World Production and Economic Importance of Rice

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World production of rice has risen steadily from about 200 million tons of paddy rice in 1960 to 600 million tons in 2004. Milled rice is about 68% of paddy rice by weight (Wu and Tanksley, 2009). In the year 2004, the top three producers were

China (31% of world production), India (20%), and Indonesia (9%). World trade figures are very different, as only about 5-6% of rice produced is traded internationally. The largest three exporting countries are Thailand (26% of world exports), Vietnam(15%), and the United States (11%), while the largest three importers are Indonesia (14%), Bangladesh (4%), and Brazil (3%). Rice is the most important crop in Asia.

Nigeria has a leading role in rice production in west African ranking highest as both the producer and consumer of rice in the sub region with production figures is slightly above 50% (Okoet al., 2012) in Cambodia, for example, 90% of the total agricultural area is used for rice production (Wu and Tanksley, 2009).

2.4 Rice pests

Rice pests are any organisms or microbes with the potential to reduce the yield or value of the rice crop (or of rice seeds) (Xiao et al., 2002).   

2.4.1 Insects

Xiao et al.,(2002), show that flooding of rice fieldstriggers a process of decomposition and the developmentof an aquatic food web, which results in largepopulations of detritus-feeding insects (especially Chironomidand ephydrid flies). These insects emerge ontothe water surface and into the rice canopy in largenumbers, very early in the growing season, providingcritical resources to generalist predator populations longbefore “pest” populations have developed. An important consideration in rice integrated pest management is theability of most rice varieties to compensate for damage.The rice plant rapidly develops new leaves andtillers early in the season, replacing damaged leavesquickly. The number of tillers produced is alwaysgreater than the number of reproductive tillers, allowingfor some damage of vegetative tillers withoutaffecting reproductive tiller numbers.

2.4.2 Diseases

The need to grow more rice under increasingly intensivesituations leads to conditions that favour diseases.High planting density, heavy inputs of nitrogen and soil fertility imbalance result in luxuriant crop growthconducive to pathogen invasion and reproduction (Xiao et al., 2002).    Thisis compounded by genetic uniformity of the crop stand,which allows unrestricted spread of the disease fromone plant to another, and by continuous year-roundcropping that carries over the pathogen to the succeedingseasons. Reverting to the less-intense, low-yieldagriculture of the past may be out of the question, buta thorough understanding of the ecological conditionsassociated with the outbreak of specific diseases maylead to sustainable forms of intensification (Xiao et al., 2002).   

2.4.3 Weeds

Weeds have long been a consideration in lowland ricecultivation, beginning with the origin of puddling,which is thought to have been invented and/or developedin order to create an anaerobic environment that kills several weeds including weedy red rice. In most integrated pest managementprogrammes in lowland rice, weed managementhas therefore been more closely considered as part ofthe agronomic practices during puddling and, later,during aeration of the soil with cultivators (Thanhet al., 2000).  At least twohand weedings are necessary in most crops, and inmany countries are considered as economically viableowing to low labour costs or community obligations tothe landless, who are then allowed to participate in theharvest (Thanhet al., 2000).

2.4.4 Rats

Numerous species of rat occur in rice fields and cancause considerable damage throughout a community.Rats often migrate locally from usually permanenthabitats to rice areas as the food supply changesthroughout a yearly cycle (Thanhet al., 2000). The rice plant is most preferredafter the panicle has emerged. Although naturalenemies of rats do exist (especially snakes), pesticidesand other measures taken by farmers often suppresstheir populations and thus make possible the survivalof large rat populations (Thanhet al., 2000).

2.4.5 Birds

Birds can be very damaging to rice, especially whenthey occur in large flocks. The red-billed quelea(Queleaquelea) in sub-Saharan Africa and variousspecies in Asia are known as persistent problems inrice ecosystems. In most Asian countries and in Chad,netting is used to trap large numbers of birds for sale asfood. Mass nest destruction is also possible for somespecies. In Asia, these methods have effectively reducedpest bird populations to very low numbers. InAfrica, the capture method may bring benefits to localpeople in terms of income or additional dietary protein,but the impact on pest bird populations has been minor  (Thanhet al., 2000).

