytes had either two wild-type GSL1 alleles, or one GSL5 allele in a gsl1-1 background, but gsl1-1/+ gsl5/gsl5 plants produced an extremely reduced number of viable seeds. A chromosome with mutations in both GSL1 and GSL5 rendered pollen infertile, although such a chromosome could be transmitted via the egg. As a result, it was not possible to obtain plants that were homozygous for mutations in both the GSL genes. Pollen grain development was severely affected in double mutant plants. Many pollen grains were collapsed and inviable in the gsl1-1/gsl1-1 gsl5/+ and gsl1-1/+ gsl5/gsl5 plants. In addition, gsl1-1/+ gsl5/gsl5 plants produced abnormally large pollen with unusual pore structures, and had problems with tetrad dissociation. In this particular genotype, while the callose wall formed around the pollen mother cells, no callose wall separated the resulting tetrads. We conclude that GSL1 and GSL5 play important, but at least partially redundant roles in both sporophytic development and in the development of pollen. They are responsible for the formation of the callose wall that separates the microspores of the tetrad, and also play a gametophytic role later in pollen grain maturation. Other GSL genes may control callose formation at different steps during pollen development.     

Microsporogenesis and pollen development in Leymus chinensis with emphasis on dynamic changes in callose deposition

Leymus chinensis is an economically and ecologically important grass that exhibits low seed production. To better understand the causes of its low sexual reproductivity, the microsporogenesis and pollen development of this species were investigated, with emphasis on dynamic changes in callose deposition. A variety of histochemical stains were employed, including Heidenhain's hematoxylin, decolorized aniline blue, DAPI, and acetocarmine, along with a temporary mount method. Microsporogenesis and pollen development generally took place from June 12 to 26. The meiosis of microspore mother cells (MMCs) was of the successive type and the tetrad was isobilateral in shape. Mature pollen grains comprised two sperms and a vegetative nucleus. Callose initially appeared in the center of the anther locule at the premeiotic phase, and then gradually and unevenly deposited around the MMC before the commencement of meiosis. At the onset of meiosis, the accumulation of callose enclosing the MMC peaked, accompanied by the disappearance of callose in the center of the locule. At the dyad and tetrad stages, the dyads and tetrads were surrounded by callose wall and the microspores in the tetrads were isolated by a crossed cell plate composed of callose. Microspores just released from tetrads were still enclosed in callose wall, and then callose gradually disappeared in the pollen wall. Ultimately, callose almost completely disappeared from the walls of mature pollen grains. In the large numbers of sections observed, most of the cases of meiosis of the MMCs, pollen development, and callose dynamics were normal, with only a few abnormities observed. The results suggest that microsporogenesis, male gametogenesis, and callose dynamics during these processes are generally normal in this species, and that the callose wall plays an important role in the production of functional pollen grains. The small numbers of abnormities of these processes that occurred likely do not adversely affect the production of viable pollen grains. Therefore, microsporogenesis and pollen development may not be factors in the low seed production of L. chinensis.     

Study of male sterility in Taiwania cryptomerioides Hayata (Taxodiaceae)

Summary. A study of male sterility over a period of three consecutive years on a conifer species endemic to Taiwan, Taiwania cryptomerioides Hayata (Taxodiaceae), was done for this article. With the aids of fluorescence and electron microscopic observations, the ontogenic processes in the fertile and sterile microsporangia are compared, using samples collected from Chitou Experimental Forest and Yeou-Shoei-Keng Clonal Orchard of the National Taiwan University, Nantou, Taiwan. The development of male strobili occurred from August to the end of March. Microsporogenesis starts with the formation of the archesporium and ends with the maturation of 2-celled pollen grains within the dehiscing microsporangium. Before meiosis, there was no significant difference in ultrastructure between the fertile and sterile microsporangia. Asynchronous pollen development with various tetrad forms may occur in the same microsporangium of either fertile or sterile strobili. However, a callose wall was observable in the fertile dyad and tetrad, but not in the sterile one. After dissolution of the callose wall, the fertile microspores were released into the locule, while some sterile microspores still retained as tetrads or dyads with intertwining of exine walls in the proximal faces. As a result, there was no well developed lamellated endexine and no granulate ectexine or intine in the sterile microspores. Eventually, the intracellular structures in sterile microspores were dramatically collapsed before anthesis. The present study shows that the abortion in pollen development is possibly attributed to the absence of the callose wall. The importance of this structure to the male sterility of T. cryptomerioides is discussed. Correspondence and reprints: Department of Life Science, National Taiwan University, 106 Taipei, Taiwan.     

