|Salinity Studies in Rice-96
Project Leader and Principal UC Investigators
Stephen R. Grattan - Extension Plant-water Relations Specialist Department of Land Air and Water Resources UC Davis
Steven C. Scardaci - University of California Cooperative Extension Farm Advisor Colusa/Yolo/Gletin counties
Stacey R. Roberts - Postgraduate Researcher, Department of Agronomy and Range Science, UC Davis
Rachel Hochman, Postgraduate Researcher, Department of Agronomy and Range Science, UC Davis
William Y. Thomas - Postgraduate Researcher, Department of Agronomy and Range Science, UC Davis
James E. Hill - Extension Agronomist and Chair, Department of Agronomy and Range Science, UC Davis
|Irrigation practices that have helped to curtail herbicide
runoff into the Sacramento River may be creating salinity problems for some California
rice growers. A survey conducted between 1993 and 1995 in Glenn and Colusa counties
produced "strong evidence" that suggests salinity is reducing rice yields in
several locations. While decreasing pesticide concentrations in tailwater runoff, new
irrigation practices - static flow and recirculating systems - are apparently increasing
the potential for field salinization by allowing salts to concentrate in lower basins.
Last year in Colusa County scientists working on this project conducted experiments with eight-foot aluminum rings - a procedure they expressed a high level of confidence in as appropriate for analysis of salinity tolerances in Sacramento Valley rice fields. Small plots like this are necessary because of the numerous measurements and intense management necessary, as well as the amount of salt required for the experiments. Because of a late seeding date, the very early variety M-103 was aerially seeded on May 28, 1996. Ten days later the rings were flooded with irrigation water containing varying levels of salinity. The plants were then observed at subsequent growth stages.
In general, salt-stressed seedlings were smaller, had fewer tillers, less root mass and had shorter, thinner, chlorotic leaves than non-stressed plants. Salt-stressed plants demonstrated an extended vegetative phase, which may explain delayed and reduced tillering. Salinity also delayed panicle initiation. At the highest levels, it took rice plants an additional 12 days to reach 50 percent heading compared to controls.
Salinity had profound effects on yield and yield components. Salinity decreased grain yield in a linear fashion, despite its having very little - or perhaps even a slightly positive - effect on straw yield. Salinity can reduce grain yield in various ways. As previously noted, stressed plants partition more of their energy into vegetative tissue than reproductive tissue. Second, salinity decreases the number of filled grains per panicle and increases the percentage of sterile florets. Third, salinity reduces individual grain size. Samples from salt-stressed plants in the study averaged just 16 grams, compared to 22 grams in control plots - a 25 percent reduction! Researchers point to these two factors - reduced grain size and increased sterile florets - as the primary cause for reduced grain yields.
Researchers found that even a small increase in salinity reduced grain yield. Further, individual grain size was reduced at salinity levels less than the current salinity threshold guidelines indicate for rice. Rice is known to be very sensitive to salinity. Current guidelines, based on limited research, indicate that crop yields can be reduced by 12 percent for every increase in unit of electrical conductivity above 3.0 deci-Siemens/meter (dS/m) in the field water. In these experiments, however soil salinity levels in excess of only 2.0 dS/m adversely affected yields and seedling establishment at even lower levels. Since climate and environmental factors can also affect salt tolerance, researchers stress the importance of additional field salinity experiments in the Sacramento Valley to modify the guidelines for this area. Additional experiments are planned for 1997 to verify first year findings. Similar results would strongly suggest salinity thresholds for rice grown in the Sacramento Valley need to be adjusted and future water policy decisions would need to take this into account.