Base and optimum temperatures vary with genotype and stage of development in wheat

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Abstract
Some assumptions concerning development in wheat (Triticum aestivum, L.) were examined. These are that (i) the rate of development towards anthesis increases linearly with temperature, (ii) the base temperature is 0°C, (iii) the optimum temperature is above the range at which wheat is normally grown, (iv) base and optimum temperatures do not change with development, and (v) the relationships for different cultivars are similar. We tested these assumptions in studies using a naturally lit phytotron with four cultivars and six temperature regimes between 10 and 25°C. Seedlings were vernalized for 50 d and then grown under a photoperiod of 18 h to avoid confounding the responses to vernalization and photoperiod with those to temperature. In cultivars Sunset and Rosella, the rate of development for the full period to anthesis increased linearly between base and optimum temperatures. However, 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°C, but temperatures higher than 19°C had little or no fürther accelerating effect. When a linear relationship was fitted by ignoring data for temperatures above 19 7deg;C, base temperatures calculated for the full period to anthesis were c. 5.5, 5.5,4.0 and 2.5°C for Sunset, Condor, Rosella and Cappelle Desprez, respectively (i.e. an average value of c. 4 7deg;C). The full period to anthesis was subdivided into three phases for fürther analysis. These were (i) from the beginning of the experiment to terminal spikelet 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 of cultivar and developmental phase. However, both base and optimum temperatures calculated from the relationships increased as development progressed from (i) to (iii). Averaging across cultivars, base temperatures for the three phases were ‐1.9, %1.2 and %8.1°C, respectively, while optimum temperatures were 25°C, respectively. Cultivars differed substantially in all these parameters. The progressive increase in optimum temperature with phasic development was apparently the main reason why linear fits for the three sub‐phases became a curvilinear fit for the full phase to anthesis.