Year: 2016
Type: dissertation
Abstract: Bottle gourd [Lagenaria siceraria (Molina.)Standl.] is potentially a useful food crop that could contribute to crop diversification among smallholder farmers and the alleviation of malnutrition in South Africa.However, the crop can be considered as being among one of the most neglected, underutilized and under-researched.The crop is widely grown by smallholder farmers in the drier regions of South Africa who use predominantly landrace varieties.There is no genetic improvement programme in the country which impacts on breeding and strategic conservation of the crop.Therefore, the genetic diversity of this crop needs to be studied and documented to enhance its production and productivity.This knowledge will be essential for effective bottle gourd pre-breeding and breeding programs in South Africa.Evaluation of the available genetic resources of the crop using agro-morphological and horticultural attributes and molecular markers will be useful in the selection of promising genotypes for the development of unique varieties with improved yield, yield components and quality and for resistance breeding against abiotic and biotic stresses.Therefore, the objectives of this study were: 1) to determine genetic diversity present among bottle gourd landraces widely grown in South Africa using qualitative and quantitative morphological traits, 2) to determine genetic diversity among bottle gourd landraces using single sequence repeat (SSR) markers; 3) to apply correlation and path coefficient analyses to determine level of association among qualitative and quantitative traits and subsequently to select unique genotypes with suitable traits for breeding and strategic conservation; 4) to determine drought tolerance of a diverse set of bottle gourd landraces using drought tolerance indices and to identify promising genotypes for direct production or breeding and 5) to determine the relationship between cucurbitacin accumulation with leaf gas exchange and chlorophyll fluorescence in bottle gourd under water stress conditions.In the first study, the genetic diversity present among 36 diverse landrace selections of bottle gourd was investigated using morphological qualitative and quantitative traits.Phenotypic differences were observed among bottle gourd landraces.Principal component analysis (PCA) of qualitative and quantitative traits identified five and seven principal components (PCs) which accounted for 78 and 87% of the total variation, respectively.Among qualitative traits, presence or absence of ii fruit neck, fruit shape, degree of neck bending and fruit neck length positively correlated with PC1, which accounted for 31.9% of the total variation.Presence or absence of seed lines and seed texture was highly correlated with PC2 which accounted for 14.9% of the total variation.For quantitative traits, plant height, number of nodes on the main stem, total number of nodes, number of male flowers, number of branches, fruit neck diameter positively correlated with PC1, which accounted for 39.6% of the total variation.Fruit mass, fruit width, fruit size, shell thickness, seed length, seed width, seed size and hundred seed weight highly correlated with PC2 which accounted for 13.7% of the total variation.Unique genotypes such as BG-16, BG-25, BG-09, BG-37 and BG-10 showing suitable qualitative traits and BG-07, BG-13, BG-67, BG-12, BG-09 and BG-06 showing suitable quantitative traits were identified for genetic improvement.In the second study, the genetic diversity of 67 bottle gourd landraces was determined using 14 selected polymorphic simple sequence repeat (SSR) markers.The markers amplified a total of 86 alleles with allele sizes ranging from 145 to 330 base pairs (bp).Number of effective alleles (Ne) ranged from 1.58 to 6.14 with a mean of 3.10.Allelic richness varied from 3.00 to 8.90 with a mean of 5.23.Expected heterozygosity (He) values ranged from 0.37 to 0.84 with a mean of 0.65.The mean polymorphic information content (PIC) was 0.57.Jaccard's coefficient of similarity values ranged from 0.00 to 1.00, with a mean of 0.63.Analysis of molecular variance revealed that 79%, 17% and 4% of the variation was attributable to among landraces, within landraces and between populations, respectively.Cluster analysis classified the bottle gourd landraces into three major clusters namely: cluster I, cluster II and cluster III.Overall, landraces such as BG-04, BG-06, BG-08, BG-09, BG-15, BG-55, selected for breeding and systematic conservation.In the third study, simple correlation and path coefficient analysis were used to determine the level of association between qualitative and quantitative traits among 36 bottle gourd landraces.Simple correlation analysis showed that fruit width, number of seeds per fruit, seed width, seed size and hundred seed weight significantly and positively correlated with fruit yield and seed yield, respectively.Highly significant and negative correlations (P < 0.001) were observed between primary fruit colour, secondary fruit colour, fruit texture, fruit shape, degree of corrugation, neck bending degree and fruit neck length with either fruit and seed yield.Path coefficient analysis iii revealed high direct positive path coefficient values between number of female flowers, seed size, and number of seeds per fruit and fruit width with fruit yield, respectively.High direct negative path coefficient values were observed between primary fruit colour and fruit shape with seed yield and presence or absence of fruit neck, fruit texture and neck bending degree with fruit yield, respectively.The identified quantitative and qualitative traits are useful for improving the yield potential of bottle gourd.The focus in the fourth study was to evaluate selected bottle gourd landraces for drought tolerance and select genotypes for breeding using yield-based selection indices.Significant differences (P < 0.05) were observed amongst bottle gourd landraces with respect to edible fruit number and fruit yield under drought stress (DS) and non-stress (NS) conditions.The mean fruit number under DS and NS conditions were 15 457 and 31 088 ha -1 , respectively.The mean fruit yield under DS and NS were 8.75 t ha -1 and 22.4 t ha -1 , respectively.Drought stress reduced fruit number and yield by 49% and 62%, respectively.Landraces such as BG-79, BG 31, BG-48, BG-67, BG-52, BG-78 and GC were promising for large scale production or as parents for drought tolerance breeding.Correlation and principal component analyses revealed the significance of drought tolerant indices such as geometric mean productivity, stress tolerance, mean productivity, yield index and harmonic mean which allowed discrimination of drought tolerant bottle gourd landraces.Lastly, twelve selected bottle gourd landraces were grown under non-stressed and water-stressed conditions under glasshouse conditions to determine whether the accumulation of cucurbitacins could be indicative of drought tolerance in bottle gourd.Leaf gaseous exchange and chlorophyll fluorescence parameters were measured and correlated to cucurbitacin content.Stomatal conductance, transpiration rate, net CO2 assimilation rate, CO2 assimilation rate/intercellular CO2, intrinsic water-use efficiency and instantaneous water-use efficiency declined under water stress conditions.Intercellular CO2 concentrations, ratio of atmospheric and intercellular CO2 were significantly increased by water stress.The maximum photosystem II activity, quantum yield of photosystem II, photochemical quenching and non-photochemical quenching were not affected by water stress; whereas, electron transport rate, electron transport to oxygen molecules and alternative electron sink declined.Cucurbitacin E and I were detected under water stress condition in several bottle gourd landraces.Significant and positive correlations were observed between iv cucurbitacin I content with electron transportation to O2 molecules and alternative electron sinks suggesting their possible role in the regulation of photorespiration and photoprotection against oxidative stress.Accumulation of cucurbitacin I could be considered as a tool for selection of bottle gourd genotypes for drought tolerance.Overall, the present study identified valuable bottle gourd genetic resources useful in the development of improved cultivars for large-scale production in South Africa or related environments.v
Author: Jacob Mashilo
Language: English
Subfield: Genetics
Domain: Life Sciences
SDG: Zero hunger
Open Access status: green
OpenAlex ID: W2618177904