Pollen-wall formation in Arum alpinum

BACKGROUND AND AIMS: Arum alpinum has a quite uncommon pollen wall. A sporopolleninous ektexine is missing. The outermost pollen wall layer is formed by the endexine which is covered by polysaccharidic ornamentation elements. An ontogenetical investigation was accomplished to clarify pollen-wall development, with special reference to callose and pollen-wall development. METHODS: Plants of Arum alpinum grown in their natural habitat were collected once a week within the vegetative period and processed for semi- and ultra-thin sectioning. KEY RESULTS: At any stage of pollen-wall formation callose is missing. Microspores are released from the tetrad by invagination of the amoeboid tapetum. The polysaccharidic wall ornamentations are formed by the tapetum. CONCLUSIONS: There appears to be no truth in the dogma that callose is essential for microspore separation and release from the tetrad. The lack of callose does not influence fertility but could be the reason for the uncommon pollen wall, where a sporopolleninous ektexine is missing.     

Heterostyly promotes compatible pollination in buckwheats: Comparisons of intraflower,intraplant, and interplant pollen flow in distylous and homostylous Fagopyrum

Premise of the Study

Heterostyly, the reciprocal positioning of stigmas and anthers in different floral morphs, has long been thought to promote intermorph pollination. However, extensive intramorph pollination occurs commonly in heterostylous species, leading to recurrent questions about the functional and evolutionary significance of heterostyly.

Methods

To identify the sources of stigmatic pollen (autogamous [intraflower], geitonogamous [intraplant], vs. interplant), we emasculated either one flower or entire plants in experimental populations of the two closely related buckwheat species, distylous Fagopyrum esculentum and homostylous F. tataricum. Differences in pollen size allowed unambiguous identification of pollen on stigmas.

Results

Only 2.4% of F. tataricum pollen and 1.5% of F. esculentum pollen arrived successfully on compatible stigmas of other plants. In the former (homostylous) species, 71.3% of the pollen load on stigmas was autogamous, 10.8% was geitonogamous, and 17.9% was interplant. In the latter (distylous) species, 37.45% of the pollen on stigmas was autogamous, 13.8% was geitonogamous, 17.0% was intramorph, and 31.75% was intermorph. The amount of incompatible pollen arriving on stigmas was greatly decreased by both one‐flower and whole‐plant emasculations, and thus, the proportion of compatible pollen deposited increased with one‐flower emasculation and increased even more with whole‐plant emasculation.

Conclusions

Our quantification of pollen‐donor sources in these two species indicated that heterostyly in Fagopyrum esculentum provided a nearly 2‐fold fitness advantage (in terms of compatible pollination) over expected (random) pollen transfers between morphs. Because of reduced herkogamy, the homostylous F. tataricum was highly autogamous.     

Temperature and water stress affect plant–pollinator interactions in Borago officinalis (Boraginaceae)

Climate change alters the abiotic constraints faced by plants, including increasing temperature and water stress. These changes may affect flower development and production of flower rewards, thus altering plant–pollinator interactions. Here, we investigated the consequences of increased temperature and water stress on plant growth, floral biology, flower‐reward production, and insect visitation of a widespread bee‐visited species, Borago officinalis. Plants were grown for 5 weeks under three temperature regimes (21, 24, and 27°C) and two watering regimes (well‐watered and water‐stressed). Plant growth was more affected by temperature rise than water stress, and the reproductive growth was affected by both stresses. Vegetative traits were stimulated at 24°C, but impaired at 27°C. Flower development was mainly affected by water stress, which decreased flower number (15 ± 2 flowers/plant in well‐watered plants vs. 8 ± 1 flowers/plant under water stress). Flowers had a reduced corolla surface under temperature rise and water stress (3.8 ± 0.5 cm² in well‐watered plants at 21°C vs. 2.2 ± 0.1 cm² in water‐stressed plants at 27°C). Both constraints reduced flower‐reward production. Nectar sugar content decreased from 3.9 ± 0.3 mg/flower in the well‐watered plants at 21°C to 1.3 ± 0.4 mg/flower in the water‐stressed plants at 27°C. Total pollen quantity was not affected, but pollen viability decreased from 79 ± 4% in the well‐watered plants at 21°C to 25 ± 9% in the water‐stressed plants at 27°C. Flowers in the well‐watered plants at 21°C received at least twice as many bumblebee visits compared with the other treatments. In conclusion, floral modifications induced by abiotic stresses related to climate change affect insect behavior and alter plant–pollinator interactions.     

