Deepwater Rice rice grown in more than 50 cm water for one month or longer during the cropping season. Based on stature and depth of water, these are of two types: (i) traditional tall, and (ii) floating. Traditional tall cultivars are tall with long leaves, and grown at water depths between 50 and 100 cm; floating rice is grown in 100 cm or deeper situations. In Bangladesh most of the rice grown in the low lying areas during monsoon are floating rice, generally called as deepwater rice, locally known as broadcast aman, jolidhan, poushdhan, etc.
Deepwater rice grown in Asia including Bangladesh is the cultigen of Oryza sativa, probably evolved from perennial grass, Oryza rufipogon, via annual wild relative, Oryza nivara. Since Bangladesh is situated in the centre of the east-west belt where rice may originally have been domesticated in Asia, it is believed that Asian deepwater rice evolved in this belt. Deepwater rice grown in Africa is a different species, Oryza glaberrima.
Growth of deepwater rice cultivars largely depends upon factors such as, flooding regime, microrelief, soil type, etc. There are more than 2,000 deepwater rice cultivars in Bangladesh and more than 6,000 in Asia. Almost all the deepwater cultivars are strongly photoperiod sensitive. Photosensitivity fixes flowering time at a favourable point in the flooding period, enables the plant to escape the adverse effect, of low temperature in the reproductive phase, and usually ensures crop maturity as soon as floods have receded. The panicle is often waned, spreading type, and seeds are prone to shatter. Seeds have dormancy period lasting for several weeks.
Deepwater (floating) rice has three special adaptations: (i) ability to elongate with the rise of water levels; (ii) develop nodal tillers and roots from the upper nodes in the water; and (iii) the upward bending of the terminal part of the plant called 'kneeing' that keeps the reproductive parts above the water as the flood subsides. Deepwater rice grows under rainfed dry land conditions for 2-4 months before the onset of flood, when plant produces basal tillers. With inundation the plant becomes an emergent macrophyte and grows in an aquatic environment for the remaining 3-5 months of its life. Nodal roots absorb nitrogen, phosphorous and possibly other nutrients from floodwater. Stem elongation is stimulated by partial submergence; it results from cell division and elongation of cells in the intercalary meristem due to an interaction of the plant hormones, under the control of two complementary genes.
There is an increase in number of elongated internodes with the increase in water depths. Majority of deepwater rice cultivars in Bangladesh is of strong elongators. Stem may reach 5-6 m in very deepwater (3-4 m) situations. Deepwater rice does not normally survive at water depths greater than 4 m.
Until in late 1960s, deepwater rice was important crop in Bangladesh that occupied about 2.09 million ha (21% of the total rice area). The cropping area then shrunk to about 0.85 million ha because of the initiation of cultivation of high yielding varieties (HYV) under irrigation in deepwater rice fields during the dry season (boro). Consequently, about 1.24 million ha now remain fallow during the monsoon.
Deepwater rice is usually seeded dry in the field during March-April following the first monsoon shower. In some areas farmers establish deepwater rice by transplanting of seedlings following dry season rice. Until the onset of flooding in June/July, the crop depends on rain. Very little fertiliser is used and weeds are controlled by harrowing and hand weeding, twice before flood. Crop matures between mid-October and mid-December, depending on the degree of photoperiod sensitivity of the cultivar.
Depending upon the cultivars, the plant population usually reaches to the highest level of 200-400 stems per sq m at the maximum tillering stage in the preflood period. During flooding period some stems may be damaged by submergence and pest attack. Grain production tends to increase with the increase in biomass up to 12 m tons/ha (dry weight). The harvest index typically, varies from 0.12 to 0.16 and tends to increase in shallow water situation. Preflood plant stand densities are not adequately related to grain yield, but stand densities and yields are highly correlated from the late elongation stage. Heaviest panicles are borne on the main tillers followed by basal tillers, and lightest panicles by nodal tiller. Panicle density usually varies between 50 and 120 per sq m. Average grain yields are 2.3 m tons/ha and some cultivars have the potential of producing yields more than 3.0 m tons/ha. [Zahirul Islam]