2.5AbakalikiRice Cultivars

Abakalikirice cultivars are often classified by their grain shapes and texture. For example, Onuekerice is long-grain and relatively less sticky, as long-grain rice contains less amylopectin than short-grain cultivars, also usually serve long-grain as plain unseasoned steamed rice. Ikworice are shortgrain (Olufowoteet al., 2008). The abakaliki rice is a sticky, shortgrain rice. Onuebonyirice cultivars include long-grained and aromatic, long and medium-grained Patna rice and short-grained Masoori (Olufowoteet al., 2008).

In Ebonyi North,the most prized cultivar is 'ponni' which is primarily grown in the delta regions of Ebonyi River. Kaveri is also referred to as ponni in the South and the name reflects the geographic region where it is grown. Aromatic rice have definite aromas and flavours; the most noted cultivars are the aforementioned basmati, Patna rice, and a hybrid cultivar from Abakalikizone sold under the trade name, Mast. It is a cross between Abakalikiand other rice producing sat long-grained rice that is creating great controversy. Both Basmati and Texmati have a mild popcorn-like aroma and flavor (Xiao et al., 2002).  

2.6 Plant Secondary Metabolites

Plant secondary metabolites are those metabolites which are often produced in a phase of subsequent to growth, have no function in growth (although they may have survival function), are produced by certain restricted taxonomic of plants, have unusual chemicals structures, and are often formed as mixtures of closely related members of a chemical family. The simplest definition of secondary products is that they are not generally included in standard metabolic charts. A metabolic intermediate or product, found as a differentiation product in restricted taxonomic groups, not essential to growth and the life of the producing organism, and biosynthesis from one or more general metabolites by a wider variety of pathways than is available in general metabolism (Aliyuand Fawole, 2000).

Secondary metabolites are not essential for growth and tend to be strain specific. They have a wide range of chemical structures and biological activities. They are derived by unique biosynthetic pathways from primary metabolites and intermediates. Plants produce as amazing diversity of low molecular weight of secondary metabolites.  Of the estimated 400,000 – 500,000 plant species around the globe, only a small percentage has been investigated phytochemically and the fraction subjected to biological or pharmacological screening is even lower (Aliyuand Fawole, 2000).

The ability to synthesize secondary metabolites has been selected through the course of evolution in different plant lineage when such compounds address specific needs such as;

1.     Floral scent volatiles and pigments have evolved to attract insect pollinators and thus enhance fertilization.

2.     Synthesizesof toxic chemical to ward off pathogens and herbivores or to suppress the growth of neighboring plants. Chemicals found in fruits prevent spoilage and act as signals (in the form of color, aroma, and flavor) of the presence of potential rewards (sugars, vitamins and flavor) for animals that eat the fruit and thereby help to disperse the seeds.

3.      Other chemicals serve cellular functions that are unique to the particular plant in which they occur (e.g. resistance to salt or drought) (Aliyuand Fawole, 2000).

2.7 Dihydrochaclone

The chemistry of dihydrochaclone has generated intensive scientific studies throughout the world. Especially interest has been focused on the synthesis and biodynamic activities of dihydrochaclone. The name “Chalcones” was given by Kostanecki and Tambor. These compounds are also known as benzalacetophenone or benzylideneacetophenone. In dihydrochaclone, two aromatic rings are linked by an aliphatic three carbon chain. dihydrochaclone bears a very good symphony so that variety of novel heterocycles with good pharmaceutical profile can be designed (Zhang et al., 2006).Dihydrochacloneare unsaturated ketone containing the reactive ketoethylenic group –CO-CH=CH-. These are coloured compounds because of the presence of the chromophore -CO-CH=CH-, which depends in the presence of other auxochromes (Zhang et al., 2006).

2.8 Dihydrochaclone Biosynthetic Pathway

Chaclone Synthase is present in all plants, thus the distribution and specificity of downstream tailoring enzymes varies dramatically across different species, within different tissues, and at different times during the life cycle of an individual plant (Aliyuand Fawole, 2000). Due to this biochemical diversity, most plants maintain multiple copies of chs genes which are expressed in various tissues at different developmental stages. Some of these isoenzyme genes are constitutively expressed, while others are transcriptionally induced by environmental stress including UV light, wounding, and pathogen infestation.These diverse expression patterns reflect the wide range of biological roles fulfilled in plants by products of the downstream flavonoid metabolic grid (Aliyuand Fawole, 2000).

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