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Role of Deepwater Rice (Oryza sativa) to Maintain Aquatic Environment in Deeply Flooded WetlandsDr. Q. R. Islam and Dr. Sara L. Bennett, Beneficiary Participation & Project Management Technical Assistance Project, Small Scale Water Resources Development Sector Project, Local Government & Engineering Department, Dhaka, Bangladesh
This paper was originally published in: Proceedings, 10th European Weed Research Society Symposium on Aquatic Weeds 1998, Lisbon. NB: this online version is for general information only. Published version is authoritative.
SummaryReplacement of deepwater rice (DWR) by modern rice in wetlands seriously impedes plant diversity and conservation in Bangladesh. Many DWR varieties have been lost and the remainders are in danger of extinction. Modern rice production has increased chemical fertilizers and pesticide uses. Due to continuous submergence, deepwater habitat is characterized by anaerobic conditions which inhibit normal plant growth. The DWR plants are adapted to withstand these extreme conditions. They survive deep flooding with rapid elongation. The plant elongation involves production of internodes on culm from early growth stage and elongation of the internodes with gradual rise of water level. The present study suggests that the elongation capacity can be evaluated by culm length, number of elongated internodes and plant age when elongation can start. DWR provides adequate supply of easily digestible protein with high biological value. The DWR fields provide shelter for fish and their food, refuge for wildlife and base of food chain. The aquatic environment prevailing in wetlands has great potential for extensive cultivation of DWR. Selection of DWR varieties according to the elongation ability and their cultivation would be especially beneficial to meet the continuously expanding needs of food production, conserve plant diversity, and stabilize traditional ecosystem in wetlands.
IntroductionThe deeply flooded wetlands cover 2.7 million hectares in Bangladesh (MPO 1987). These lands receive water from the east, west, and north through major rivers which flow from the Tripura, Assam and Meghalaya Hills in India. Depth of water ranges from 1 m to more than 5 m during the monsoon season (Brammer et al., 1988). The major physical properties of the natural environment in these wetlands include indigenous floating or deepwater rice (DWR) varieties (Oryza sativa L.), wild fish species, natural plant communities of submerged and rooted floating species, swamp forest trees, waterfowl and other wildlife (NEMREP, 199; Zaman, 1994).
In recent years, there is a major difference in wetland environment. Increase in food demand with growing population changes crop production system. The spread of fertilizer responsive modern rice varieties in early 1960s has replaced DWR in more than a million ha during the past three decades (BBS, 1976; BBS 1996). Many native varieties have been lost. The modern rice production with irrigation in winter season has introduced chemical fertilizers and pesticide use. Capacity to elongate is the most important morphological feature of DWR plants. This specialised character enable the plants to adapt in deeply flooded conditions. The present study examines the relationship between morphological characteristic and elongation ability of DWR plant and its role in the management of natural resources in wetlands.
Material and methods Seeds of six DWR varieties were collected from the field during the harvest period in December 1992. The varieties were three each from the flooded depressions known as Shanir haor (Naogaon Village) and Kawadighi haor (Fatehpur village) at the foot of the Mehgalaya Hills and Tripura Hills in India, respectively. Both the haors are in the central part of the Northeast region of Bangladesh, between latitudes 24° 27¢ and 24° 44¢ North and Longitudes 90° 52¢ and 91° 40¢ East. The major differences between the two depressions are that the flooding is earlier and deeper in Shanir haor than in Kawadighi haor (MPO, 1991; NERP, 1996). After the collection of seeds, the number of elongated internodes on the main culm or stem of the plant and the length of the main culm were recorded at field.
In May 1993, the seeds were grown to determine the plant age at which internode elongation can start under submerged condition. For each variety five germinated seeds were sown in a pot containing 4 kg of clay soil. The pots were watered every day. When the plants reached second to third leaf emerging stage, the pots were transferred to a water tank and the plants were submerged with water. The water level was raised everyday up to around the base of the highest leaf blade of most plants. An internode longer than 1 cm was considered to be elongated internoded (Hosoda & Iwasaki, 1960). In the present study, the shortest elongated internode was found to be 5.7 cm.The plant age at which the plant can start to elongate was determined by the lowest position of two leaves between which the first elongated internode was identified. For example, when an elongated internode was found between 6 th and 7 th leaves, the 6 th leaf was used to express the plant age. Only main culms were investigated in the present study. Data on the protein content of grain and protein quality of 491 Bangladesh DWR varieties were taken from a previous study at the Laboratory of Applied Plant Physiology, Kyushu University Japan (Islam, 1990).
