Wheat spike temperatures in relation to varying environmental conditions

Most field studies investigating the effect of temperature on growth proces ses use temperatures recorded within a Stevenson screen. These are likely t o deviate from temperatures within the plant. This investigation reports a comparative study of methodologies and applications for measuring temperatu res in the field during grain development by comparing Stevenson screen, am bient (air temperatures within the crop canopy), and wheat spike temperatur es. Miniature sensors were inserted into wheat spikelets located midway on the spike of a primary tiller at anthesis. Located also within the crop can opy, and at the same height as the spike sensors, were sensors to measure a mbient temperatures. Stevenson screen temperatures were also recorded at the site. Temperatures were recorded automatically every 12 min during grain filling from anthesis to maturity. Plants were grown in dryland and irrigated conditions within the same location, with the aim of determining differences in plant tempera tures between stressed and non-stressed plants. Stevenson screen temperatur es did not relate closely to ambient or spike temperatures. Plants growing in adequate soil moisture conditions had spike temperatures lower than ambi ent temperatures, but in some dryland trials, where soil moisture was limit ing, spike temperatures equalled ambient temperatures, indicating that the plants were under moisture stress. Temperature differences of up to 5 degre es C were observed between the spikes of irrigated and nonirrigated crops o n a hot day. Neither ambient nor screen temperatures gave an accurate measu rement of spike temperature on hot days. Spike temperature differences betw een 2 cultivars, awned and awnless, were investigated. Trends were not cons istent over both years; however, in 3 of the 4 environments, the maximum sp ike temperatures were higher for the awned cultivar (Hartog) than the awnle ss cultivar (Halberd). On very hot days, when ambient temperatures exceeded 40 degrees C, spikes of Hartog were cooler than those of Halberd.