Wheat Responses to a Wide Range of Temperatures: the Hot Serial Cereal Experiment.

In order to determine the likely effects of global warming on field-grown wheat (Triticum aestivum L.), a “Hot Serial Cereal” experiment was conducted -- so-called “Cereal” because wheat was the crop, “Serial” because the wheat was planted about every six weeks for two years, and “Hot” because infrared heaters were deployed on six of the planting dates in a T-FACE (temperature free-air controlled enhancement) system, which warmed the canopies of the Heated plots. Average degrees of warming for the infrared-heated plots were 1.3 and 2.7°C during day and night, respectively. Air temperatures ranged from -3 to +42°C, so the wheat was exposed to a very wide range of temperatures. When plotted against growing-degree-days computed from canopy temperatures, vegetation index curves from heated and control plots coalesced, which gives confidence that the infrared-heater treatment simulates natural warming. IR warming increased leaf-level net assimilation rate by 5% but had only a nominal effect on stomatal conductance. A regression-based analysis of simulations with the CSM-CROPSIM-CERES model showed that effects of T-FACE on phenology were similar to what would be expected from equivalent changes in air temperature. Warming had no effect on grain yield (average of 697 g m^-2) for normal winter planting dates.  However, for spring plantings when temperatures were above optimum,  warming decreased grain yield from 510 to 368 g m^-2.  Supplemental heating had the greatest effect in the early fall plantings when mid-season  frost limited the yield of unheated plots to only 3 g m^-2 whereas yield of heated plots was 435 g m^-2.  Thus, future increases in temperature may decrease wheat yield for late plantings and shift optimum planting windows to earlier dates in lower latitude areas of the world.