Cardinal temperatures for wheat leaf appearance as assessed from varied sowing dates and infrared warming

Accurate data on crop responses to temperature are essential for predicting the potential impacts of climate extremes. Air temperature can be precisely regulated in controlled environment chambers, but chambers seldom provide realistic radiation, photoperiod, wind and humidity regimes, which raise concerns as to whether responses quantified in such environments accurately reflect field performance. Field experiments employing sowing date (SD) and artificial warming treatments can provide a wide range of temperature regimes under otherwise natural field conditions. We analyzed temperature effects on main stem leaf appearance for the spring wheat (Triticum aestivum L.) cultivar Yecora Rojo using 15 sowing dates at Maricopa, AZ, USA. Six dates included infrared-based temperature free-air controlled enhancement (T-FACE) warming treatments. Mean air temperatures over the 15 periods of measurement varied from 11.6 to 33.2°C. Our objective was to characterize the effect of temperature on leaf number, emphasizing air temperatures above 20°C, a value often cited optimal for wheat development. An underlying concern was how different shapes of temperature responses functions might affect estimates of cardinal temperatures. For comparisons among four segmented linear functions, a quadratic function and two forms of the beta function, the best fit to the data was for a two-segment function with a base temperature (T _base) of 1.9°C and an optimum (T _optl) of 22.2°C. In attempting to estimate a second, upper temperature for maximum development (T _optu), the estimation process failed. This likely reflected the low frequency of data from mean air temperatures over 25°C and possible severe stress responses at extreme low and high temperatures. The results further demonstrated the value of growing crops under a wide range of temperature regimes, which can be attained under field conditions through use of planting date and T-FACE treatments.