Pollen Morphology Germination and Pollination Biology

Piper spp. exhibit much uniformity in pollen morphology, except for the variation in size. Pollen grains are small, the diameter along the equatorial axis ranges from 8.018.0 ¡m and along the polar axis, between 6.5-15.0 ¡m. Grains are monosulcate, the sulcus extending upto the lateral extremities. Very rarely the grains are non aperturate or porate (Rahiman 1981). The sulcus is in the form of a narrow slit with thick borders all along the periphery. The exine surface is reticulate, the bronchi being large and irregular on the distal polar surface. In P. nigrum the pollen grains measure along the equatorial axis from 9.5-13.0 ¡m, the mean being 11.0 ¡m; and along the polar axis 7.0-10.5 ¡¡m having a mean of 8.84 ¡m (Rahiman 1981). Here the grains are spheroid to suboblate, rarely pyramidal, sulcus measures 7.0-11.0 ¡m in length and 1.0-2.0 ¡m in breadth. Iljas (1960) and Martin and Gregory (1962) also reported that pollen grains are small, the mean diameter being approximately 10 ¡m irrespective of cultivars. Pollen morphology of Piper was also reported by Mitrou (1970).

Anther dehiscence

Anther dehiscence is controlled to a very great extent by temperature and relative humidity (Iljas 1960, Martin and Gregory 1962). De Waard and Zevan (1969) found that in Sarawak the opening usually takes place between 12.00 and 14.00 h on days when relative humidity of 60 per cent is attained and at a temperature of 32°C, combined with bright sunshine. He also mentioned that the mass of pollen may spill freely over adjacent stigmas and other parts of the spike. Under Kerala conditions anther dehiscence takes place around 4 PM.

Pollen production

The total amount of pollen per anther varies with the cultivar. Marinet (1953) reported that in Indian cultivars each spike yielded 500,000-700,000 pollen grains, each 10 ^m in diameter. Martin and Gregory (1962) estimated 100,000-300,000 pollen grains per spike.

Pollen dispersal and mode of pollination

Barber (1906) and Anandan (1924) attributed pollination to splashing of rain, the latter reported that rain drops help in scattering pollen grains in different directions, either wash down the pollens to lower spikes or carry them to neighbouring vines. He ruled out the role of insects in pollination. In Sarawak, in the cv. Kuching, fresh pollen appeared in glutinous clusters dispersable in water (De Waard and Zevan 1969). Accumulation of dew may cause the disintegration of the pollen lumps (Martin and Gregory 1962). Drops collected from the spikes were reported to contain considerable quantities of pollen (De Waard and Zeven 1969). It appears that water is a medium for pollen distribution. Iljas (1960) on the other hand reported the presence of dry, powder like pollen in the cv Bangka and suggested the possibility of direct gravitational distribution (geitonogamy).

Barring a couple of reports there are very little studies on the pollination biology of pepper. One of the earliest reports was that of Martin and Gregory (1962) who studied the mode of pollination and factors affecting fruit set in Puerto Rico. Semple (1974) made a study on pollination in Piperaceae in Costa Rica. The following probabilities exist for natural pollination.

Insect pollination

Pepper flowers are not adapted for insect pollination. However presence of insects have been reported by Martin and Gregory (1962) and Semple (1974) suggestive of their role in pollination. Semple (1974) found that in Costa Rica several species of Trigona bees are the most common visitors on Piper spp. and that they were found to collect "large amount of pollen in their baskets" by working up and down the spikes collecting pollen and then flying from spike to spike. No direct evidence for insect pollination came from other sources, though Marinet (1953) reported poor yield following insect control operation. However studies conducted in Kerala, the centre of diversity for black pepper, rule out the role of insects in pollination. Insect activity is rare in pepper plants except for ants and certain other crawling insects and insect pests such as pollu beetle. Bees do visit male spikes of certain Piper species. The spikes of certain species are fragrant; male spikes of P. hymenophyllum and P. argyrophyllum have faint aromatic small, while a Piper sp. maintained in the germplasm conservatory of IISR has fragrant spikes.

Wind pollination

Studies have shown that pollen transportation by wind is negligible under Indonesian situation (Iljas 1960) and practically absent under Indian condition. But Martin and Gregory (1962) based on their study in Puerto Rico, indicated that 32-64 per cent of the pollen on the spike may be dispersed to the air within 24 h after exposure.

Geitonogamy

Geitonogamy is a self pollination mechanism which involves gravitational descending of pollen grains combined with action of rain water or dew drops. This is an effective mechanism in plants having long pendent inflorescences. Heavy rains often have an adverse effect on pollination. Similarly lack of rain during the flowering period result in poor fruit set. (Anandan 1924, Govinda and Venkateswaran 1929, Marinet 1953). Iljas (1960) reported geitonogamy from Indonesia. He found that free hanging spikes isolated inside polyethylene bags displayed good fruit set, irrespective of insects or rain water. Pepper is adapted for such a mode of pollination. Pendent hanging spikes, spiral arrangement of flower, sequential ripening of stigmas and prolonged receptivity of stigmas are all favourable to geitonogamy.

The protogynous condition of pepper affects fruit set. The temporal separation of female and male phases can vary very much in bisexual cultivars from 0-30 days or even more, and it was found to be season and climate dependent. Suppression of anther emergence was noticed in certain high elevation areas where the temperature is low. Fruit setting and yield are reduced considerably in such areas (Ravindran, unpublished). In cultivars where male and female phases mature simultaneously autogamy may result. Inter-spike pollination may also be of frequent occurrence, as it is inevitable in a pepper plant that produces large number of spikes.

