Rice belongs to the family of Graminaeae and the genus Oryza. Orzyae coantains about 20 different species, of whic only two are cultivated: Oryza sativa L. ('Asian rice') and Orzya glaberrima Steud or 'African rice'. The species O. sativa comprises of two main types: indica and japonica. Another variety of rice is javanica, originated from Javanese province in Indonesia.
The indica rice type is characterized by a long, wide to narrow, light green leaves, profuse tillering, usually long and thin grains as well as several secondary ramification or a small branches in the panicle holding the grains. On the other hand, japonica variety can be associated with thin, light green leaves with medium tillering. It has a short to intermediate size and its grain is rather short and round.
In general, rice plant has around and hollow stems, flat leaves and panicle at the top of the plant. The plant comprises vegetative organs such as the roots, stems, leaves, and reproductive organs or panicle made up of spikelets. Below are the descriptions of each organ:
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Root: a shallow root system, found at the below of the rice plant bark which absorbs water and nutrients
Stem: made up nodes and internodes, transport water and nutrients and bring air to the roots
Internodes: characterized by hollow and smooth surface, with lower internodes are shorter than the upper internodes. Shorter internodes provide a resistance to lodging.
Nodes: contained a leaf and bud which will develop into a tiller
Leaf: grow alternately on the stem, a leaf for each node, panicle leaf is a type of leaf which covered the panicle, produce carbohydrates based on photosynthesis, photo respiration and transpiration also occurred on the surface of the leaves
Reproductive organs (panicle and flower)
Panicles: found on the top of the rice plant, which branches into pedicels that carried spikelets
Flowers: made of male and female reproductive parts, normally undergo self-fertilization
Season of planting and harvesting rice
Regarding planting season for rice, no article has been found to explain the possible planting season. Regarding harvesting time, from the Journal of Agriculture and Forestry, the suitable time to harvest is when the yield and quality of rice are at its best. The growing period of rice is 150-160 days. In order to obtain maximum rice yield and total milled rice, it is essential to sow and harvest it just on time. Early harvesting may reduce the field yield of paddy and head rice yield due to presence of immature kernels. Late harvesting may also reduce rice yield because of grain shattering and lodging (Halil SÜREK, 1996). Optimum harvesting time for lowland rice was between 28 and 34 days after heading during the dry season, and between 34 and 38 days after heading during the wet season in the tropical areas (Nangju, D. 1970). Harvesting is conducted on a range of 27 -39 days after flowering at high moisture content (18-23%) gave maximum head rice recovery in India (Grovindaswami, S. 1968). In Japan, 20-25 days after heading was found the best time to harvest (Eikichi, I. 1954). In California, some rice growers reported high rice yields, harvesting at a range between 22-26% moisture. In addition, Arkansas, rice is harvested at 18-22% moisture content (Huey, B.A. 1977). Maximum paddy yield and total milled rice with minimum breakage during milling, acceptable moisture content,minimum green paddy in crop, and minimum chalky kernel in milled rice were obtained at 32 days after flowering in Bangladesh (Biswas, S.K.. 1984). In Pakistan, the optimum harvesting time is determined as 30-35 days after flowering and this time 80% of the grains turn into a yellow colour (Shulten, G.G.M. 1985). An additional study has been conducted on two areas with different season in India. During summer season, it was found that rice should be harvested between 32-42 days after flowering (panicle moisture content 18-23%) to get the maximum yield of rice (Varshney, A.C. 1988). For rainy season, the results of the experiment indicated that grain hardness and yield were highest, and percentage of broken grains was lowest, when harvested at 32 days after flowering (Sajwan, K.B. 1992).
