BASE AND OPTIMUM TEMPERATURES VARY WITH GENOTYPE AND STAGE OF DEVELOPMENT IN WHEAT

Some assumptions concerning development in wheat (Triticum aestivum, L .) were examined. These are that (i) the rate of development towards a nthesis increases linearly with temperature, (ii) the base temperature is 0 degrees C, (iii) the optimum temperature is above the range at w hich wheat is normally grown, (iv) base and optimum temperatures do no t change with development, and (v) the relationships for different cul tivars are similar. We tested these assumptions in studies using a nat urally lit phytotron with four cultivars and six temperature regimes b etween 10 and 25 degrees C. Seedlings were vernalized for 50 d and the n grown under a photoperiod of 18 h to avoid confounding the responses to vernalization and photoperiod with those to temperature. In cultiv ars Sunset and Rosella, the rate of development for the full period to anthesis increased linearly between base and optimum temperatures. Ho wever, in cultivars Condor and Cappelle Desprez, a linear fit was not statistically acceptable. For these cultivars, the rate of development towards anthesis increased rapidly with increase in temperature from 10 to 19 degrees C, but temperatures higher than 19 degrees C had litt le or no further accelerating effect. When a linear relationship was f itted by ignoring data for temperatures above 19 degrees C, base tempe ratures calculated for the full period to anthesis were c. 5.5, 5.5, 4 .0 and 2.5 degrees C for Sunset, Condor, Rosella and Cappelle Desprez, respectively (i.e. an average value of c. 4 degrees C). The full peri od to anthesis was subdivided into three phases for further analysis. These were (i) from the beginning of the experiment to terminal spikel et initiation, (ii) from terminal spikelet initiation to heading, and (iii) from heading to anthesis. When these sub-phases were analysed a linear relationship was found to be appropriate for all combinations o f cultivar and developmental phase. However, both base and optimum tem peratures calculated from the relationships increased as development p rogressed from (i) to (iii). Averaging across cultivars, base temperat ures for the three phases were -1.9, +1.2 and +8.1 degrees C, respecti vely, while optimum temperatures were <22, 25 and >25 degrees C, respe ctively. Cultivars differed substantially in all these parameters. The progressive increase in optimum temperature with phasic development w as apparently the main reason why linear fits for the three sub-phases became a curvilinear fit for the full phase to anthesis.