Rates and cardinal temperatures for processes of development in wheat: Effects of temperature and thermal amplitude

This paper addresses three issues relating to development in wheat. Whether thermal amplitude changes rates of leaf appearance and phenological development, whether the responses of leaf appearance to a wide range of temperature can be acceptably described in terms of rates and cardinal (base and optimum) temperatures, and whether similar rates and cardinal temperatures can be applied to parallel processes of development. The two studies used a total of 22 naturally-lit growth chambers and 44 treatments, and the disparate wheat cultivars Sunset, Condor, Rosella and Cappelle Desprez. All plants were vernalised and then grown under photoperiods extended to 18 h. In the first study, thermal amplitudes varying between 0 and 14 degrees C around a common mean of 19 degrees C did not change rates of leaf appearance and had no consistent effect on phenological development at any stage to anthesis. Consequently, conclusions from data collected under one amplitude at this mean temperature can be extrapolated with confidence to another. In the second study, with six mean temperature regimes of 10-25 degrees C, rate of leaf appearance was significantly increased progressively by increasing temperature up to 19 degrees C. All cultivars had an optimum temperature for this process which approximated 22 degrees C but they differed in their base temperatures which ranged from -5.7 to -1.9 degrees C. Optimum and base values for the concurrent phase of phenological development ranged between similar extremes, but the rankings of base values amongst genotypes differed markedly for leaf appearance and phenology. For the process of culm extension, temperature significantly, but not linearly, affected final length but not internode number. Length was unchanged by temperatures between 10 and 16 degrees C but was significantly reduced above 19 degrees C. Cultivars differed in their respective optimum and base temperatures for rate of elongation, ranging from ca 19 to 21 degrees C and from 2 to 5 degrees C. The concurrent phenological process had a higher optimum of 22-25 degrees C but again a similar range in base temperature with differing cultivar rankings. We conclude from the second study that cardinal temperatures for leaf appearance, culm elongation and phenology were different, and for each process there were cultivar differences.