Plant materials were collected from the jasmine germplasm of the Department of Floriculture and Landscape Architecture, TNAU, Coimbatore located at 11°02’ N latitude and 76°57’ E longitude at an altitude of 426.72 m above MSL. The weather condition at Coimbatore during the study was moderately warm with hot early summer months during March-May. In open field conditions, the maximum temperature fluctuated between 25°C and 35°C with a mean of 30°C. The minimum temperature ranged between 17°C and 23.5°C with a mean of 20°C. The annual rainfall was 750 mm and relative humidity ranged between 60 and 90 per cent with a mean of 75 per cent.

The study involved nine genotypes of jasmine belonging to eight Jasminum species. The commercial species category included four genotypes namely, J. sambac cv. Ramanathapuram Gundumalli, J. auriculatum cv. CO.1 Mullai, J. grandiflorum cv. CO.1 Pitchi and J. grandiflorum (White). The lesser known (underutilized) species category included five genotypes namely, J. calophyllum, J. flexile, J. multiflorum (Pink flowered type), J. nitidum and J. rigidum.

The methods adopted to study the palynological aspects of the above listed Jasminum species are briefly discussed below.

For assessing the morphological characteristics of pollen, flower buds at mature bud stage were involved. In the laboratory, anthers were isolated from the flower buds in petri-dishes and were maintained for 24-48 hours at room temperature (24°C) to facilitate release of pollen. Then the petri- dishes with pollen were transferred to a desiccator, where they were kept until analysis. Preparation of pollen for analysis was performed by mounting two-layer transparent tape on the object carrier on the microscope and applying pollen with a brush. The prepared samples were observed under a Scanning Electron Microscope (SEM) at a magnification of 2000X (whole grain) to15,000X (exine pattern). The pollen size was measured and the range was recorded for a sample of 30 pollen grains from each of the nine genotypes of jasmine.

Pollen production per flower was estimated using Haemocytometer as suggested by Sathiamoorthy (1973). Three samples of anthers from each Jasminum species were collected just prior to dehiscence. The anthers were crushed with a small glass rod in a vial containing 2.5 ml of distilled water and a drop of teepol for obtaining a good suspension of pollen grains in water. The contents were thoroughly shaken and two drops of it were pipetted out and placed on each of the two counting chambers of a Spencer bright line Haemocytometer. The number of pollen grains in each of the eight “corner squares” was recorded. This was repeated five times for each sample and was designated as sub samples. The average number of pollen grains per square multiplied by 2500 gave the quantity of pollens per anther.

The pollen viability and fertility were assessed by the following two methods.

1. (a) Acetocarmine test

   Freshly dehisced pollen grains were collected from each of the Jasminum species in sterilized petri-dishes. The pollen grains were dusted on the cavity slide and a drop of Acetocarmine stain was placed on the pollen grains. Deeply stained, normal and plump pollen grains were considered viable while shriveled, deformed and weakly stained pollen grains were considered as sterile ones. Pollen viability was assessed for three days viz., first day, second day and third day of anther dehiscence and expressed in percentage.

2. (b) In vitro pollen germination

   Pollen from freshly dehisced anthers were collected and tested for germination on the day of anther dehiscence. Pollen was dusted in cavity slide in which 10 per cent of sucrose and 10 ppm of boric acid solution were dropped over gently with a pointed dropper and pollen grains were mixed thoroughly in the solution with the help of a needle. The cavity slides were kept in petri dishes over small glass rods containing moist filter paper at the bottom and closed properly. These were then incubated at ambient room temperature (25oC) for optimum germination. The slides were examined under microscope after one hour, two hours and three hours. Germinated and non-germinated pollen grains were counted separately in several random fields containing a total of 100 pollen grains and the length of the pollen tube was measured in micrometer (μm).

The details of pollen morphology of the nine genotypes of jasmine are furnished in Table 1 Scanning Electron Microscope (SEM) images of the Jasminum genotypes indicated wide variations in shape of pollen among the genotypes.

The pollen size ranged between 36.2 μm and 57.9 μmin J. sambac cv. Ramanathapuram Gundumalli, 33.0μm and 42.9 μm in J. auriculatum cv. CO. 1 Mullai,32.32 μm and 41.22 μm in J. grandiflorum cv. CO.1 Pitchi, 33.0 μm and 46.2 μm in J. grandiflorum(White), 53.5 μm and 68.4 μm in J. calophyllum,30.2 μm and 53.2 μm in J. flexile, 30.5 μm and 53.5μm in J. multiflorum (Pink), 33.0 μm and 52.8 μm inJ. nitidum and 39.6 μm and 49.5 μm in J. rigidum (Table 1).