Result A wide variation was observed in the main culm length of DWR plants. The length ranged from 142 to 359 cm in the varieties examined in the present study. Plant with longer culm length was observed in Shanir haor where flooding depth is higher and occurs earlier than in Kawadighi haor (Table 1). The culm length was found to be depends on the total number of elongated internodes on the main culm. The number varied from 10 to 20. Variety with highest number of internodes was observed in Shanir haor. The earliest plant age at which DWR plant can start to elongate was 6 th leaf stage. It was found in the variety which was collected from Shanir haor. Variety which starts to elongate at 8 th and 9 th leaf stage was found in both areas. However, the plant elongation appeared to start slightly delay in Kawadighi haor. Longer culm length, higher number of elongated internodes and internode elongation at early plant stage show that DWR plants from Shanir haor have higher elonagation ability.
Table 2 shows that a strong relationship exists between the morphological
characteristics of DWR plants. The culm length is positively correlated
with total number of elongated internodes. The correlation is negative
between plant age at which elongation can start and culm length and total
number of internodes. This shows that the plant with ability of producing
more elongated internodes and longer culm length initiate internode elongation
at very early growth stage.
Grain protein content in 491 DWR varieties ranged from 3.8 to 11.5% with a mean of 7.0%. The coefficient variation was 0.154. Analysis of the polypeptide composition of grain protein showed that alcohol soluble protein body (PB) I is low in DWR 53.2% varieties. The PB I is not easily digestible compared to acid soluble PB II ( Tanaka et al., 1975; Kumamaru et al., 1988).
Discussion DWR grows in the wet season on the rich alluvial soils around margins of flooded bowl-shaped depressions (known as beels) between the natural levees of rivers or in abandoned river channel (Zaman, 1994). The beels vary in size from few hectares to many thousands of hectares (NEMREP, 1993). In many cases two or more neighbouring beels link up to form larger water bodies known as haors. They are subject to early floods and a rapid rise in flood-levels. More than 600 DWR varieties were grown in these wetlands before the introduction of irrigated modern rice (Zaman, 1975). The number has decreased significantly. The DWR growing area has declined from 2.01 million ha in late 1960s to 0.8 million ha in early1990s (BBS, 1976 & 1996). On the other hand, the modern rice production has introduced fertilizers use by 280 kg/ha and pesticide use by 1.85 kg/ha (DAE, 1992).
The DWR seeds are sown after first raining in March/April. Seedlings are often damage by early flooding and rapid rise of floodwater. Thus, DWR varieties with the elongation ability at an early growth stage are important to farmers. The DWR plant flowers with the advent of shortened day length in the late monsoon season (October) when floodwater begins to recede from the cropped land. The growth duration of the rice is 250 to 270 days. The plant age at which internode elongation starts and the rate of internode elogation are the most important characteristic for the survival of DWR plant. The present study suggests a strong relationship between flooding depth and the plant height, number of elongated internode on main culm and plant age when plant elongation can start. These morphologcal characteristics can be best indicators to determine elongation ability of deepwater rice plants. The DWR yields 1.50 to 2.13 ton/ha which is about 35 to 60% lower than irrigated modern varieties (BBS 1994 and NERP 1996 ). However, the grain protein content and quality of DWR is superior to non-floating rice and root and tuber crops (Islam, 1990 & 1996). This suggests that DWR production can directly enhance protein supplies to low-income people in wetland areas.
Historically, wetlands in Bangladesh provides a well-protected sanctuary for various species of fish, indigenous rice and a wealth of biodiversity. Currently these support major rice growing industry, subsistence and commercial fisheries, and abundant natural aquatic vegetation which provides a source of food, fuel and fertiliser for local people (Karim, 1993; Islam 1996; NERP 1996). In these wetlands, DWR can provide shelter for fish and their food in the form of periphyton on submerged stems and leaves, refuge for wildlife, overwintering refuges for the larger mother fish, fish spawning and base of food chain through decomposition of plant material. Selection of DWR varieties according to the elongation ability and their cultivation in deeply flooded area would be especially beneficial to maintain aquatic environments in wetlands and prevent loss of genetic base. This can protect the remaining DWR varieties from the danger of extinction and maintain and conserve plant diversity.
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