Pollination, Fertilization and Fruit Formation in Eggplant (Solanum Melongena L.)

ACTA AGROBOTANICA Vol. 61 (1): 107–113 2008

POLLINATION, FERTILIZATION AND FRUIT FORMATION IN EGGPLANT (SOLANUM MELONGENA L.)

Agnieszka Sękara1, Monika Bieniasz2

1Departament of Vegetable Crops 2Department of Pomology and Apiculture Agricultural University in Kraków, 29 Listopada 54, 31-425 Kraków, Poland e-mail: [email protected]

Received: 15.09.2007

Summary does not need a pollinator to transfer it in the fi eld. It is The aim of the investigations was to determine the re- a partly self-incompatible species and needs cross-ferti- gularity of pollination and fertilization as dependent on the phe- lization for greater fruit set (Amoako and Yeboah- notype of fl ower in eggplant. The experiment was conducted in Gyan, 1991). In hot humid climate, cross-pollination 2005 and 2006 at the Agricultural University in Kraków, Poland. may occur up to 20%, mainly due to heterostyly, even The object of the investigations was eggplant (Solanum melon- though it is treated as a self-pollinated crop (S warup, gena L.) ‘Epic F1’, grown in the open fi eld. Flower phenotype 1995). Quagliotti (1979) estimated the amount of (long-, medium- or short-styled), fruit setting and the number cross pollination in eggplant at between 0 and 46%, but of seeds per fruit were recorded on experimental plants during cross pollination at distances of 50 m or more was non- the fl owering period. The number of pollen tubes in the middle existent. The fl ower remains open for 2-5 days without of the style and the number on fertilized ovules in ovaries were closing at night. Anthesis and pollen release occur in evaluated for all types of fl owers. The course of pollination was the morning, but these phenomena are infl uenced by the different in particular types of eggplant fl owers. Differences concerned only the number of pollen tubes and fertilized ovules of daylight, temperature, and humidity. Pollen viability is short, medium- and long-styled fl owers and the number of seeds retained for 8-10 days at a temperature of 20-22°C with in the fruit. There was not observed any incompatibility in the a relative humidity of 50-55% (Chen, 2001). In egg- growth on pollen tubes in the styles of all types of eggplant fl o- plant, fertilization is completed within 3 days after polli- wers. Stereomicroscopy observations of cross sections of the ova- nation and gene expression 3 days after anthesis is indu- ries do not show differences in the formation and position of the ced by stimuli of both pollen tube elongation and fertili- ovules on the placenta. Short-styled fl owers were characterized zation. Ovaries of unpollinated fl owers began to senesce by signifi cantly lower pollination and fruit set effi ciency and they 3 days after anthesis, although half of them had the po- produced fruits with a signifi cantly lower number of seeds. The tential to set fruit (Nagasawa et al. 2001). low number of pollen tubes, fertilized ovules and seeds can be The variation in style length and its position to- a straight consequence of the morphology of short-styled fl owers: wards stamens was widely described in eggplant. Long- small-sized stigmas and spatial separation of anther pores and stigmas, which makes pollination diffi cult. styled fl owers (the stigma is above the stamen or on the same level as the stamen) are always more appreciable in the number than short-styled fl owers (the stigma is Key words: Solanum melongena, heterostyly, pollination effi - below the stamen). Sometimes fl owers with the stigma ciency, fertilization effi ciency on the same level as the stamen are defi ned as medium- styled ones. K rishnamurthi and S ubrama- INTRODUCTION niam (1954) described four types of fl owers: 1) long styled with large ovary, 2) medium styled with medium Flowering and fruit set are two most important sized ovary, 3) pseudoshort-styled with rudimentary factors determining the yield of cultivated eggplants. ovary, and 4) true-short-styled with very rudimentary Genotype and environmental factors infl uence the co- ovary. Pseudo- and short-styled fl owers fail to set fru- urse of fl owering and the fl ower morphology, especially its, whereas long- and medium-styled fl owers produce the style length which determines the effectiveness of fruits. Fruit set rates are higher for long-styled fl owers fruit setting. Eggplant is a self-pollinated plant – pollen compared with short-styled fl owers, so short-styled ones 108 Agnieszka Sękara, Monika Bieniasz are not totally sterile (Chen, 2001). In long-styled fl o- MATERIALS AND METHODS wers, the stigma may project beyond the anthers, what The experiment was conducted in 2005 and 2006 favours cross-pollination. Nothmann et al. (1983) at the Agricultural University in Kraków, Poland. The stressed that stylar heteromorphism affected fruit set object of investigations was eggplant (Solanum me- more than did the position of the fl ower in the cluster. longena L.) ‘Epic F ’, grown in the open fi eld, in the According to Passam and Bolmatis (1997). the 1 Experimental Station in Mydlniki, near Kraków, South proximity of the stigma to the anther pores at anthesis Poland. Seeds were sown on 4th March 1995 and 3rd infl uences fruit set, fruit size and seed content, but not March 2006 in seed-boxes fi lled with peat substrate and seed quality of eggplant. Fruit weight and seed forma- placed in a controlled environment: temperature 27 ± tion were maximal in fl owers with the stigmata at ma- 1°C and RH 80%. After emergences, temperature was turity situated close to the anther pores. There are some maintained at 26 ± 2°C (day) and 21 ± 1°C (night), RH investigations designed to determine the infl uence of at 80%. When the fi rst true leaf was expanding, uniform agronomical and environmental factors on the occur- seedlings were pricked out into plastic containers Ø 8 rence of different types of fl owers in eggplant. Pas- cm, fi lled with peat based substrate. Uniform transplants sam and Bolmatis (1997) showed that the style with 5-6 leaves were fi nally transplanted into open fi eld length in eggplant is a varietal characteristic. B obadi at the beginning of May, 70 and 62 days after sowing and van Damme (2003) stated that nitrogen failed (in 2005 and 2006, respectively), in a spacing of 0.75 to infl uence style length and type of fl owers of eggplant. × 0.60 m. Eggplant was grown in standard agronomic According to Kowalska (2003), eggplant formed the conditions. The observations were conducted on 20 highest number of fl owers with a long pistil and much plants during the whole fl owering period. Single fl owers less fl owers with medium and short pistils regardless of were numbered according to the order of appearance on the pollination method (self pollination, using bumble each plant. Flower phenotype (long-, medium- or short- bees) and fl ower hormonization. styled), fruit setting and the number of seeds per fru- The production of short-styled fl owers with it were recorded for all plants. Results were evaluated a small and highly reduced gynoecium, often infertile or by ANOVA, Tukey’s test, at p = 0.05. At full fl owering with small possibilities of setting fruits, seems to be the (17.07.2005 and 26.07.2006), 60 fl owers of each type fi rst step to functional andromonoecy – the production (long-, medium- and short-styled) were collected. Only of staminate and hermaphroditic fl owers. Andromono- pollinated fl owers were taken into consideration. In the ecy is widespread in the genus Solanum (Dulber- fi rst part of the investigations, 30 styles of each above ger et al. 1981; Solomon, 1986; Diggle and mentioned type were isolated and fi xed immediately in Miller, 2004). The aim of the present investigations FAA (formalin-acetic-alcohol), according to Martin’s was to discuss the reasons and benefi ts of eggplant sty- method (Martin, 1959). Germination of pollen on lar heteromorphism and to determine the regularity of stigmas, growth of pollen tubes and fertilization of pollination and fertilization as dependent on the type of ovules were examined under fl uorescence microscopy. fl ower.

