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Oryza sativa L.

Oryza sativa L.

Family

Gramineae

Synonyms

Oryza glutinosa Lour., Oryza montana Lour., Oryza praecox Lour., Oryza aristata Blanco.

Vernacular Names

Malaysia Padi.
English Rice.
Indonesia Padi (General), pari (Javanese), pare (Sundanese).
Papua New Guinea Rais.
Philippines Palay.
Burma (Myanmar) Sabar-bin.
Cambodia Srö:w.
Laos Khauz.
Thailand Khao.
Vietnam l[us]a.
French Riz.

There are specific for the rice grain, unhulled grain, polished rice, cooked rice (also depending on how it is cooked), left-over rice and even rice stuck to the bottom of the pot. The crop and the unhusked grain are known as a paddy.

Geographical Distributions

Oryza sativa evolved along the foothills of the Himalayas and was probably first cultivated in ancient India. It has been cultivated for about 9000 years. In Indonesia, Malaysia and the Philippines cultivation of O. sativa began some time after 1500 BC. The earliest evidence of cultivated O. sativa is found in the Ban Chian excavation in Thailand, dating cultivated rice at 500-900 BC. O. sativa is planted throughout the humid tropics and in many subtropical and temperate areas with a frost-free period longer than 130 days.

Description

O. sativa is an annual grass that can grow up to 50-130 cm tall and measures up to 5 m long in deep-water rice that forms small tufts. The roots are fibrous and arise from the base of the shoots. The stem (culm) is erect to ascending, smooth, composed of a series of nodes and internodes while the number depends on cultivar and growing season. Each node is with a single leaf, and sometimes also with a tiller or adventitious roots. The internodes are usually short at the base of plant and progressively increasing towards the top.

The leaves are in two ranks. The sheaths are initially enclosing each other, form a pseudostem and enclose the internodes later. The ligule is triangular to linear-lance-shaped, measures 1-1.5 cm long and often split. The auricles are often present, falcate, measure 1-5 mm long and hairy. The blade is linear, measuring 24-60 cm x 0.6-2.2 cm, hairless, smooth to scabrous and often with spiny hairs on the margin.

The inflorescence is a terminal panicle, measures 9-40 cm long and with 50-500 spikelets that depend on the cultivar. The spikelets are single, borne on a short pedicel, oblong to lance-shaped, measure 7-11 mm long and about 2-3 times longer than wide. They contain a single bisexual flower, with 2 small glumes, a large boat-shaped lemma measures 6-10 mm long that sometimes with an awn measuring up to 15 cm long, and likewise palea with a very short awn, 6 stamens, with a broad ovary, and 2 plumose stigmas.

The fruit (caryopsis, grain) varies in size, shape and colour, ovoid, ellipsoid or cylindrical, measuring 5-7.5 mm x 2-3.5 mm and often whitish-yellow or brown to fuscous.

Ecology / Cultivation

O. sativa is grown as far north as 53°N in Moho, northern China and as far south as 35°S in New South Wales, Australia. It grows on dry or flooded soil. Tolerant cultivars can withstand varying degrees of low temperature. Traditional cultivars are generally photoperiod sensitive, and flower when daylengths are short (critical daylength of 12.5-14 hours). Many modern cultivars are photoperiod insensitive and flower at any latitude, provided temperature is not limiting. O. sativa yields higher when solar radiation during the reproductive and ripening phases is high, so that generally the grain yields are higher during the dry season           than during the wet season. Low temperature limits the range of the rice crop. The average temperature during the growing season varies from 20-38°C. O. sativa is most susceptible to low temperature at the stage of panicle initiation, when temperatures below 15°C at night can cause spikelet sterility. Low temperature can also result in poor germination or death of seedlings, yellowing of leaves, low tiller number, degeneration of spikelets, high sterility, stunting, and poor panicle exsertion causing low grain yields. Low soil and floodwater temperatures also affect the nutrition, growth and grain yield of rice. Temperatures above 21°C at flowering are needed for anthesis and pollination.

The chief factor limiting the growth of O. sativa is the water supply. However, the water regime in which O. sativa is growing and the water requirements are variable. Upland Oryza, grown as a rainfed crop, requires an assured rainfall of at least 750 mm over a period of 3-4 months and does not tolerate desiccation. Lowland Oryza tends to be concentrated in flat lowlands, river basins and deltas. In Southeast Asia countries, the average water requirement for irrigated rice is 1200 mm per crop or 200 mm of rainfall per month. Relative humidity within the crop canopy is high, since there is standing water in most Oryza crops. A low relative humidity above the canopy during the dry season aggravated by strong winds can cause spikelet sterility.

O. sativa is generally grown at sea level but also in mountainous areas of Southeast Asia countries. Cold-tolerant cultivars are grown up to 1230 m altitude in the Mountain Province of the Philippines and up to 2300 m altitude in the northwestern Himalayas. No direct effect of altitude is evident. Cold-tolerant cultivars do not differ morphologically from other cultivars. However, such cultivars can withstand 12°C water temperature at seedling stage, 15-17°C night temperature during panicle initiation and 21°C day temperature during anthesis. O. sativa does best in fertile heavy soils. It can be planted in dry soil or puddled soil and grown like an upland crop, or in inundated soils. The soils on which Oryza grows vary greatly. The texture ranges from sand to clay, organic matter content from 1-50%, pH from 3-10, salt content from almost 0-1%, and nutrient availability from acute deficiencies to surplus. The optimum pH for flooded soil is 6.5-7.0. Because land management depends on the soil, climate, water supply, and socio-economic conditions of the area, there is a considerable range in the pedogenetic and morphological characteristics of rice-growing soils. O. sativa is grown primarily in submerged soil, and the physical properties of the soil are relatively unimportant as long as sufficient water is available. Pore spaces are important physical properties as they influence the retention and movement of water and air. The soil pH before and after lowland fields have been flooded is an important determinant of soil fertility and the management of rice soils. In submerged soil, the pH tends to be neutral, i.e. the pH of acid soils increases whereas the pH of calcareous and sodic soils decreases; ions of Fe, N and S are reduced; the supply and availability of the elements N, P, Si and Mo is improved, whereas the concentration of water-soluble Zn and Cu decreases; toxic reduction products such as methane, organic acids and hydrogen sulphide are formed. The chemical composition of the soil varies among regions, countries, and areas.

The flooding of rice soils creates a favourable environment for anaerobic microbes and the accompanying biochemical changes. As a result, the decomposition rate of organic matter decreases. However, a thin surface layer generally remains oxidized and sustains aerobic microbes. The main biochemical processes in flooded soil are a series of successive oxidation-reduction reactions mediated by different types of bacteria. Nitrogen fixation takes place in paddy soils by Azotobacter and blue-green algae. The lowland rice and deep-water rice may be subjected to drought or complete submergence. There is varietal tolerance of such adverse conditions.

Line Drawing / Photograph

Oryza_sativa

References

  1. Plant Resources of South-East Asia No. 10: Cereals.

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