Agro-morphological and Nutritional Characterization of Tomato Landraces (Lycopersicon Species) in Africa
Tomato (Solanum lycopersicum), often associated with a wide range of health benefits due to its rich nutritional quality, is an important fruit vegetable crop in Kenya. However, varieties grown in Kenya are few and highly susceptible to biotic and abiotic stresses that adversely affect productivity. Wild relatives and unimproved accessions of crops are often better adapted to biotic and abiotic stresses and serve as a source of desirable genes for crop improvement in this respect. The main objective of this study was to characterize 69 tomato ecotypes from the World Vegetable Centre and the National Genebank of Kenya. The specific objectives were: (1) to evaluate the African tomato landraces for morphological and agronomic traits (2) to determine the effect of water stress on growth, yield and nutritional quality of selected African tomato landraces. Field and greenhouse experiments were laid out in a randomized complete block design with three replications at the University of Nairobi’s Kabete Field Station, Kenya, in 2014 and 2015. Characters were evaluated based on the International Plant Genetic Resources Institute tomato descriptor list of 14 agronomic and 10 morphological traits at flowering and fruiting stages. Twenty (20) accessions were selected for their desirable agronomic traits from the initial list of 69 accessions and subjected to four watering levels: 100%, 80%, 60% and 40% of the field capacity (FC). During growth, accessions from all watering levels were evaluated for agronomic and physiological traits. Fully ripe fruits were harvested from the 20 accessions at 100% and 60% FC respectively and evaluated for β-carotene, vitamin C, minerals, simple sugars, total phenolics and total antioxidant activity. Analysis of variance from the first experiment indicated significant differences (P<0.05) in the accessions for all the agronomic traits evaluated. Accessions with the highest and the least number of fruits recorded means of 8.3 and 442.3 fruits per plant respectively. Similarly, fruit weight varied widely within the range of 565 g to 2759 g per plant. Yield showed positive and significant correlation with fruit length (r=0.42), fruit width (r=0.51), fruit weight (r=0.50) and stem girth (r=0.41). The first three components of principal component analysis explained 78.2% of total variations among the genotypes. The characters contributing most to variability were growth type, foliage density, fruit size and fruit cross sectional shape. Cluster analysis using unweighted pair group method with arithmetic mean grouped the genotypes into two clusters. Cluster I contained 63 accessions while cluster II had 6 accessions. Results from the second experiment showed significant (P<0.05) interactions among accessions and water levels for both agronomic and physiological traits evaluated. Water stress significantly reduced fruit yield xiv which ranged from 127.3 to 1487.7 g at 60% FC compared to 521.0 to 2404.3 g at 100% FC. Similarly, reductions in stem girth, plant height and leaf area were recorded for the agronomic traits. Water stress reduced stomatal conductance which ranged from 74.0 to 100.1 mmol/m2s at 60% FC compared to 207.7 to 287.5 mmol/m2s at 100% FC. Similar reductions were also observed for SPAD value and leaf relative water content under water stress conditions. However, water stress significantly increased leaf canopy temperature for all the accessions. Water stress significantly increased total phenolics which ranged from 3.2 to 11.3 garlic acid equivalent (GAE)/100 g at 60% FC compared to 1.5 to 4.9 GAE/100 g at 100% FC. Total antioxidant activity increased with water stress from 17.9 to 38.3% inhibition at 60% FC compared to 13.3 to 29.3 % inhibition at 100% FC. Increased levels of fructose, glucose and sucrose were recorded at 60% FC compared to 100% FC. On the contrary, significantly lower levels of mineral nutrients (potassium, zinc, magnesium, iron and sodium), β-carotene and vitamin C were recorded at 60% FC than at 100% FC. Thus, this study revealed significant variations in morphological, agronomic, physiological and nutritional diversity among the African tomato accessions. This rich diversity could be exploited in future tomato improvement programmes.
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