Breeding system and bumblebee drone pollination of an explosively pollen‐releasing plant,Meliosma tenuis (Sabiaceae)

• Explosive pollen release is a mechanism used by some angiosperms that serves to attach pollen to a pollinator's body. It is usually adopted by species with zygomorphic tubular flowers and pollinated by birds and bees. The tree genus Meliosma (Sabiaceae, Proteales) has unique disc‐like flowers that are externally actinomorphic, but internally zygomorphic, and release pollen explosively.
• To elucidate the adaptive significance of explosive pollen release, we observed flowering behaviour, the breeding system and pollinator visits to flowers of the Japanese species Meliosma tenuis in a temperate forest. Flowers bloomed in June and were nectariferous and protandrous. Explosive pollen release was triggered by slight tactile stimuli to anther filaments or staminodes in male‐stage flowers.
• Because pollen cannot come into contact with the pistils enclosed by staminodes, M. tenuis is functionally protandrous. Artificial pollination treatments revealed that M. tenuis is allogamous. The dominant flower visitors were nectar‐seeking drones of the bumblebee species Bombus ardens (Apidae). The drones’ behaviour, pollen attachment on their bodies and fruit set of visit‐restricted flowers suggest that they are the only agent triggering the explosive pollen release mechanism, and are the main pollinator of M. tenuis.
• The finding that bumblebee workers rarely visit these flowers suggests that the explosive pollen release has another function, namely to discourage pollen‐harvesting bumblebee workers.

     

Development of the floral shoot and pre‐anthesis cleistogamy in Hydrobryopsis sessilis (Podostemaceae)

The reproductive biology of Hydrobryopsis sessilis (Podostemaceae, subfamily Podostemoideae), a reduced, threatened, aquatic angiosperm endemic to the Western Ghats of India, was examined. This is the first report on the transition from the vegetative to the reproductive phase in this plant, describing floral ontogeny, pollination and the breeding system. The cytohistological zonation of the apical meristem of the reproductive thallus is identical to that of the apical meristem of the vegetative thallus. The floral shoots do not replace vegetative shoots (i.e. the vegetative shoots never bear flowers), but form at new sites at the tip of the flattened plant body. Each floral shoot meristem is tiny, deep‐seated and concave and arises endogenously following lysigeny. The floral shoot meristem gives rise to four to six bracts in a distichous manner. The development of spathe, stamens and carpels is described. The ab initio dorsiventrality of the carpels and the occurrence of endothelium in the ovules are reported. The mature stigmas and anthers lie close to each other. The pollen germinates within undehisced anthers and the pollen tubes enter the stigmas in the unopened floral bud, leading to pre‐anthesis, complete, constitutional cleistogamy under water. The seed set is 63.2%. A significant finding is the penetration of several pollen tubes into the filaments of stamens in 16% of the flower buds, indicating a trend towards cryptic self‐fertilization. The Indian Podostemoideae appear to show a shift from xenogamy or geitonogamy or autogamy in a chasmogamous flower to complete autogamy in a cleistogamous flower. The floral modifications leading to cleistogamy in H. sessilis have been identified. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 159 , 222–236.     

Ingenious floral structure drives explosive pollination in Hydrilla verticillata (Hydrocharitaceae)

• In explosive pollination, many structures and mechanisms have evolved to achieve high‐speed stamen movement. The male flower of the submerged plant Hydrilla verticillata is reported to be able to release pollen explosively some time after leaving the mother plant time, but the mechanism of stamen movement and the related functional structure in this species are unclear.
• In this study, we observed the male flower structure and pollen dispersal process of H. verticillata. We analysed the stamen movements during the pollen dispersal process and conducted several controlled experiments to study the process of storage and release of elastic potential energy in explosive pollination.
• When the male flower of H. verticillata is bound to the united bracts, the sepals accumulate elastic potential energy through the expansion of basal extensor cells. After the male flower is liberated from the mother plant, the stamens unfold rapidly with the sepals under adhesion and transfer the elastic potential energy to the filament in seconds. Once stamens unfold to a critical angle, at which the elasticity of the filament just exceeds the adhesion between sepals and anthers, the stamens automatically rebound and release pollen in milliseconds.
• These results reveal that Catapult‐like stamens, spoon‐shaped sepals and enclosed united bracts in the spathe together constitute the functional structure in rapid stamen movement of H. verticillata. They ensure that the pollen can be released on the water surface, and thus adapt successfully to the pollen‐epihydrophilous pollination.