Table 1 The effectiveness of fertilization and fruit setting as dependent on the type of ﬂ ower in eggplant.

Long- Medium- Short- Long- Medium- Short- Type of ﬂ ower LSD LSD styled styled styled 0.05 styled styled styled 0.05

Year 2005 2006

Pollination 44.2 39.9 17.3 7.86 38.8 50.8 30.2 8.84 efﬁ ciency (%)

Fruit set 58.7 54.6 8.6 6.56 61.2 51.8 36.8 27.47 efﬁ ciency (%)

Number of seeds 372.5 358.5 141.0 70.52 376.5 376.3 217.8 44.15 per fruit Pollination, fertilization and fruit formation in eggplant (Solanum melongena L.) 109

The number of pollen tubes in the half of the style and les of each abovementioned type were observed under the number of fertilized ovules were evaluated. In the stereomicroscopy to evaluate the regularity of ovule second part of observations, the cross sections of 30 sty- formation.

I. II. III. Fig. 1. Stylar heteromorphism in eggplant: I. Long-styled fl ower, scale bar = 1 mm, II. Medium-styled fl ower, scale bar = 1 mm, III. Short-styled fl ower, scale bar = 1 mm.

I. II. III. Fig. 2. Position of the ovules on the placenta (cross sections of the fragment of eggplant ovary): I. Long-styled fl ower, scale bar = 100 μm, II. Medium-styled fl ower, scale bar = 100 μm, III. Short-styled fl ower, scale bar = 100 μm.

Fig. 4. Terminal pores of the stamens in long-styled fl ower of Fig. 3. Bumble bees on eggplant fl owers. eggplant, scale bar = 0.5 mm. 110 Agnieszka Sękara, Monika Bieniasz

RESULTS dium-styled fl owers were localized closest to the stigma of the style. Stylar heteromorphism was a distinct phenome- Pollination effi ciency of the long- and medium- non of eggplant ‘Epic F ’ in the conditions of the present 1 styled fl owers was the greatest and was contained in the experiment. The plants produced three types of fl owers: range of 38.8-50.8% (Tab. 1). Pollination effi ciency of long-styled, medium-styled and short-styled (Figs 1, the short-styled fl owers was signifi cantly lower (17.3 and I-III). Stereomicroscopy observations of cross sections 30.2%, in subsequent years of the experiment). In 2005, of the ovaries do not show differences in the formation fruit set effi ciency of the long- and medium-styled fl owers and position of the ovules on the placenta (Figs 2, I-III). was much higher as compared to the short-styled ones. In Bumble bees (Bombus terrestris) were widely observed 2006, signifi cant differences in fruit setting between the on turned down fl owers (Fig. 3). There were no morpho- investigated types of fl owers were not observed. Fruits logical differences in the forming of anthers among the set from the long- and medium-styled fl owers contained long-, medium- and short-styled fl owers. The stamens a comparable number of seeds (371.0 seeds per fruit, on formed a cone like a tube around the style and dehisced average). Fruits set from the short-styled fl owers contained at the terminal pores (Fig. 4). Stamen pores of the me- about two times less of seeds (179.4 seeds per fruit). Percent of obserwations Percent of obserwations

