Variation in time of ear emergence of wheat (Triticum aestivum L.) : physiology, genetics and consequences for yield

The timing of ear emergence in wheat is largely controlled by photoperiod and vernalization. Large genotypic differences in sensitivity to photoperiod and vernalization exist.Studies on the response to vernalization are complicated by growth and development during the vernalization treatment. It was shown that this growth and development could be adequately equated to time at higher non-vernalizing temperatures by comparing the increase in number of primordia during a vernalization treatment with that under non-vernalizing conditions. Alternatively the vernalization treatment could be given to developing grains in the ear, transforming vernalization sensitive into insensitive grains (chapter 2).Controlled environment experiments with reciprocal crosses between monosomic lines of two vernalization sensitive wheats, the early variety Spica from Australia and the late variety Bersee from Prance, showed that a photoperiod insensitivity allele was present on the PPd 2 locus in Spica, and that both varieties carried a vernalization insensitivity allele on chromosome 5B. Factors independent of photoperiod and vernalization, the effects of which are described as earliness perse and in this experiment were thought to influence rate of development and number of spikelets, were found on chromosomes 4B, 4D, 3A and 6B (chapter 3).Experiments in controlled environments showed that even after vernalization and in long days, genotypes sensitive to photoperiod and vernalization reached ear emergence later than insensitive genotypes. The variation in ear emergence within phenotype classes, however, was shown to be large,, and this was considered to be due to earliness perse .The results of the controlled environment studies were compared with data on the wheat accessions in the USDA Small Grain Collection, and a close correlation between the two sets of data was found, both for differences in sensitivity to photoperiod and vernalization and for differences in ear emergence independent of photoperiod and vernalization. This indicates that the results of controlled environment experiments can be used to predict the timing of ear emergence under field conditions (chapter 4).The effects of differences in ear emergence on yield were tested with F 5 lines selected from a cross between Norman, a late semi-dwarf winter variety from Great Britain, and Talent, an early winter variety from France. The selections represented early and late, and short and tall genotypes in equal numbers. Earliness was shown to be linked with photoperiod and/or vernalization insensitivity (chapter 5), and short stature with the presence of the Rht2 dwarfing allele. In field trials in 1981-2 and 1982-3 at PBI in Cambridge, and in 1982-3 at The Murrays experimental farm of the Scottish Crop Research Institute near Edinburgh, the short selections gave yields greater or equal to the tall selections, and the early selections gave yields greater or equal to the late selections. Earliness was thought minimize yield loss by enabling the crop to escape drought stress around and after ear emergence, and this was considered to be especially important for the short selections (chapter 6).It has been shown that both genes and climatic factors such as daylength and winter temperatures can have similar effects on ear emergence and subsequently on yield. Mien it is considered beneficial to alter time of ear emergence, either vernalization sensitivity, photoperiod sensitivity or earliness perse factors could be varied. Changing photoperiod or vernalization sensitivity will be relatively easy because the effects of the genes involved can be recognized. However, the climate might not allow a change in photoperiod or vernalization sensitivity. Variation for earliness perse will affect time of ear emergence independently of vernalizing temperatures and daylength but is difficult to identify and as yet little is known about the underlying physiology and genetics.