Rice.   Rice (Oryza sativa) belongs to the genus Oryza. There are 18 valid wild species distributed mainly in Asia, Africa and America. Of the two cultivated species Oryza sativa and Oryza glaberrima, the former is cosmopolitan and the latter is confined in Africa.

Excavations from NonNok Tha in Thailand had yielded carbonized rice glumes, probably dating back to 3,500 B.C. or earlier. The carbonized grains obtained in India could be dated around 2,300 B.C. Samples from Lothal (extension of the Harrappan civilization) excavations in India reveal the use of paddy husk in potteries, bricks, etc.

The Asian rice (O. sativa) evolved from the ancestral wild progenitor over a broad region stretching from the Gangetic plains below the Himalayan foothills across north-east India, upper Burma, northern Thailand, Laos, Vietnam and south China. It is generally felt that the domestication had occurred independently, and India is one of the oldest regions where domestication began. There is mention of nivara, a wild form of rice, in Vedic literature. 'Atharva Veda' mentions vrihi, probably meaning wild rice.

The extension of rice culture, extensive contacts between the people and selection pressure, both natural and artificial had led to the differentiation of races. Two major sub-species or races, i.e. indica and japonica, are recognized. All the rice varieties of India belong to the former group, though types resembling japonicas or with those of japonica type grains have also been reported from India and Nepal.

The haploid number of chromosomes in is 12 and most varieties have 2n=24 chromosomes. Of the wild species, eight are tetraploids and the rest are diploids. Cultivated forms are diploid. Rice is considered a secondary and balanced polyploid.

The rice grain is caryopsis. The kernel is enclosed by the husk or hull or lemma and palea. The outer layer of the kernel is the pericarp which may be white or colored. The pericarp consists of the epicarp, the mesocarp, and the cross layer. Attached to the pericarp and on its inner side is thin layer of testa and below it is the aleurone layer. The bulk of the kernel is constituted by the endosperm which may be glutinous or starchy. The embryo situated at the lower end of the ventral side of the kernel consists of the radicle, the plummule, the epiblast and the scutellum.

The endosperm is composed mostly of starch. The protein is concentrated in the aleurone layer and in the scutellum. The protein content is around 7 to 8 percent. In some varieties, protein bodies are also found distributed in the endosperm cells.

Seventeen amino acids are normally present. The amino acid composition of the proteins determines the nutritional value. However, the biological value of protein depends on the ratio of eight essential amino acids to the total amino acids.

Rice cultivation in the world extends from 39 degrees S latitude (Australia) to 45 degrees N latitude (Japan) and 50 degrees N latitude (China) in India, it stretches from 8 degrees N latitude to 34 degrees N latitude. Rice is also grown even in areas below the sea-level, as in Kuttanad region of Kerala. It is also grown well at altitudes above 1,979 m, as in parts of Jammu and Kashmir. It is cultivated as a purely rain-fed upland crop in West Bengal, Bihar, Uttar Pradesh, etc., where the monsoon is precarious and its distribution is often erratic. On the other hand, in parts of West Bengal and Bihar, shallow and deep water (4.75-6 m) rices are also grown.

Equally elastic is its duration. The earliest varieties are harvested in 85 to 90 days. In contrast, there are rices that take 240 days to mature. Rice is considered a short day plant. Varieties may be sensitive or insensitive to day length, temperature or to both.

Rice shows considerable variability in grain size from 9 to 14 mm. It may be round, oval or elongated. The color of the hull varies from straw to dark purple, brown or black, with various intergrades. Rice is normally white, but red and purple rices are grown in the country. The color is due to either anthocyanin or non-anthocyanin. Variability is also considerable in other morphological characters, such as the root-system, plant height, tillering ability, leaf characteristics, panicle features and in physiological attributes, such as photosynthetic ability, degree of sensitivity to day length and temperature and the translocation potential.

There are two main seasons for growing rice in India, though three crops are taken in Tamil Nadu and Kerala. The kharif season is characterized by a gradual fall in temperature, more numerous cloudy days, low light intensity, a gradual shortening of the photoperiod, high relative humidity and cyclonic weather. During rabi, there is a gradual rise in temperature, bright sunshine, near-absence of cloudy days, a gradual lengthening of photoperiod, and a lower relative humidity.

