Investigations on cytogenetics , flowering and seedset in ginger

Abstract

Investigation on cytogenetics, flowering and seedset in ginger was carried out at the Department of Plantation Crops and Spices, College of Horticulture, Vellanikkara with the following objectives. 1. To workout the karyomorphology of ginger varieties and to study the cytogenetical polymorphism in relation to the plant morphology. 2. To study the meiotic irregularities in relation to pollen sterility in different varieties. 3. To study the effect of media and irradiation on the pollen germination and pollen tube growth in different varieties. 4. To work out the pollen – pistil interaction by fluorescence microscopy to decide on the presence of incompatibility mechanism in ginger. 5. To study the factor responsible for absence of fruitset and seedset in ginger. The study was carried on nine ginger varieties representing different geographical regions in India. Both morphological and cytogenetical characters were recorded. Studies were also conducted on pollen morphology, pollen-pistil interaction and on methods to over come the barriers in seed set. Absence of morphological difference between varieties adds to the use of cytogenetical investigations for the characterisation of varieties of ginger. The miotic index was found to be maximum during 5-6 AM. All the nine ginger varieties studied showed a chromosome number of 2n = 22. The karyotype of nine varieties of ginger studied showed considerable difference in their morphological features such as length of chromosomes, centromere position, total chromatin length and total chromosome volume. Such differences could have occurred through translocation, inversion and deletion of chromosome segments. Classification of karyotypes in ginger varieties fell in to primitive 1a and 1b group. During meiotic studies it was seen that the genome of ginger is highly unstable. Abnormalities like bridges and laggards were also present which will lead to the formation of micronuclei. Ginger flowers carry enough pollen grains in the single bilobed anther. But 60-84 per cent of the pollen grains were sterile. The high amount of meiotic irregularities may be leading to high percentage of pollen sterility. Out of the seven different media tried for pollen germination, the medium containing 8% sucrose + 60 ppm boric acid + 1% gelatin produced the highest percentage of germination of pollen grains (11.81%). The sterility and pollen germination was also correlated. Irradiation of pollan grains has been suggested to be effective in breaking the barriers in seedset. But it was seen that the irradiation of pollen grains was deteremental for the germination of pollen grains in ginger. Proper interaction between pollen and pistil is quite significant for the seed production. The study by using uv microscopy revealed the presence of spiny stigmatic surface which will prevent proper adherence, contact and germination of pollen grains. It is hard to see whether self and cross incompatibility is operating in ginger. In the present investigation many techniques for breaking incompatibility (if existing) such as bud pollination, artificial sibbing, artificial cross pollination between varieties, chemically aided pollination, mixed pollination, mentor pollination, stigma removal and artificial pollination failed to produce fruitset and seedset. Hence, based on the result the existence of incompatibility mechanism operating in ginger is quite complicated. Structural chromosomal aberrations such as translocation, deletions, inversions etc. leads to the sterility of microspores and megaspores. Pollen sterility ranging from 60 – 84 per cent might be one of the serious limitation for the setting of fruit and seeds in ginger. The spines present on the stigmatic surface prevent the pollen grain to get attached to the stigmatic surface during the act of pollination. Sterility is correlated with pollen germination and it is only upto 14.61 per cent in variety having maximum fertility. The germination of pollen grains was taking place very slowly so that flowers are withered before the pollen tube could reach the ovules. Coiling of pollen tube was also noticed during the advanced stages of pollen germination. The style length was very long (39,000 µ m) in ginger. But the pollen tube attained only 108 µ m in the best case under in vitro condition after 24 hours. So the possibility of pollan tube reaching the ovule is remote by the time flower will be withered and dehisced. The flowers were found to remain on the plant for less than 12 hours after anthesis. In essence, the absence of fruit set and seed set in ginger dose not seem to be controlled by a single factor, but an array of factors make this challenging problem more complex.