Factors contributing to obtain high yield
Several factors are identified that can affect the yield of rice plant. The factors are water resource, nutrient availability, spikelet position, night temperature and CO2 concentration. Limited water resources can inhibit the growth of paddy plant thus reducing the yield, irrespective to availability of fertilizer or vice versa. Nitrogen is an essential element for the plant. PK treatments indicated that P and K were rarely limiting yield without N application. But, if N was applied, application of P and K did increase average yields significantly. Nitrogen fertilizer alone had on average an effect similar to that of farmyard manure (treatments N and FYM). Controlled-release N fertilizer combined with P and K applications did not improve yields compared with standard NPK fertilizer (treatments CR-NPK and NPK), and the high yield level of the FYM-NPK treatment was almost matched by the NPK treatment including micro-elements. (S.M. Haefele, 2006). On interaction between water and nutrient, water stress does interfere with nutrient uptake and nutrient-use efficiency in the plant, which makes it rather unlikely that this effect is limited to a specific nutrient supply situation (O'Toole and Baldia, 1982). Relationship between spikelet position and high night temperature is discussed below. The number of productive tillers (per unit ground area), spikelet sterility and grain weight are important components of yield (Sheehy et al., 2001) that are affected by the cultivation system and by environmental factors among which temperature is considered to be agronomically important (Singla et al., 1997). Production of tillers is sensitive to temperature (Mitchell, 1953). Furthermore, tiller number in the small grains is positively correlated with panicle dry weight per area (Paulsen, 1987).. Elevated night temperature (>29 â-¦C) increases spikelet sterility of rice with a subsequent reduction in seed-set and grain yield (Satake and Yoshida, 1978; Ziska et al., 1996). Under stress conditions, different grain length and width can be seen on the tip and base sections of panicle. Competition between grains in a spikelet also is possible. In addition, high night temperature also causes increased vegetative respiration rates (Frantz et al., 2004) and leaf membrane injury (Reynolds et al., 1994). All this can affect yield. Next is the correlation between high night temperature and CO2 concentration. Elevated CO2 accelerated rice development, and increased leaf photosynthesis by 30-70%, canopy photosynthesis by 30-40%, and crop biomass yield by 15-30%, depending on genotype and environment under optimum conditions (Horie et al, 2000). CO2 increased spikelet susceptibility to high-temperature damage under both ambient and elevated (Kim et al 1996). However, CO2 does not interact with night-time heat on spikelet sterility. In contrast, under low night temperature, elevated CO2 enhanced photosynthesis rate.
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Rice cultivated in Malaysia and the market value
Aromatic and long kernel in rice
Aromatic and kernel traits are linked based on genetic analysis. Long kernel trait can be found on genotype E2 , E11, Gharib, E6, E26, E34, E35, E36, E19, E20 and E27. Aromatic trait can be observed on 34 genotypes, which 10 superior types are the E11, Sadri, Gharib, E7, Kasturi, Rambir Basmati, E21, E13, E24, and Rato Basmati. Positive correlation (r = 0.59, p_0.05)) was observed between aroma and kernel elongation in these selected 10 genotypes. Aromatic rice can have a high market value and represents but local and national identity. Longer kernel contained more biomass and such the yield is higher in term or production. Long grain, when cooked properly tends to be much fluffier and less sticky. It produces a "drier" rice result which means the rice, when not overcooked, is easily separated. Due to lower gluten in long grain styles of rice, flour made from this rice may be an excellent substitute for people on low gluten or gluten free diets.
Emasculation technique is a removal of male part of the plant and transferred artificially to the female part on the same plant in a controlled manner and environment. Emasculation is conducted in the morning before the rice plant begins to bloom, or less often in the afternoon after the daily blooming period has passed (Jones, J. W. 1924). The technique involved removal of 15-20 spikelets from the female panicle on early morning. The glumes on the remaining spikelets are clipped off at an angle of about 45 degrees. These will expose half of the upper part of the lemma, but only the end and sometimes none of the palea. By clipping the glumes in this manner all 6 anthers are exposed and removed with a forcep. Removing anther should starts at the upper spikeket and proceed down to the lower part to prevent the pollen from falling into an open flower. Then, emasculated panicles are tagged and bagged. The male panicles are examined to locate the anthers on the glume apex. These matured anthers are then placed on female floret. The anthers need to be break to release the pollen onto stigma. Only mature pollen should be used. Finally, emasculated floret is bagged and left till the seed is mature. Another method is to utilize heated water (41-43.5Â°C) to emasculate florets. The florets will open followed by bagging two panicles together for pollination. The setback of hand emasculation is lower seed set (Baldi, 1967) whereas heat emasculation according to Jennings et al. (1979) was slower and had lower seed set than hand emasculation because culms were bent to introduce panicles in a thermos bottle and often got broken, and because florets open only for a short time, when pollen must be available. The benefits of heat emasculation are hot water immersion acts simultaneously upon the whole panicle: when temperature and panicle stage are appropriate, emasculation is 100% effective. In addition, emasculation on 45Â°C reduces the rate of self-pollination (Yoshida, 2008).
Chromosome involved in elongation and aromatic rice
The flavor or aromatic rice is associated with the presence of 2-acetyl-1-pyrroline. A recessive gene (Fgr) on chromosome 8 of rice has been linked to this important trait (Yoshihashi et al, 2002). To know the genome of rice, particularly the gene involved in important traits such as aromatic gene, some cytogenetic methods are applied such as mapping rice centromeric genes onto wheat aneuploid stocks, followed by RFLP analysis, alignments of nucleic acid sequence and database sequence for comparison, BAC library screening and finally BAC-FISH analysis (Lili Qi, 2009). Other method involves GISH or genomic in-situ hybridization, by applying direct visual method for distinguishing parental genomes and analyzing genome organization in interspecific hybrids, allopolyploid species and interspecific introgression lines (Jiang J, 1994).