STRATEGIES FOR DEALING WITH MICRONUTRIENT DISORDERS AND SALINITY IN LOWLAND RICE SYSTEMS

In wetland rice, the most commonly observed micronutrient disorders ar e Zn deficiency and Fe toxicity. Salinity is the main obstacle to high yields in coastal areas in the humid tropics and in arid and semiarid areas where evaporation exceeds precipitation. To develop sustainable rice-production systems in areas where these stresses occur, cultivar tolerance is often essential although improved management also plays an important role. In the last 15 years, IRRI research has focused on germplasm improvement. Intensive screening and breeding have resulted in high tolerance to Zn deficiency in IRRT's late elite Lines; screeni ng methods are available, but tolerance mechanisms and genetics are no t yet fully understood. Recent work has shown the importance of root-i nduced changes in the rhizosphere for solubilizing Zn and increasing i ts plant uptake. Repeatable screening methods are not yet available fo r isolating genotypes with tolerance to Fe toxicity. Pot and field met hods earlier developed do not allow screening of sufficient numbers of rice lines for an effective breeding program. A greenhouse screening procedure using a 400 mg l(-1) iron solution is being tested. Genetic studies are required but may only be feasible when additional knowledg e of mechanisms is available. Prebreeding efforts for salinity have re sulted in adequate knowledge on mechanisms, genetics and reliable scre ening techniques. Salt-tolerant rice cultivars with targeted agronomic characteristics and yield potentials are now available. (C) 1998 Else vier Science B.V.