Chapter 6 General conclusions
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Cassava is one of the most reliable crops that can be grown under adverse growth conditions that are often unsuitable for other crop production. The production advantages of cassava are, however, partly offset by the rapid deterioration of the roots, which can begin as quickly as 24 hours after harvest. Most varieties of cassava deteriorate within three to four days of harvest. This rapid deterioration is due to physiological processes which are initiated at sites of mechanical damage. Physiological deterioration of cassava roots can lead to substantial quantitative and qualitative post-harvest losses causing high production and market risks. The short shelf-lite of cassava has played a major role in the evolution of cultural and post-harvest management practices.
In many regions cassava is moving rapidly from being a subsistence crop to becoming an income-generating crop. Marketing problems are becoming exacerbated as increasing urbanization is placing both distance and time between producers and consumers. Processing of cassava is important for extending the versatility and economic viability of the crop. A continuous supply of roots is needed for efficient processing operations, especially in regions where the trend is towards larger cooperative or commercial units. Physiological deterioration places serious constraints on the crop's suitability for modern production, processing and marketing practices and consequently has an impact on all levels of income generation.
In recognition of the important role of cassava in developing countries and the increasing need to improve its storage potential to deal with changing market needs, FAO held a workshop on post-harvest deterioration of cassava in Rome during December 1991. The meeting was under the general auspices of FAO and was cosponsored by the Rockefeller Foundation. The principal objectives of this meeting were to provide a forum for scientists to review the biochemistry and physiology associated with post-harvest deterioration of cassava and to explore the possibilities of developing cassava varieties with superior storage potential.
Existing post-harvest management practices were discussed and their application to the changing market needs of cassava examined. Storage techniques that have been developed, such as packing in moist media, freezing, waxing and canning were reviewed. In general, these methods were considered to be either technically or economically unsuitable for most marketing needs. However, it was recognized that CIAT and NRI have developed a simple technique based on plastic-bag storage of fungicide-treated roots which increases storage time to about two weeks. This approach was considered feasible and worth pursuing in countries where sufficient infrastructure, such as market channels, farmers' cooperatives and extension services, exist. However, in many developing areas, including Africa, the plastic-bag storage approach developed in Latin America may not be either commercially viable or desired by the consumer.
The team of scientists present at the workshop considered that solving the problem of physiological deterioration of cassava would provide very significant benefits to farmers, traders and processors, irrespective of their scale of operation, as well as providing consumers with a better quality product and enhanced food security.
After reviewing both the current state of knowledge and the potential technology that could be applied, it was agreed that developing an alternative method for extending cassava storage should be pursued. It was emphasized that any new methods proposed should be environmentally acceptable and appropriate to a wide variety of cultivars and production systems. Storage needs were discussed in view of the existing and potential role of cassava. Increasing the storage life of cassava roots to a minimum of two weeks could have a substantial impact on cassava utilization and potentially resolve an estimated 90 percent of the deterioration constraints associated with current cassava marketing and utilization practices.
Despite the fact that the mechanisms involved in physiological deterioration of cassava have yet to be elucidated, the group considered that this problem could be resolved. The fact that cassava can be left intact in the soil for over a year was viewed as demonstrating the long-term storage potential of the crop.
Information available to date has demonstrated that a fairly limited range of genetic variability exists in cassava for physiological deterioration.
Results indicated that only a very small proportion of the clones evaluated did not deteriorate one week after harvest and demonstrated a high degree of variability caused by environmental factors. Wild species evaluated showed responses similar to that of cultivated cassava. However, the evaluation method used was considered insufficiently precise to critically assess the genetic variability for physiological deterioration in cassava. For quantifying this trait, the group considered that there was a need to further define and develop a screening assay method. This method once developed could then be used to evaluate the important core germplasm collections available in Latin America and Africa to determine more fully genetic variability and stability of this trait.
It was thought possible that conventional breeding could achieve two weeks storability, using recurrent selection methods. However, the highly heterozygous nature of cassava was considered to be a major problem in the use of conventional breeding techniques for resolving this problem. For widespread impact through conventional breeding. it was recognized that major efforts would be required for incorporating the trait into different cultivars without altering the desired characteristics of the parent genotypes. Since each of these will also have to be bred for many other traits, the effort required was not considered tenable.
Genetic manipulation using molecular techniques was considered the most applicable method for resolving this problem. There is at present no information available on the genes involved in the biochemical pathways that are associated with physiological deterioration in cassava. However, because of their implication in the process of post-harvest deterioration, the genes and gene products associated with the synthesis and degradation of phenylpropanoids were considered principal targets for study and manipulation. A substantial literature exists to serve as a basis for this approach.
Many of the genes of the phenylpropanoid pathway associated with woundinduced responses have been isolated and characterized from various other plants. These genes could be used to isolate the corresponding genes from cassava. Detailed genetic studies could then provide an insight into the deterioration process and would therefore be integral to developing strategies for genetic manipulation approaches. Genetic engineering has beneficially achieved repression or overexpression of these genes in a number of agriculturally important crops.
A molecular genetic approach could in theory increase the storage potential of cassava roots to a minimum of two weeks, without the use of post-harvest treatments. This could be achieved by suppressing the development of physiological deterioration and enhancing the wound-healing responses to prevent the onset of microbial deterioration.
The introduction into cassava of discrete gene constructs by genetic manipulation offers the unique advantage of adding new traits to elite genotypes without altering other desired characteristics.
The development of a reliable variety-independent methodology for transformation of cassava is particularly important because it is a highly heterozygous tetraploid and regeneration of parental types by back-crossing approaches is not feasible. To develop an effective programme on cassava it was considered essential to develop a reliable genotype-independent transformation method for cassava and obtain additional data on the underlying biochemical mechanism of physiological deterioration to enable identification of target genes.
In view of the number of related achievements with other crops, the molecular genetic approach to controlling physiological deterioration of cassava was considered the most promising research strategy. It was recommended that FAO should collaborate with appropriate institutes to facilitate a comprehensive initiative in this research area.
The post-harvest behaviour of cassava has played a major role in the evolution of cultural and post-harvest management practices. As with any innovative intervention the socio-economic impact of extending the storage potential of cassava should be carefully evaluated to reflect the different requirements of potential beneficiaries. It is clear that innovations that affect deterioration characteristics will both cause and require major adjustments in existing practices and social structures. An evaluation and appreciation of these impacts is essential for interactively shaping any research proposed.
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