     

Temperatures during flower bud development affect pollen germination,self‐incompatibility reaction and early fruit development of clementine (Citrus clementina Hort. ex Tan.)

• One of the key environmental factors affecting plant reproductive systems is temperature. Characterising such effects is especially relevant for some commercially important genera such as Citrus. In this genus, failure of fertilisation results in parthenocarpic fruit development and seedlessness, which is a much‐prized character. Here, we characterise the effects of temperature on flower and ovary development, and on pollen–pistil interactions in ‘Comune’ clementine (Citrus clementina Hort. ex Tan.).
• We examine flower bud development, in vitro pollen germination and pollen–pistil interaction at different temperatures (15, 20, 25 or 30 °C). These temperatures span the range from ‘cold’ to ‘hot’ weather during the flowering season in many citrus‐growing regions.
• Temperature had a strong effect on flower and ovary development, pollen germination, and pollen tube growth kinetics. In particular, parthenocarpic fruit development (indicated by juice vesicle growth) was initiated early if flowers were exposed to warmer temperatures during anthesis.
• Exposure to different temperatures during flower bud development also alters expression of the self‐incompatibility reaction. This affects the point in the pistil at which pollen tube growth is arrested and confirms the role of sub‐ and supra‐optimal temperatures in determining the numbers of pollen tubes reaching the ovary.

     

Enigmatic morphology of Djinga felicis (Podostemaceae – Podostemoideae), a badly known endemic from northwestern Cameroon

Djinga is a monotypic genus restricted to the Cameroon Ridge (‘Dorsale Camerounaise’) of NW Cameroon. Besides the type locality Mount Djinga (Adamawa Province, near Tignère), it also grows in waterfalls near Mount Oku (NW Province). This paper describes the structure and development of Djinga felicis using scanning electron microscopy and microtome sections. Cusset's protologue is enriched considerably. Roots are green, dorsiventrally flattened and adherent to submerged rocks. They are broad ribbons or crusts (up to 1 cm broad) which branch exogenously. Root‐borne shoots (up to 17 cm) have filamentous leaves with sheaths and (occasionally) attached stipules. Flowers are borne as part of reproductive short shoots which arise exogenously along the stems and endogenously along the roots. Djinga shows non‐axillary stem branching. Reproductive shoots along the main stem usually stay short. They arise from the abaxial side of double‐sheathed leaves, a key innovation of Podostemoideae. These short shoots shift into pocket‐like stem positions clearly below their associated leaves. Each flower bud inside a spathella is erect, but with an inclined ovary. The flowers are unistaminate with a broad connective. Unlike many other Podostemoideae, the tricolpate pollen grains are arranged in both dyads and monads. The ovary is globular and unilocular. The mature 8‐ribbed capsule dehisces by two slightly unequal valves, releasing 40–50 seeds. © 2009 The Linnean Society of London, Botanical Journal of the Linnean Society, 2009, 160 , 64–81.     

Flower developmental stage and organ sensitivity of bell pepper (Capsicum annuum L.) to elevated temperature

High temperatures adversely affect crop productivity of several plant species including bell pepper (Capsicum annuum L. var. annuum). The objectives of this study were: (1) to determine whether flower ontogeny is adversely affected by high temperature during different phases of development, including pre‐ and post‐pollination events; (2) to determine the duration of high temperature exposure necessary to cause reduction in fruit set; and (3) to determine whether injury to the pistil or stamen during development is responsible for reduced fruit set during high temperature. We determined that flower buds at <2·5 mm in length, corresponding to microspore mother cell meiosis to tetrad dissolution, and flowers that reached anthesis during the high temperature exposure had reduced fruit set when exposed to 33 °C for 48 or 120 h. Flower buds at <2·5 mm in length also had reduced pollen viability when exposed to 33 °C for 120 h. Morphological examination demonstrated that meiocytes initiated tetrad formation, but after dissolution the microspores remained small and clumped without a thick exine. High temperature exposure at a late‐development, pre‐anthesis stage did not affect pistil or stamen viability, but high post‐pollination temperatures inhibited fruit set, suggesting that fertilization is sensitive to high temperature stress.