FOR more than half of humanity, rice is the staple diet and is one of the most important cereals in the world. It also occupies an important position in the agro-based economy of Pakistan. Besides meeting the dietary requirements of the people, rice has emerged as a major export commodity contributing about 13 per cent to the total valuable foreign exchange earnings. Rice accounts for 6.6 per cent value-added in agriculture and 1.6 per cent in GDP in the country. It is grown on an area of 2,377 thousand hectares with the average yield of 2,021 kg ha-1.
Food security in the world is challenged by increasing food demand and threatened by declining water availability. More than 75 per cent of the rice supply comes from 79 million ha of irrigated land. Thus, present and future food security depends largely on the irrigated rice production systems.
However, the water-use efficiency of rice is low, and growing rice requires large amount of water. In Asia, irrigated agriculture accounts for 90 per cent of total diverted freshwater, and more than 50 per cent of this is required to irrigate rice. Until recently, this amount of water has been taken for granted, but now the global water scarcity is threatening the sustainability of irrigated rice production. The reasons for this are diverse and location-specific, but include decreasing quality (chemical pollution, salinization) and resources (e.g. falling groundwater tables, silting of reservoirs), and increased competition from other sectors such as urban and industrial users.
The costs of its use and resource development are increasing as well. Therefore, farmers and researchers alike are looking on one hand for ways to decrease water use in rice production and on the other to increase its use efficiency.
A fundamental approach is to start at the field level, where water and rice interact. For farmers with no control over the availability or distribution of water beyond their farm gates, the crucial question to be addressed is ?what are the options to cope with decreasing water supply (or the increasing costs of it) at the farm or field inlets??
Such an approach to reduce water inputs in rice is to grow the crop like an irrigated upland crop such as wheat or maize. Upland crops are grown in non-puddled aerobic soil without standing water. Irrigation is applied to bring the soil water content in the root zone up to field capacity after it has reached a certain lower threshold (e.g. halfway between field capacity and wilting point). The amount of irrigation water should match evaporation from the soil and transpiration by the crop.
The increasing water scarcity, which threatens the sustainability of lowland rice production, has lead to a new way of cultivating rice that requires less water than lowland rice. It entails the growing of rice in aerobic soil, with the use of external inputs such as supplementary irrigation and fertilizers, and aiming at high yields. Yields under aerobic conditions were 2.4-4.4 t ha-1, which were 14-40 per cent lower than under flooded conditions.
The total water input from transplanting to harvest was 650-830 mm under aerobic conditions and about 1,350 mm under flooded conditions. Because water use decreased relatively more than yield, water productivity under aerobic cultivation increased by 20-40 per cent (in one case even 80%) over that under flooded conditions. Rice transplanting requires a large amount of labour, usually at a critical time for labour availability, which often results in shortage and increasing labour costs. In addition, under the changing socio-economic environment in Pakistan, workers are not available or reluctant to undertake tedious operations like transplanting.
These situations further escalate labour costs. Alternate methods of establishing crops, especially rice, that require less labour and water without sacrificing productivity are needed. Considering water availability and opportunity cost of labour, dry seeding of rice is an appropriate alternative for traditional transplanting method. Methane emission from the puddle field is also a threat to environment. The concentration of atmospheric methane has increased from about 0.74-1.73 ?mol mol-1 during the last 150 years. Rice cultivation is the largest anthropogenic source of CH4 efflux. Studies have shown wide differences (about 1 to 60 mg m-2 h-1) in CH4 efflux densities from rice cultivation through the world. This primarily is attributed to differences in rice culture in Europe and America (dry) and China (flooded). Methane emission from Chinese rice fields have been reported to be 4 to 10 times from European and American fields. There is also a linear relationship between CH4 efflux and density of wetland systems.
Sub-optimum plant population and uneven crop stand resulting from poor nursery seedlings is one of the most important yield limiting factors in the traditional rice production system. Farmers in Pakistan have been practicing traditional seed soaking for rice nursery sowing since decades that result in poor and delayed germination. Nursery seedlings thus raised can be transplanted when they are 40-45 days old.
While, 30 days old seedlings are considered ideal for transplanting. Older seedling results in lower tilling capacity thus reducing the final yield. Direct seeding (aerobic rice) is an attractive alternative to the traditional production system but poor germination and uneven crop stand and high weed infestation are among the main constraints to its adoption. Selective herbicides are now available for effective weed control but germination problem is still unsolved.
Improved seed invigoration techniques are being used to reduce the germination time, to get synchronized germination, improve germination rate, and better seedling stand in many horticultural and field crops like wheat, maize and more recently rice. These treatments can also be employed for earlier and better nursery stand establishment, which will result in the improved performance of traditional rice production system. Seed hardening also called wetting and drying or hydration-dehydration, is performed by repeated soaking and drying of seeds in water. The beneficial effect of seed hardening is primarily due to pre-enlargement of the embryo, biochemical changes like enzyme activation, and improvement of germination rate particularly in old seeds. Different priming tools have also successfully been integrated. Seeds are hardened always in tap or distilled water; furthermore, seed priming is performed in single cycle of wetting and drying. Most recently, the department of Crop Physiology, University of Agriculture, Faisalabad has introduced a new technique for rice seed invigoration in which both seed hardening and osmoconditioning have been successfully integrated. This soaking is named as osmohardening. Seeds of both coarse and fine rice were hardened in various salt solutions instead of tap or distilled water. Osmohardening in CaCl2 (having osmotic potential -1.25 M Pa) solution was the best for vigor enhancement compared with other salts.
After a series of experiments, it was concluded that pre-sowing seeds treatments can not only improve the nursery seedlings and performance of improved nursery seedlings but may also enhance the performance of direct seeded rice in both coarse and fine rice. Osmohardening with CaCl2 in fine rice and osmohardening with KCl in coarse rice were the most effective priming techniques. Growth, yield and quality of the transplanted and direct seeded rice were also improved owing to seed priming. Priming resulted in improved root proliferation that resulted in improved nitrogen uptake, and enhanced (-amylase activity that increased starch hydrolysis, which resulted in increased contents of total and reducing sugars. These might be the basis for improved performance of primed seeds.