HEAT STRESS Effects of Night Temperature, Spikelet Position and Salicylic Acid on Yield and Yield-Related Parameters of Rice (Oryza sativa L.) Plants A. R. Mohammed & L. Tarpley Texas AgriLife Research and Extension Center, Beaumont, TX, USA Introduction Rice (Oryza sativa L.), the world’s most important staple food crop, is cultivated under a wide range of environ- ments (Khush 2005). Throughout these environments, rice is experiencing increases in night temperature that could lead to decreased rice yields, thereby decreasing world rice production. Global circulation models project that the global temperature will increase by 1.4–5.8 °C because of projected increases in the concentrations of all greenhouse gases by the end of the 21st century (Houghton et al. 2001). Much of this increase in average global temperature is due to a projected increase in night temperature as a result of less radiant heat loss due to increased cloudiness (Alward et al. 1999). Most of the studies on crop growth and grain yield are based on daily mean air temperature, which assumes no difference in the influence of day vs. night temperature (Peng et al. 2004). Year-to-year variation in rice grain yield has been attrib- uted to night temperature; high night temperatures (HNT) are considered a major constraint to rice produc- tivity (Peng et al. 2004). Although the physiological effects of high day temperatures are well understood (Ziska et al. 1996), the effects of HNT associated with global warming on rice production are poorly understood (Peng et al. 2004). Productive tillers (per unit ground area), total number of grains per panicle, spikelet fertility (SF) and grain weight are important components of yield (Sheehy et al. 2001) that are affected by the cultivation system and by environmental factors among which temperature is considered to be agronomically important (Singla et al. 1997). Production of tillers is sensitive to temperature (Mitchell 1953) and tiller production in rice is an impor- tant agronomical trait (Li et al. 2003). Furthermore, tiller number per unit area in the small grains is positively Keywords high night temperature; rice; rice grain characteristics; salicylic acid; spikelet fertility; spikelet position Correspondence A. R. Mohammed Texas AgriLife Research and Extension Center 1509 Aggie Dr. Beaumont, TX 77713 USA Tel.: +1 4097522741 x 2430 Fax: +1 4097525560 Email: abdulrazack@neo.tamu.edu Accepted June 29, 2010 doi:10.1111/j.1439-037X.2010.00439.x Abstract Periodic episodes of heat stress and seasonally high night temperatures (HNT) are predicted to occur more frequently in the current changing weather environment. These events affect aspects of crop growth and development, including oxidative-stress damage, reducing crop yield and quality. Salicylic acid (SA), a naturally occurring phenolic compound, associated with thermo- tolerance, prevents oxidative damage by detoxifying superoxide radicals and altering antioxidant capacity in plants. A study was conducted to determine the effects of HNT and SA on multiple rice yield-determining parameters with special emphasis on effects of HNT and SA at different spikelet positions in the panicle. Plants were grown under ambient night temperature (27 °C) or HNT (32 °C) in the greenhouse. They were subjected to HNT through use of contin- uously controlled infrared heaters, starting from 2000 h to 0600 h. The HNT did not affect productive tillers, main-stem panicle length or number of primary branches per panicle; however, reduced yield resulted from significant negative effects on spikelet fertility (SF), grain length and width. The grains located at the base of the panicle showed decreased fertility, length and width. Application of SA increased antioxidant capacity, thereby preventing damage to membranes, hence increasing yield by predominantly affecting SF in rice plants. J. Agronomy & Crop Science (2011) ISSN 0931-2250 40 ª 2010 Blackwell Verlag GmbH, 197 (2011) 40–49