Table 1
Pollen morphology, pollen size (µm) and pollen output in Jasminum species

Pollen output (average number of pollen produced/ flower) for the Jasminum species ranged between 625 and 28,660. The pollen output was 28,660 in J. rigidum which was the highest among the nine genotypes and 625 in J. sambac cv. Ramanathapuram Gundumalli which was the least among the genotypes. The pollen output was 14,175 in J. auriculatum cv. CO.1 Mullai, 17,920 in J. grandiflorum cv. CO.1 Pitchi, 23,816 J. grandiflorum cv. White, 8,769 in J. calophyllum, 13,778 in J. flexile, 17,002 in J. multiflorum (Pink) and 21,056 in J. nitidum and (Table 1).

Makde (1982) opined that in Jasminum species, a majority of pollen grains were characterized by large scale vacuolation and scanty cytoplasm resulting in degeneration of pollen grains prior to dehiscence, leading to low pollen output.

Data on pollen viability are furnished in Tables 2 and 3 and pictorial details are furnished in Fig. 1 to 3. The percentage of fertile pollen in J. sambac cv. Ramanathapuram Gundumalli was 5.67 per cent on the day of dehiscence and it gradually decreased to 0 per cent on the third day. The percentage of fertile pollen in J. auriculatum cv. CO.1 Mullai was 94.87 per cent on the day of dehiscence and it gradually decreased to 43.33 per cent on the third day. The percentage of viable pollen in J. grandiflorum cv. CO.1 Pitchi was 77.17 per cent on the day of dehiscence and gradually decreased to 18.67 per cent on the third day. The percentage of viable pollen in J. grandiflorum cv. White was 95.50 per cent on the day of dehiscence and it gradually decreased to 18.67 per cent on the third day. (Table 2).

In J. flexile, the percentage of viable pollen was 90.67 per cent on the day of dehiscence and it gradually decreased to 22.33 per cent on the third da y. The percentage of viable pollen in J. calophyllum was 77.17 per cent on the day of dehiscence and it gradually decreased to 18.67 percent on the third day. The percentage of viablepollen in J. multiflorum (Pink) was 54.67 per centon the day of dehiscence and gradually decreasedto 17.33 per cent on the third day. The percentageof fertile pollen in J. nitidum was 53.33 per centon the day of dehiscence and it gradually decreasedto 18.33 per cent on the third day. The percentageof fertile pollen in J. rigidum was 95.33 per centon the day of dehiscence and it gradually decreasedto 27.67 per cent on the third day (Table 3).

Fig. 1
Pollen morphology Jasminum species visualized under SEM

The mean germination percentage in Jasminum species ranged between 0 per cent and 94.34 per cent, with the highest in J. rigidum. No germination was observed in J. sambac cv. Ramanathapuram Gundumalli. The mean germination percentage was 42.56 per cent in J. auriculatum cv. CO.1 Mullai, 25.3 % in J. grandiflorum cv. CO. 1 Pitchi, 59.9 % in J. grandiflorum cv. White, 31.72 % in J. flexile, 24.0 % in J. calophyllum, 1.39 % in J. multiflorum (Pink) and 8.86 % in J. nitidum (Table 4).

In the present study, pollen tube growth as measured in terms of the length of pollen tube in the Jasminum species ranged between 0 μm and 1917.10 μm. The maximum pollen tube length of 1917.10 µm was recorded in J. rigidum. The mean length of pollen tube was 447.81 μm in J. auriculatum cv. CO. 1 Mullai, 552.61 μm in J. grandiflorum cv. CO. 1 Pitchi, 449.82 μm in J. grandiflorum cv. White, 222.15 μm in J. flexile, 641.15 μm in J. calophyllum, 884.27 μm in J. nitidum. Pollen tube was not conspicuous in J. multiflorum (Pink).

Pollen viability is an important factor, since the probability of fertilization usually declines when pollen grains with low viability are deposited on the stigma (Wilcock and Neiland, 2002; Zhao et al., 2004; Teng et al., 2012). Lai (1995) and Deng et al. (2014) attributed low pollen fertility and Deng et al. (2016) attributed low pollen viability for poor fertilization injasmine. Deng et al. (2016) also reported that among15% of anthers, tetrads formed pollen mass insteadof free microspores, and only one tube grew normallyfrom the tetrad pollen.

Table 2
Pollen of commercial jasmine genotypes

Table 3
Pollen viability of underutilized jasmine genotypes

Table 4
In vitro pollen germination in Jasminum species (after 3 hr incubation)

Fig. 2
Pollen stainability in Jasminum genotypes

Fig. 3
In vitro pollen germination in Jasminum genotypes

The differential pollen viability of the jasmine species recorded in the present study is attributable to the varied ploidy levels and chromosomal forms. In J. sambac cv. Ramanathapuram Gundumalli, higher pollen sterility is attributed to its triploid status which leads to meiotic abnormalities. These observations are supported by the earlier reports of Raman (1955) and Deng et al. (2017) in Jasminum species.

In the various jasmine species studied, pollen germination percentage was low to moderate (Plate 2). This might be associated with the irregular meiosis leading to defective pollen and egg cells, ultimately resulting in sterility. Datta et al. (1960) elucidated that structural changes lead to loss of genes as expressed in the suppression of the female reproductive development in J. grandiflorum.