Genetic studies on drought tolerance and grain shattering in rice.
Grain yield of rice can be enhanced by breeding for drought tolerance and optimum shattering. New generations of rice, the interspecific fixed lines known to carry more drought tolerance traits, are a potential source for improvement of the Ugandan landraces but they frequently carry genes for grain shattering. Information gathered through surveys to understand rice production constraints and farmers’ preferred rice traits are limited and they are generally unranked. The objectives of this study were seven-fold. They were to 1) determine farmers’ perceptions on the key production constraints and preferences for rice in Uganda, 2) develop and appraise cheap and efficient rice breeding equipment, 3) determine anthesis and the grain shattering pattern of rice, 4) characterize for drought tolerance and grain-shattering ability in rice populations in Uganda, 5) investigate crossability between O. sativa and the new interspecific fixed lines, and 6 and 7) determine the inheritance and gene action controlling drought tolerance and grain shattering traits. The experiments were conducted at Namulonge, Uganda, from 2006 to 2009. Data on farmer’s perceptions of the key production constraints and preferences for rice in Uganda were obtained through participatory rural appraisal (PRA) methods and a formal household survey of 380 farmers from three regions in Uganda during March 2007 to June 2008. The results showed that the variety NERICA 4 was the most preferred by farmers. Rice yellow mottle virus disease, rice blast and African rice gall midge were major constraints under lowland conditions, while drought was the major constraint under upland conditions. The preferred traits in traditional varieties included aroma characteristics, grain size and the texture of cooked grains. Cultivars that combine high yield potential, early maturity, drought tolerance, aroma characteristics and special market attributes should be included in the breeding objectives. A new anther suction device that uses an ordinary household vacuum cleaner was fabricated. Its overall units include a pipette tip, silicone tubing, hose tubes, a metallic stand, a rubber stopper and a household vacuum cleaner of 1600 w capacity. It is efficient, cheaper, less bulky and has lower replacement costs than the commercial one. In addition, a hand-held single grain tester, which is efficient, cheaper and more suitable for field studies than the laboratory tester, was developed and appraised. Studies to determine the grain-shattering pattern in rice were conducted by making assessments at 10, 20 and 30 days after anthesis. The results revealed that the highest level of discrimination among the varieties was between 10 days and 20 days after anthesis. Among the genotypes tested, NERICA rice attained anthesis earlier than CT rice and O. sativa attained anthesis later than both rice groups. Appropriate methods of planting the parental lines were proposed for pollination to be synchronised. Interspecific rice and O. sativa were characterized in the field for drought tolerance and grain-shattering ability. There was high variability wihin species in grain shattering and drought tolerance traits, suggesting that selection could be effective. The distribution of grain shattering scores was not normal, suggesting that some selection had taken place for the trait, unlike the drought tolerance traits and leaf roll scores where normal distributions were observed. Results of the crossability test between O. sativa and interspecific fixed lines revealed that some genotypes had high crossability when used as males, or as females, or as both females and males. The best male genotype combiners were CT 16334(2)-CA-2-M, WAB 365-B-1H1-HB and IRAT 257, while the best females combiners were CT 16334(2)-CA-11-M, CT 16317-CA-4-M and IRAT 325. In addition, additive effects were more important that non-additive effects for pollen viability and seed set in the crosses. Genotypes with acceptable crossability and varying levels of drought tolerance and grainshattering ability were selected for further studies. The selected genotypes were crossed in a North Carolina II mating scheme, and as pairs of crosses, and used to understand the genetic mechanisms of drought tolerance at the reproductive and vegetative stages. Evaluation of the populations revealed that additive effects for filled grains were more important under drought stress and non-drought stress conditions than the non-additive effects. There were high heritability estimates for leaf roll under drought stress, implying that additive effects were more important. There was a higher contribution of additive effects for grain-shattering ability than nonadditive effects in the populations. Allelic tests for the shattering trait revealed that a single gene was in 46% of the crosses; two genes in another 46% of the crosses had duplicate dominant effects as well as complimentary dominant/recessive effects; and there were three genes in four percent of the crosses. Segregation ratios of 3:1 and 1:3 were also found in two crosses. This could be due to a reversal of the function of the genes. Another surprising finding was that the segregation ratio of 3:1 was found in a cross with loose grain attachment to the pedicel in an allelism test. This segregation was probably due to unfixed loci in some of the interspecific lines. This study identified rice breeding priorities for Uganda, prepared breeding tools and provided relevant information for generating lines that have improved drought tolerance and optimum grain-shattering ability by determining the modes of gene action for these two traits.
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