1.2 Roots and Tubers
1.2.1 Roots
The root is the part of a plant body that bears no leaves and therefore lacks nodes. It typically lies below the surface of the soil. Edible roots mainly include cassava, beet, carrot, turnip, radish and horseradish. Roots have low protein and dry matter compared to tubers. Moreover, the major portion of dry matter contains sugars. The major functions of roots include absorption of inorganic nutrients and water, anchoring the plant body to the ground and storage of food and nutrients.
1.2.2 Tubers
Tubers are underground stems that are capable of generating new plants and thereby storing energy for their parent plant. If the parent plant dies, then new plants are created by the underground tubers. Examples of tubers include potatoes, water chestnuts, yam, elephant foot yam and taro. Tubers contain starch as their main storage reserve and contain higher dry matter and lower fiber content compared to roots. Various tropical roots and tubers are presented in Figure 1.2.
Figure 1.2 Various tropical roots and tubers.
The production of roots and tubers can be grouped into annuals, biennials and perennials. The perennial plants under natural conditions live for several months to many growing seasons, as compared to annual or biennial. The main points of difference among annuals, perennials and biennials are presented in Table 1.2. The perennials generally contain a greater amount of starch as compared to biennials.
Table 1.2 Annual, biennial and perennial roots/tubers
― | Life cycle | Limiting aspects | Benefits
Annual | Takes 1 year to complete its life cycle. | Growth can be a limiting factor in excess/scarcity of water for annual plants. Insect and disease problems are of minor concern. | Lesser benefits ascompared to perennials and biennials.
Biennial | Takes 2 years to complete its life cycle. | Early growth and quality is affected by late-season moisture stress. | Provides lesser benefit as compared to perennials in agriculture.
Perennial | Takes more than 2 years to complete its life cycle. | No specific period for growth. But by providing early and modified irrigation practices, production can be improved. | They can hold soil to prevent erosion, do not require annual cultivation, reduce the need for pesticides and herbicides, and capture dissolved nitrogen.
1.3 Requirements for the Higher Productivity of Tropical Roots and Tubers
The factors that need to be focused upon to meet the objectives of food security, sustainable farming and livelihood development are farming systems, pest and pathogen control systems, genetic systems and strategies for improvement, together with marketing strategies and the properties of the products and constituents.
1.3.1 Farming Systems
Tropical roots and tubers are generally grown in humid and sub-humid tropics, which are not suited for cereal production. Significant differences exist in the farming system perspectives of tropical root and tuber crops, varying from complex systems of production to intercropping farming systems. These systems are important to consider when studying the variation of different crop farming systems. The increasing production in the Pacific region has depended largely on farming more land rather than increasing crop yields. This is contrary to the projections of FAO that the 70 % growth in global agricultural production required to feed an additional 2.3 billion people by 2050 must be achieved by increasing the yields and cropping intensity on existing farmlands, rather than by increasing the amount of land brought under agricultural production (Hertel, 2010).
Farming systems need to be carefully looked after, by protecting and raising the production of tropical roots and tubers. For this purpose, various changes in attitudes and agricultural practices are desirable. Additional investments are required to reduce the impact of climate change and to overcome the disastrous effects of soil erosion. Diversity in the production of tropical roots and tubers and increasing production surface area may be adopted for higher productivity and better quality of tropical roots and tubers. Proper organization among small farmers, effective investment in mechanization, and improved storage and processing facilities can improve the productivity of tropical roots and tubers.
1.3.2 Pest and Pathogen Systems
The pest and pathogens of different tropical roots and tuber crops are varied. Roots and tubers are generally produced by small-scale farmers, debarring a few exceptions using traditional tools and without the adequate input of fertilizers or chemicals for pest and weed control. Therefore, the correct use of less expensive and effective dosages of pesticides and fertilizers is important to increase the productivity of these crops. Moreover, the activities need to be designed to reduce environmental degradation. Biochemical approaches need to be followed to reduce the damage due to pests and pathogens. The assessment of loss caused by pests and pathogens cannot be overlooked, which otherwise affects the production of tropical roots and tubers. In addition, pest and pathogens are of particular concern because of their direct effect on human and animal health. The effect of climatic conditions on the damaging action of pests and pathogens needs to be highlighted. Therefore, proper crop protection, involving different management practices, needs to be followed to reduce the damage due to pests and pathogens and to enhance the productivity of tropical roots and tubers.
1.3.3 Genetic Systems and Strategies for Genetic Improvement
The genetic system of roots and tubers widely differs, so the strategies for genetic improvements also differ. The breeding of root and tuber crops is primarily done sexually. The fact is that the different genetic systems suffer from many breeding complications along with limited opportunities for genetic development and further modifications (Mackenzie, 1995).
Some of the tubers, such as sweet potato and potato, may benefit from breeding cultivars, which are adapted to shorter growing seasons, while other crops (e.g. cassava) may need to fit into some other system, as they have contrasting growing cycles (Mackenzie, 1995). Hundreds of genetically distinct varieties of the roots and tubers are known to exist. Therefore, a focus is needed to genetically improve and develop the variety of roots and tubers, depending upon the requirement to achieve the required target. The dissemination of knowledge to the field is also a great concern in the area. Other considerations (e.g. crop management practices and crop diversification) specify that the decision-making should be carried out in individual breeding programs so as to benefit from these advancements. The needs for improvement in the programs are actually unique for a specific crop, rather than to the group of these crops classified as tropical roots and tubers.
Higher production can be achieved by exploring the genetic yield potential and by gaining knowledge about the genetic background of tropical roots and tubers (Okoth et al., 2013). Proper plant breeding approaches and genetic modification need to be followed for creating new genetic varieties. Overall, modern breeding technologies open up new possibilities to create genetic variation and to improve selection, but conventional breeding techniques remain important to improve the production of these crops.