Fig. 5. Number of pollen tubes in the styles of diff erent types of eggplant fl owers. Pollination, fertilization and fruit formation in eggplant (Solanum melongena L.) 111 Percent of obserwations Percent of obserwations

Fig. 6. Number of fertilized ovules in the pistils of different types of eggplant ﬂ owers.

There was not observed any incompatibility in DISCUSSION the growth on pollen tubes in the styles of all types of eggplant fl owers. Pollen tubes grew into the style with Analysis of the recent reports on the fl owering seven ducts. Long-, medium-, and short-styled fl owers biology of cultivated eggplants brings the question if the were differentiated only in relation to the number of pol- eggplant heterostyle is a transitional stage between her- len tubes observed in the styles. There was found 10-300 maphroditism and andromonoecy and what the benefi ts pollen tubes in the middle of the style in the short-styled of eggplant heterostyly are. Andromonoecy has been fl owers, 150-900 in medium-styled, and 200-1000 in mentioned as a frequent phenomenon for cultivated egg- long-styled ones (Fig. 5). The ovaries of the short-sty- plants (H awkes et al. 1979; Anderson and S y - led fl owers contained 0-70 fertilized ovules. Many more mon, 1989). Differences between medium- and long- fertilized ovules were found in the remaining types of styled fl owers concerned only the pistil morphology; the fl owers: 40-400 in medium-styled and 100-400 in long- pollination and fruit set effi ciency as well as the number styled ones (Fig. 6). of seeds per fruit of the mentioned types of fl owers were 112 Agnieszka Sękara, Monika Bieniasz similar. In the conditions of the present experiment, the primarily as possible near-distance attractors and as the short-styled hermaphroditic fl owers of eggplant were source of pollen for reproduction. In Solanum palina- not functionally pistillae. The short-styled fl owers were canthum, the yellow anthers were shown to be more fertile and able to set fruits but fruit set rates were great- important attractants than was the corolla (Coleman er for the medium- and long-styled fl owers, as it was re- and Coleman, 1982). ported by Chen (2001) and Kowalska (2003). The The incompatibility in the growth on pollen tubes high frequency of short-styled fl owers was coincident in the short styles was not observed, so a smaller num- with the peak fl owering period of the investigated egg- ber of pollen tubes, noticed in short-styled fl owers, may plant cultivar (data not shown). It may be an adaptation have a morphological substratum: the above mentioned in preventing excessive fruit set. Passam et al. (2000) pollination diffi culties and small sized stigmas of short- found that the transitory reduction in style length, due styled fl owers which absorb less pollen. A low number to the presence of the fruit in the stage of growth, may of pollen tubes, fertilized ovules and seeds can be a stra- lead to a temporary reduction in fertility of eggplant. ight consequence of short-styled pistil morphology and Authors suggested that fruit load and development af- a way of pollination. R ylski et al. (1984) showed that fect fl ower mass and style length of eggplant, whereas pistils with a style length of less than 0.5 cm tend to be auxins infl uence the number of fl owers. The fl owers of smaller than medium- or long-styled ones and carry plants that did not set fruits showed only minor fl uc- a relatively small stigma with undeveloped papillae and tuations in fl ower and pistil mass and style and anther a low sugar content that precludes pollen germination length. and thus results in pollen drop. Passam and Bol- Sambandam (1964) stated that 30-40% of matis (1997) found that fl owers of style length <0.5 the eggplant fruit set is attributed to pollination by con- cm had signifi cantly smaller ovaries and failed to set tact, gravity and wind, the rest is dependent on insects. fruits. In fl owers with style length greater than 0.51 cm, Chen (2001) stated that the cone-like formation of the ovary size was quite similar. eggplant anthers favors self-pollination; but since the stigma ultimately projects beyond the anthers, there is ACKNOWLEDGEMENTS ample opportunity for cross-pollination. Such kind of The study was supported by the project grant of the androecium indicates the adaptation to the buzz-pol- the Ministry of Science and High Education No. 2P06R lination mechanism. In buzz-pollination, insects grasp 06429. the anther cone and transmit vibrations through their bodies to the anthers, thus producing pollen expulsion from the anther apices (Buchman, 1983). This fea- REFERENCES ture restricts pollination to a group of insects which are A moako J., Yeboah- Gyan K., 1991. Insect pollination of able to vibrate the anthers, i.e. bumble bees (Bezerra three Solanaceous vegetable crops in Ghana with special and Machado, 2003). 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