Responses of three wheat [Triticun aestivum] genotypes to high soil temperature during grain filling

High temperatures limit wheat (Triticum aestivum L.) production in many areas around the world. Soil temperatures near the root zone could be as high as the air temperature during wheat grain filling. The objectives of this study were to investigate the effects of high soil temperature on grain yield and related traits of wheat genotypes and to examine their differential responses. Three genotypes, Imam, Fang and Siete Cerros were grown under three temperature conditions in the daytime during grain-filling period: (i) normal air temperature/normal soil temperature (26/26 deg C), (ii) normal air temperature/high soil temperature (26/38 deg C) and (iii) high air temperature/high soil temperature (38/38 deg C). The night temperature was 18/18 deg C in all treatments. Temperatures during the daytime were increased at a rate of 4 deg C hr sup(-1) from 18 deg C to the designated temperature, which was kept for at least 4 hr around noon. The 26/38 deg C and 38/38 deg C treatments significantly decreased the chlorophyll content (SPAD) of flag leaves, grain-filling duration, and increased carbohydrate remobilization or loss from the stem and the root, but with varying degrees among genotypes. Grain yield, biomass, grain weight, grains number spike sup(-1) and harvest index at the 38/38 deg C treatment were significantly lower than at the other two treatments. In Imam, the grain yield was lower at 26/38 deg C than at 26/26 deg C, while in Siete Cerros the grain yield, grain weight, grain number spike sup(-1) and harvest index were lower at the 38/38 deg C treatment than at the other two treatments. These results indicated that high temperature of soil alone (26/38 deg C) or high temperature of both air and soil (38/38 deg C) decreased the chlorophyll content and grain-filling duration, and increased carbohydrate remobilization. Genotypic differences in the responses to high soil temperature (26/38 deg C) and high air/soil temperature (38/38 deg C) were also observed.