Rice is essentially a short day plant. A combination of temperatures, photoperiod and light intensity, however, determines the growth period, crop performance and productivity. Yields, in general, are higher in rabi than in kharif.

Rice occupies an area of 38 million hectares in the country. Included in this total is seven million hectares whose soil is either saline, alkaline, saline-alkaline or flood-prone. Only 25 per cent of the rice acreage in the country has assured irrigation. Nearly 55 per cent of the area is ill-drained or water-logged. The rest of the area constitute the rain-fed uplands where the rainfall is marginal to moderate and its distribution is sometimes erratic. The area, production and yield per hectare in different states (1976-77) in the country are given in Table 2.

Rice cultivation is conditioned by temperature parameters at the different phases of growth. The critical mean temperature for flowering and fertilization ranges from 16 degrees C to 20 degrees C, whereas during ripening, the range is 28 degrees C to 32 degrees C. Temperatures beyond 35 degrees C affect not only pollen-shedding, but also grain-filling. Broadly speaking, vigorous vegetative growth, moderately high temperatures are required, whereas for panicle initiation, slightly lower temperatures (20 degrees C to 22 degrees C) would be ideal. High temperatures and high light intensity adversely affect grain-filling.

The major soil groups where rice is grown are: (i) riverine alluvium, (ii) red-yellow, (iii) red loamy, (iv) hill and submontane, (v) terai, (vi) laterite, (vii) coastal alluvium, (viii) red sandy, (ix) mixed red and black, and (x) medium and shallow black.

The country has been classified in to eight agro-climate zones. These are:

1. The humid western Himalayan region, which comprises of Jammu and Kashmir, Himachal Pradesh and Kmaon and the Garhwal Division of Uttar Pradesh - The major soil groups are submontane soils, hill and terai soils.

2. The humid Bengal-Assam basin which includes West Bengal and Assam - Riverine alluvium, terai soils, lateritic soils, red-yellow loams and red sandy soils.

3. The humid eastern Himalayan region and the Bay Islands, which include Arunchal Pradesh, Nagaland, Manipur, Mizoram, Tripura, Meghalaya and the Andaman and Nicobar Islands - Red loamy soils, lateritic soils, red-yellow soils, and alluvial soils.

4. The sub-humid Sutlez-Ganga plains, which include Punjab, Uttar Pradesh, Bihar and the Union Territory of Delhi - Calcareous alluvial soils, riverine alluvium, saline and alkaline soils, red-yellow loams, mixed red and black soils and red sandy soils.

5. The sub-humid to humid eastern and south-eastern uplands which include Orissa, Andhra Pradesh and eastern Madhya Pradesh - Lateritic soils, red-yellow loams, mixed red and black soils, deltaic alluvium, deep and medium deep black soils, red loamy soils and coastal alluvium.

6. The arid western plains, which include Haryana, Rajasthan and the Union Territory of Dadra and Nagar Haveli - Alluvial soils, red-yellow soils and medium to deep black soils.

7. The semi-arid Lava Plateaux and Central Highlands, which include Maharashtra, western and central Madhya Pradesh and the Union Territory of Goa, Daman and Diu - Riverine alluvium, coastal alluvium, mixed red and black soils and red sandy soils.

8. The humid to semi-arid Western Ghats and the Karnataka Plateaux, which include Karnataka, Tamil Nadu, Kerala, the Union Territory of Pondicherry and the Lakshadweep Islands - Lateritic soils, red sandy soils, deltaic coastal alluvium and red loamy soils.

The extreme soils conditions in which rice is grown are represented by acid peaty soils of Kerala (pH 3) and highly alkaline soils occuring in Punjab, Haryana and Uttar Pradesh (pH 10). For normal crop growth without any amendment, the pH range of 5 to 8.5 is suitable.

Soil salinity affects the yield of many crops above 4 mmhos/cm conductivity, but rice, which is known to be a fairly tolerant crop, can grow normally in soils up to 6 mmhos/cm conductivity. The tolerance to the alkalinity is governed by the salinity of the soil, and of irrigation water, but generally exchangeable sodium percentage greater than 15 may adversely affect the crop growth.

RICE-CULTURE.   The systems of rice cultivation in various rice growing areas of the country are largely dependent upon the rice growing conditions prevalent in the respective regions. The principal systems followed in India are 'dry', 'semi-dry' and the 'wet'.