Apomixis

The first report of a possible case of apomixis in black pepper was that of Gentry (1955). He found fruits on male sterile cv Uthirankotta without the presence of pollen source in the neighbourhood. But his finding was not supported by later workers. Such situation as reported by Gentry does occur frequently in nature. It is commonly observed that in forests, isolated female plants exhibit good fruit set in the absence of any pollen source. This is common in many related species as well; indicating that apomixis could be playing an important role. Such fruiting has been recorded in female plants of P. longum and P. chaba growing in the germplasm conservatory of IISR. Incidentally in the case of P. chaba only female pants are present in the conservatory and they fruit profusely and produce viable seeds. In P. longum also pollen source is not required for fruiting, indicating that apomixis may be the rule in this species. In the Western Ghat forests, small populations of either male or female plants are met with quite frequently and in such female populations good fruiting is also noticed. All evidences tend to point to the presence of apomixis in the South Indian species of Piper.

But this is not the situation in the case of cultivars, which are mostly bisexual. Apomixis seems to disappear or at least gets suppressed to a great extent with the gain of bisexuality. In a study conducted by Sasikumar et al. (1992) many aspects of the pollen biology of pepper were brought out. Highest fruit setting was observed under open pollination in the three most popular cultivars, Karimunda, Panniyur 1 and Aimpiriyan. Spike setting was complete (100%) in all the three cultivars under selfing, bagging, hand pollination and open pollination. Self fertilization was highest in cv. Aimpiriyan followed by cvs. Panniyur 1 and Karimunda. Under water-free pollination, highest spike and fruit set was observed in cv. Panniyur 1. Though Karimunda registered a slightly higher spike set than Aimpiriyan under water-free pollination, the fruit set in general was higher in Aimpiriyan. In all the three cultivars fruit set was less under water-free condition than under open pollination. However the magnitude of reduction in seed set was very much in Karimunda. The above authors have calculated the extent of autogamy in three cultivars, 86 per cent in Karimunda, 92 per cent in Panniyur 1 and 95 per cent in Aimpiriyan and the percentage self compatibility was 84 per cent, 91 per cent and 86.5 per cent respectively. These results indicate that selfing is the predominant form of fertilization in cultivated bisexual pepper. Though protogyny occurs, it appears ineffective to prevent selfing as the pendent spike is abundantly assured of pollen from upper flowers. In cv. Karimunda emasculation and bagging resulted in 7.0 per cent spike setting and 8.2 per cent fruit setting indicating the occurrence of apomixis to a small extent. The fruit setting in predominantly female cultivars such as Cholamundi, Uthirancotta and Karuvilanchy may be entirely due to apomixis.

Assisted pollination, hybridization

Intervarietal hybridization is an extremely important approach for crop improvement and enhancing crop yield. Such crossing involves pollen transfer from the pollen parent to the stigmatic lobes of the female parent. Nambiar et al. (1978) used emasculation, isolating the emasculated spikes in butter paper bags and pollination. Emasculation is carried out by scooping out the anthers from either side of the ovary using a fine pointed needle. The unemasculated portion of the spike is trimmed off and the spike covered with paper bag. Pollen grains are collected by washing the spikes with distilled water and collecting the washings. Pollination is carried out by applying the pollen suspension on the emasculated spike using an ink filler or brush. The setting has been reported to be around 6-12 per cent. Martin and Gregory (1962) described two techniques of pollination in pepper which were also not very successful. In one technique ripe anthers were opened by means of a scalpel and pollen was scooped up and applied on the stigma of selected spikes. In the other technique spikes from male and female parents were brought together and brushed with a camel hair brush. Emasculation was tried by alcohol, hot water and excision. Iljas (1960) suggested emasculation by employing a suction pump.

In Sarawak, De Warrd (1969) developed a method of hand pollination which made use of the extended period of protogyny in the cv. Kuching. Here prior to cross pollination all spikes present on the receiving plant (female parent) are removed to prevent geitonogamy. At three or four locations branches were selected, and they were bagged in cheese cloth bags and allowed to develop spikes. As soon as stigmas are receptive in three or four spikes in the selected branch, pollen from a male parent is transferred to the stigma. For this, a portion of spike having freshly opened anthers are cut off and placed on the end of a long pin and the entire pollen cluster is gently brought into contact with the young stigma. This method was reported to give 50-75 per cent success.

Ravindran et al. (1981) developed an efficient crossing technique making use of the protogyny. They raised dwarfened pepper plants (bush pepper) of various cultivars by rooting the lateral fruiting, plagiotropic branches. These dwarf plants flowering in pots are used for establishing a crossing block in green house (Fig. 2.21). These plants can be kept isolated either spatially or mechanically using nylon cloth or rain proof bolting cloth or poly bags. In such isolated plants spikes are selected for crossing, and an soon as the stigmas become receptive pollination is carried out. For this, sufficient number of anthers are collected from the male parent the previous afternoon into a small vial and is kept in a desiccator over calcium chloride for dehiscence. The

Figure 2.21 Crossing block established at I.I.S.R. using bush peppers grown from lateral, plagiotropic fruiting branches.

next morning a few drops of distilled water is added to the vial, shaken well and this suspension of pollen is brushed onto the stigma or applied using an ink filler. For successful pollination high concentration of pollen grains in the suspension should be ensured, and this can be checked by observing a drop of suspension under a microscope. The process of pollination is continued for a few days till anther emergence is noticed in the spike, when pollination is stopped and the unfertilized portion of the spike is trimmed off. This method is very successful and upto 82 per cent setting was recorded. Only limitation is that it is not applicable to cultivars having no or only very brief interval between the male and female phases, where emasculation becomes necessary.

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