Responses of field-grown irrigated rice cultivars to varying levels of floodwater salinity in a semi-arid environment

Shallow saline water tables, naturally saline soils and variations in climatic conditions over the two growing seasons, create a harsh environment for irrigated rice production in the Senegal River Delta. At the onset of the growing season, salts accumulated by capillary rise in the topsoil are released into the soil solution and floodwater. Rice fields often lack drainage facilities, or drain from one field to the other, thus building up salt levels during the season. Salt stress may, therefore, occur throughout the growing season and may coincide with susceptible growth stages of the rice crop. The objectives of the present study were to (i) determine varietal responses to seasonal salinity in both the hot dry season (HDS) and the wet season (WS) and (ii) derive guidelines for surface water drainage at critical growth stages. We evaluated responses of three rice cultivars grown in the region to floodwater salinity (0-2, 3, 6, 8 mS cm(-1)), applied either at germination, during 2 weeks at crop establishment, during 2 weeks around panicle initiation (PI), or during 2 weeks around flowering. Floodwater electrical conductivity (EC) reduced germination rate for the most susceptible cultivar by as much as 50% and yield by 80% for the highest salinity level imposed. Salinity strongly reduced spikelet number per panicle, 1000 grain weight and increased sterility, regardless of season and development stage. The strongest salinity effects on yield were observed around PI, whereas plants recovered best from stress at seedling stage. Floodwater EC <2 mS cm(-1) hardly affected rice yield. For floodwater EC levels >2 mS cm(-1), a yield loss of up to 1 t ha(-1) per unit EC (mS cm(-1)) was observed for salinity stress around PI (at fresh water yields of about 8 t ha(-1). Use of a salinity tolerant cultivar reduced maximum yield losses to about 0.6 t ha(-1) per unit EC. It is concluded that use of salinity tolerant cultivars, drainage if floodwater EC >2 mS cm(-1) at critical growth stages, and early sowing in the WS to avoid periods of low air humidity during the crop cycle, are ways to increase rice productivity in the Senegal River Delta. (C) 2001 Elsevier Science B.V. All rights reserved.