Polyploidy and its Application

The possibility of using polyploidy in crop improvement was also explored. A natural triploid (2n (3x)=78) was located among the germplasm and this has very bold fruits and low fruit setting. This triploid plant was used for generating a series of cytotypes. But none of these chromosomal variants so far has shown any agronomically useful characters.

An induced tetraploid was developed in Panniyur-1 by treating the seeds with colchicine. (Nair and Ravindran 1992). This tetraploid having the somatic chromosome number of 2n (4x)=104, has larger and thicker leaves but the growth was slower than the diploid parents, and is difficult to establish in the field. Ibrahim et al. (1987b) reported an extra bold variant of Panniyur 1, which probably is a tetraploid.

110 P.N.RAVINDRAN, K.NIRMAL BABU, B.SASIKUMAR and K.S.KRISHNAMURTHY Mutation Breeding

Induced mutations were used in pepper mainly for broadening the genetic base, especially in Malaysia, and also for producing mutants tolerant to P. capsici and nematodes. In Malaysia, the mutation breeding work was limited to the cultivar Kuching. In India, mutation studies were carried out by Irulappan et al. (1982) and Ravindran et al. (1986). Ravindran et al. (1986) used gamma rays for inducing variability in Karimunda, Panniyur 1, Kuthiravally, Kalluvally (Pulpally), Kalluvally (Malabar), Thommankodi and Aimpiriyan and 1-4 kr gamma rays were used for irradiation. Irradiation adversely affected the germination of seeds, as the dose increased, germination was delayed. The Gr50 level (50 per cent germination of seeds) was reached in control by 40th day in Karimunda, while at 3 kr the Gr50 was reached by 65 days and at 4 kr the germination never reached 50 per cent. The difference between control and 3 kr to reach Gr50 was three days in Kalluvaly (Malabar), six days in Thommankodi, 15 days in Kuthiravally, 25 days in Karimunda and 28 days in Panniyur-1. Only in Kalluvally (Malabar) all the treatments gave above 50 per cent germination.

The percentage reduction in germination from 0-4 kr was high in Aimpiriyan (52%), Karimunda (52%), Kalluvally (Malabar 39%), Kalluvally (Pulpally 70.5%) and low in Kuthiravally (36%), Panniyur 1 (25%) and Thommankodi (2.7%). Thus Kalluvally (Pulpally), Karimunda and Aimpiriyan were most affected by the radiation treatment and Thommankodi the least. The M1 population showed certain morphological abnormalities such as chlorophyll changes, twinning of seedlings, rosette leaves etc. The frequency of chlorophyll abnormalities in the Mx seedling population ranged from 13.0 per cent in cv. Karimunda to 0.1 per cent in cultivars Kuthiravally and Kalluvally. Panniyur 1 gave 0.7 per cent, Thommankodi 0.8 per cent and Aimpiriyan 0.2 per cent chlorophyll abnormalities. Karimunda gave the highest number of chlorophyll mutants (13.0%). Incidentally Karimunda gave around 3 per cent chlorophyll abnormalities even in the control population. The types of chlorophyll changes noted included albino, xantha (yellow) and variegated. The first two were lethal, whereas variegated ones later developed to normal plants. The variegated seedlings were similar to what Blixt (1961) described in peas as the "chlorotica variomaculata" type (Ravindran et al. 1986). Chandy et al. (1980) carried out preliminary studies on the effect of EMS on vegetative buds of black pepper but no mutants were noticed in the Mx and further generations were not studied.

Paulus (1993) reviewed the mutation breeding work carried out in Malaysia. Here mutation breeding was used mainly to create genetic variability especially for developing tolerant or resistant lines against P. capsici and M. incognita. Cultivars Kuching and Semongok Perak were used in mutation breeding work. The ripe seeds were irradiated with gamma rays upto 100 Gy. Both Mx and M2 were studied, mainly for locating resistance against Phytophthora.

In addition to seeds, cuttings were also irradiated and raised. Similar studies using rooted cuttings were carried out in India also. In Malaysia, a method called "cutting back method" was used in irradiation experiments with vegetative cuttings. This technique was originally used by Bauer (1957) for black current (Ribes nigrum) and is arrived at for improving the efficiency of mutagen treatment by restricting the disadvantageous formation of chimeras. Chimeras are usually formed during ir radiation of multicellular apical buds. In the cutting back technique the dormant buds are treated and the main shoot is removed at sprouting so that the MVj (first vegetative mutation generation) will originate from axillary buds which are present in the treated plant but with a comparatively reduced cell population. Further cutting backs are done in successive vegetative mutation generation (MV2, MV3 etc.). This technique has been applied in many vegetatively propagated plants.

Paulus (1993) used two nodal cuttings of Kuching with a leaf on the upper node for gamma ray irradiation ranging from 0-50 Gy. The irradiated plants were planted and MV1 and MV2 generations were studied. The LD50 was around 30 Gy. Apart from gamma rays, EMS treatment was also used for inducing mutation, the LD50 is around 0.25-0.3 per cent. The progenies were subjected to the leaf screening method (Kueh and Khew 1980) for locating tolerance against P. capsici. In this method a culture disc of P. capsici is placed together with a drop of distilled water on the abaxial surface of a recently matured leaf. The lesion diameter developed on the leaf is measured four days after inoculation. The reactions were compared with the lesion developed on Kalluvally, Belantung and Kuching leaves used as checks (representing, tolerant, less susceptible, susceptible respectively). Plants giving reactions similar to Kalluvally (lesion diameter equivalent to Kalluvally) were selected for further studies (Paulus 1993). Mass screening was also done in the M2 seedlings using a suspension of P. capsici culture. The M2 seedlings were grown in trays and inoculum at the rate of 250 ml/tray was poured. The surviving seedlings were subjected to another cycle of screening in the same way after two months. The finally survived lines were grown for further studies (Paulus 1993). Eleven lines from the irradiated population were selected for further studies and breeding. In Brazil Ando et al. (1980, 1984) carried out mutation studies using gamma radiation for developing Fusarium resistance, but could not achieve any resistant genotype.

Because of the inherent limitations, mutation breeding has not been pursued much by pepper workers. It would be a much better idea to use the in vitro system for generating variability and perhaps a combination of induced mutation, plus in vitro culture technique can provide tremendous amount of variability for eventual selection.

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