Selected Morphological And Agronomic Descriptors For The Characterization Of Cassava

Author : Rosemary Wanja Mutegi-Murori
Publisher : Cuvillier Verlag
ISBN 13 : 373693260X
Total Pages : 166 pages
Book Rating : 4.7/5 (369 download)

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Download or read book Towards Identifying the Physiological and Molecular Basis of Drought Tolerance in Cassava (Manihot esculenta Crantz) written by Rosemary Wanja Mutegi-Murori and published by Cuvillier Verlag. This book was released on 2010-02-10 with total page 166 pages. Available in PDF, EPUB and Kindle. Book excerpt: Drought is one of the most important factors limiting crop production in sub-Saharan Africa. This has detrimental effects to the people living in this region, and whose population is increasing more rapidly than their domestic food production. Noticeably, pressure on agricultural land has continued to intensify. Cassava is one of the staple crops with remarkable tolerance to drought. It is adapted to diverse and poor soil conditions, in addition to its flexibility in planting and harvesting times. Understanding its physiological and molecular basis of drought tolerance may help to target the key traits that limit crop yield under drought conditions. To improve our understanding on drought tolerance mechanisms in cassava, the project "Identifying the physiological and genetic traits that make cassava one of the most drought-tolerant crops" was initiated in 2005 by the Brazilian Agricultural Research Corporation (Embrapa) in collaboration with the International Center for Tropical Agriculture (CIAT); the International Institute of Tropical Agriculture (IITA); Cornell University and University of Goettingen. The ultimate goal of the project was to identify morphological, physiological and molecular traits related to drought tolerance mechanisms in cassava for further progress, and for their application in cassava and other crop breeding programs. The present study was conducted within the framework of this project with 31 African cassava germplasm accessions from IITA and a mapping population developed at CIAT. The objectives of this study were, 1) To develop a protocol for hardening and rapid micro-propagation of cassava plantlets under local, low-cost conditions; 2) To identify agro-morphological attributes that are related to drought tolerance in cassava; 3) To identify drought-tolerant and drought-susceptible cassava germplasm from a selection of African accessions; 4) To identify secondary traits that could be used for phenotyping breeding materials for drought tolerance; 5) To screen the CIAT mapping population with simple sequence repeats (SSR) and expressed simple sequence repeat (ESSR) markers for linkage analysis. Thirty one putative drought-tolerant and drought-susceptible African cassava germplasm accessions from IITA were micro-propagated using direct and in-direct techniques, at Kenya Agricultural Research Institute (KARI), Nairobi, Kenya. In direct micro-propagation, plantlets were hardened using vermiculite and multiplied through nodal cuttings. In in-direct micro-propagation, plantlets were first multiplied through sub-culturing and later hardened. The direct micro-propagation method had a higher multiplication rate. The number of plantlets obtained in 7 months using the direct method were 1173 as compared to 722 attained using the in-direct micropropagation. Rapid micro-propagation through nodal cuttings was cheaper in terms of consumables and an effective alternative to enhance rates of multiplication, over the in-direct method and the more conventional technique like the use of stem cuttings. Agronomic and morphological evaluation of contrasting African cassava germplasm accessions was carried out in water-stressed and well-watered environments at 5 time points. The trial was conducted at the experimental field of KARI, Kiboko Research Station in Makindu, Eastern Kenya, a site characterized by Acri-orthic Ferralsol soil. Analysis of variance was performed using the agronomic and morphological data, and broad sense heritability was estimated. In general, significant differences were observed among the accessions, suggesting a strong genetic basis for the phenotypic variation observed. Variation was also notable in water-stressed and well-watered environments for a majority of traits evaluated. This was due to the artificial water applied since, during the trial period, there was hardly any rainfall. At harvest, leaf length and width of certain accessions at the water-stressed site approached that of the well-watered treatment. On average, the estimated mean percentage leaf retention was high in the well-watered treatment. However, leaf retention in some of the accessions assessed was almost the same in both treatments. These accessions tended to produce higher yields. Thus, it may be desirable to select for higher leaf retention when developing varieties adapted to dry areas. The range of yields under stress was from 3.3 to 36.7 kg/m2, whereas, under the well-watered treatment, it was smaller: 28.3 to 53.3 kg/m2. Differences among accessions in yield and overall above-ground fresh biomass showed that these are important primary traits to phenotype germplasm under favorable and water-stress conditions. In addition, 4 accessions G26, G11, G8 and G31 were more tolerant than the rest of the genotypes evaluated, calling for further research and their involvement in agricultural experimentation under drought-prone conditions. Considering relationships between traits, genotype ability for both accumulation of above-ground fresh biomass and to partition carbon into roots (harvest index) were among the traits most correlated with root yield. However, accessions, environment and the interaction of both influenced the traits strongly. Thus, it is important that agro-morphological field trials be conducted in several locations for several seasons for effective evaluation of their influences on traits that might be relevant for phenotypically assessing drought tolerance. Unlike for agro-morphological traits, where a drought-tolerant accession could be identified from the yield, which is of primary concern, selection of an outstanding accession in a water-limited environment using metabolic traits was not achieved in this study. Maybe the well-watered plants experienced an incipient stress due to low humidity, symptomless diseases or nutrient deficiency, which was not ascertained in this study. Despite this, significant differences were observed between the waterstressed and well-watered treatments for the traits evaluated, except protein and amylose content. Performance in individuals was variable, although insignificant differences were observed between the different stress phases. Changes in sugar concentration have a role in the drought-tolerance of the accessions evaluated, although, their relative contribution to drought stress could not be determined from the available data. Further work is imperative to identify and quantify sugar concentrations in relation to osmotic adjustment in these accessions. In addition, further research to determine the time course of ABA accumulation, as a cassava plant goes from its young stage to aging, is required so as to know the stage at which ABA data are most informative. Two genetic linkage maps were constructed using a South American mapping population of 228 individuals derived from a cross between a drought-tolerant and a drought-susceptible parent. A set of 377 simple sequence repeats (SSR) and expressed simple sequence repeats (ESSR’s) were utilized for the initial polymorphism screening. Differences in map size, interval, number and mean distance between markers were apparent between the two maps. The female map had 14 linkage groups as compared to 13 in the male map. Twenty seven allelic bridges were noticeable between the two maps. In addition, 25 markers showed collineality with other available cassava maps. Forty six markers, whose map distances had not been determined previously, were mapped in this study. These maps form an important platform upon which to characterize the genetic basis of drought tolerance in cassava. Continued addition of more markers in these maps will refine the utility of the resource for future cassava breeding efforts. In conclusion, four African cassava accessions apparently have the ability to withstand severe drought. However, a majority of the accessions evaluated gave poor response in adaptability to water-limited conditions. This suggests that further agroecologically based research is required on these materials, since they represent diverse improved accessions from IITA breeding activities. This, coupled with markerassisted genetic analysis, would be an appropriate approach for the identification of drought-tolerant accessions.