PhD

In Kerala, Papaya (Carica papaya L.) is a common component of the homestead farming system. This nutritious crop is paid less attention and limited attempts have been made to evolve superior papaya varieties in our State. A study was carried out at the Department of Pomology and Floriculture, College of Agriculture, Vellayani during 2000 to 2002 to evolve hybrids of papaya with medium to dwarf stature, good dessert qualities and resistance/tolerance to pests and diseases, which can be grown as a part of nutrition garden in the limited space available in the homesteads of Kerala for dessert purpose. Attempts were also made to identify the genotypes and sex forms in papaya through molecular characterisation using Random Amplified Poymorphic DNA (RAPD) technique.

Inbreds of the four selected varieties viz. ‘ Pusa Nanha’, ‘ Pusa Dwarf’, ‘Sunrise Solo’ and ‘Coorg Honeydew’ were produced through sibmating. The inbreds developed were crossed in 4x4 diallel method (Griffings Method I ). The performance of the four parents, six F1’s and six reciprocal F1’s were evaluated.

The evaluation of the inbreds selected as parents revealed that Pusa Dwarf was the dwarfest variety and it started flowering from the shortest height from ground level. Pusa Nanha flowered earlier and produced large sized fruits with high volume. Fruit yield per plant was maximum is Pusa Nanha. Coorg Honeydew showed high percentage of fruit set and fruits with good quality attributes. Sunrise Solo gave early flowering and fruits with high TSS, total sugars, carotenoid content and maximum shelf life.

The analysis of performance of parents and hybrids proved the following. The hybrid Coorg Honeydew x Pusa Dwarf registered lowest height at all the stages of growth and recorded lowest height to first flower. Hybrid Pusa Nanha x Coorg Honeydew exhibited highest percentage of fruit set and took the least number of days to harvest. The highest fruit yield per plant was recorded by the hybrids Coorg Honeydew x Sunrise Solo and Sun rise Solo x Pusa Dwarf. Good fruit quality attributes were shown by Sunrise Solo x Coorg Honeydew (high TSS, low acidity), Coorg Honeydew x Sunrise Solo ( high total sugars), Sunrise Solo x Pusa Nanha (high Ascorbic acid content) and Pusa Nanha x Coorg Honeydew (high Carotenoids).

Based on per se performance and combining ability analysis, hybrids like Pusa Dwarf x Coorg Honeydew (dwarfness, low flowering & earliness in bearing), Pusa Nanha x Coorg Honeydew (earliness in bearing), Coorg Honeydew x Sunrise Solo (fruit yield per plant, fruit quality), Sunrise Solo x Pusa Dwarf (fruit yield per plant and yield characters like fruit weight, fruit volume) and ‘Pusa Nanha’ x ‘Pusa Dwarf’ (disease tolerance and fruit yield per plant) were found to be promising.

Using the Hayman’s Graphical Approach, the gene actions involved in the expression of the characters were found out. It also helped to assess the parental order of dominance.

Pusa Nanha carried the highest number of dominant alleles for plant height, days to harvest, number of fruits per plant, fruit yield per plant, fruit length, fruit girth, fruit volume, pulp percentage and total carotenoids. The lowest number of dominant alleles were for fruit cavity index and TSS. Pusa Dwarf had highest number of dominant alleles for plant girth, height to first flower, days to first flower, fruit weight, total sugars, and shelf life of fruits. The least number of dominant alleles in Pusa Dwarf was for the characters percentage of fruit set, days to harvest and fruit yield per plant.

Sunrise Solo carried the highest number of dominant alleles in the expression of characters like TSS and ascorbic acid and the least number of dominant alleles for plant girth, height to first flower, days to first flower, pulp percentage, total sugars and shelf life of fruits. For the expression of characters like flowers per cluster, percentage of fruit set and fruit cavity index, the parent Coorg Honeydew recorded highest number of dominant alleles while the lowest number of dominant alleles was recorded for plant height, number of fruits per plant, fruit weight, fruit girth, total carotenoids. For the characters days to first flower and days to harvest Coorg Honeydew possessed genes which are dominant as well as recessive with positive effect. The parent Pusa Dwarf possessed genes with positive effect which are dominant as well as recessive for fruit girth.

The heterosis exhibited by the hybrids over the mid parent, better parent and standard parent were analysed. The hybrids Coorg Honeydew x Pusa Dwarf (dwarfness, low flowering), Sunrise Solo x Pusa Dwarf (yield attributes, fruit yield per plant), Coorg Honeydew x Sunrise Solo (fruit yield per plant, fruit quality attributes) Pusa Dwarf x Pusa Nanha (early harvest, fruit quality characters) and Sunrise Solo x Coorg Honeydew (number of fruits, TSS) registered maximum heterosis over mid parent, better parent and standard parent.

The study revealed that the hybrids Pusa Dwarf x Coorg Honeydew, Coorg Honeydew x Pusa Dwarf, Sunrise Solo x Pusa Dwarf, Coorg Honeydew x Sunrise Solo and Sunrise Solo x Coorg Honeydew excelled all the other hybrids based on their superior performance for various horticultural traits studied.

Genomic DNA was successfully isolated from the different sex forms of the four varieties using modified Walbot method. Primers from OPA – 01 to 20 and OPB-01 to 20 except OPA – 05, OPA – 06 and OPB – 09 yielded amplification products with isolated DNA. A total of 143 bands were obtained from the 37 primers of which, 132 were Polymorphic. Primers OPA – 03, OPA-13, OPB-04 and OPB-17 gave reproducible bands with an average of ten bands per primer. Male plants of Pusa Nanha and Pusa Dwarf yielded one specific band with primer OPA-03. Similarly, hermaphrodite plants of Sunrise Solo and Coorg Honeydew yielded one specific band with primer OPA-13, OPB-04 and OPB-17.

The genetic distance was maximum between the female plants of the four varieties and the male & hermaphrodite plants. The female plants of the four varieties formed the largest cluster in the dendrogram. The male plants of Pusa Nanha and Pusa Dwarf and the hermaphrodite plants of Sunrise Solo and Coorg Honeydew formed separate cluster. The similarity Index between sexes was less than the similarity index within a sex .