The relationships between grain size and shape and milling yield were examined in samples of 11 current commercial cultivars and breeding lines of wheat grown at several sites in northern New South Wales. In addition, the effects of segregation within samples for grain size and test weight were studied. For samples that had been segregated for test weight, there was a strong positive correlation between test weight and milling yield. This correlation was much weaker for samples of different varieties grown at a particular site although over all sites the correlation was significant. Similarly, in samples stratified for grain size, there was a consistent positive correlation between seed size and milling yield as expected on theoretical grounds. In contrast, in samples of different cultivars grown at the one site, grain size was not correlated with milling yield. These contrasting results were taken to indicate that the contribution of differences in seed size to genotypic difference in milling yield were small relative to the effects of other factors. The effects of some of these other attributes of wheat grains, which may influence milling yield (e.g. the amount of germ tissue, thickness of bran, depth of crease), were also studied. Free-milling wheats appeared to differ from their poorer milling counterparts in several of these factors, each of which appeared to make a minor but cumulative contribution to differences in milling yield. No one factor studied here appeared on its own to make a major contribution to genotypic difference in milling yield.
Tolerance to pre-harvest sprouting, measured by the response of grain in intact harvested ears to a standard wetting treatment, varied substantially from season to season and in all trials declined with time after harvest ripeness. The major factor associated with the observed variation in tolerance at harvest and the decline after harvest ripeness appeared to be the level of grain germinability. Statistical analyses of meteorological data relating to the 10- and 20-day periods prior to harvest and the period from harvest to wetting treatment, indicated that most of the seasonal variation in tolerance could be explained in terms of the amount of rain during the 20-day period prior to harvest. The potential to use this relationship to develop an early warning system for wheat growers together with implications for wheat breeding and cultivar improvement programs are discussed.
TThe advanced wheat breeding line BD 159, from Victoria, exhibited a wide variation in falling number values at trial sites in 1990 when corresponding values for standard cultivars were uniformly high. The variable and unpredictable behaviour of BD 159 appears to be typical of a number of advanced lines and parental stocks from Australian breeding programs. The grain samples of BD 159 with low falling numbers had elevated levels of a-amylase which was distributed evenly in the proximal and distal halves of the grains. This distribution pattern, which was quite distinct from the steep gradient in a-amylase activity typical of germinated grains, and the absence of any evidence of sprouting indicated that the anomalous behaviour of BD 159 is a new and different form of the late maturity a-amylase syndrome previously described in wheat varieties such as Spica and Lerma 52. The high levels of a-amylase were reproduced at Narrabri in northern New South Wales when plants were transplanted from the field and allowed to ripen in a cool temperature glasshouse. Plants which were left to ripen in the field produced grain with a very low a-amylase activity.
Grain quality results for variety trials extending over 27 years (3 sites and 5 varieties) were compared with the temperature profiles during the grain filling period (56 days prior to harvest) to determine the effects on quality of high temperatures (>35�C) during this period of growth. Heat stress episodes have been frequent at two (Narrabri, N.S.W., and Turretfield, S.A.) of the three sites studied; spring temperatures were more moderate at the third site, Wongan Hills, W.A. There were highly significant (P< 0.01) correlations of heat stress (as hours above 35�C, during grain filling) with protein content (positive) and with grain yields (negative) at Narrabri for all varieties. In many combinations of site and variety, heat stress correlated negatively with loaf volume, and with dough strength (as Rmax, resistance to stretching with the Extensograph). Heat stress episodes in the Narrabri (N.S.W.) region in 1981 and 1982 gave further opportunity to examine these relationships. Results showed very clearly that high temperatures late in grain filling were associated with weaker dough properties (lower Rmax) in the resulting grain. These trends may form the basis of a predictive system by which to estimate crop quality and to interpret the results of variety trials.
Germinability in harvest-mature wheat grain showed a marked dependence on temperature. The optimum temperature for the complete germination of all grains ranged from 20�C for the non-dormant variety, Timgalen, to 10�C for the strongly dormant red wheat RL 4137, whereas the optimum in terms of the shortest lag period ranged from 25� to 15�C for the same varieties. Germinability gradually increased during post-harvest storage and, for after-ripened grain, the optimum temperature for both complete germination and shortest lag period were greater than 30�C. Germinability could also be increased by pre-treating imbibing grains at temperatures of 5�, 10� or in some cases 15�C. This treatment was only effective for grain at moisture contents >25% (dry weight) and the effect was not reversed by redesiccation. The pre-treatment temperature required for maximum germinability decreased with increasing levels of grain dormancy. Complete removal of dormancy required a pre-treatment period of c. 48 h; however, lesser periods gave the shortest lag period in the case of the dormant varieties. The implications of these results for the utilization of dormancy in the development of preharvest sprouting damage tolerant varieties and their subsequent use in practice are discussed.
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