Individual and combined effects of jointing and booting low-temperature stress on wheat yield

Low temperature greatly affects wheat production, while low-temperature pretreatments may enhance plant tolerance to subsequent low-temperature stress. However, the knowledge on grain yield formation responses to multiple low-temperature stresses under natural conditions is still limited. Here, two years of environment-controlled phytotron experiments were carried out with two wheat cultivars under different low-temperature levels, periods and frequencies, and the temperature regimes in this study were designed to be similar to the daily variation in natural temperature. Spring frost (canopy temperature fall below 0 degrees C or Stevenson screen air temperatures below 2 degrees C) had negative effects on grain yield and yield components for both cultivars. Moreover, wheat yield was more sensitive to low temperatures at booting than at jointing. The reduction in grain yield under low-temperature conditions was mainly attributed to the decreased spike number per plant (SNPP) and grain number per spike (GNPS). Furthermore, grains in the apical spikelets were the most sensitive to spring frost, followed by those in the basal and central spikelets. Compared to the non-low-temperature-pretreated plants, the low-temperature-pretreated plants did not have an increased grain yield, except at T4 (T-min/T-max/T-mean - 6 degrees C/4 degrees C/ -1 degrees C). Grain yield under T4 probably increased because severe low temperatures killed parts of the tillers, which resulted in carbon and nitrogen relocation to the roots, young tillers and axillary buds. As a result, the young and new tillers in the axillary buds grew rapidly and eventually formed spikes.