Frost resistance in cereals after EAR emergence

Spring radiant frost damage to cereals post head-emergence causes significant crop losses in Australia and internationally. The problem arises in areas where the heat and drought of summer restrict the cropping season to winter and spring. Typically, during the growing season, day-time temperatures are ideal for growth but night-time temperatures can fall to potentially damaging levels. Varieties with improved frost resistance post head-emergence would reduce the occurrence of sporadic frost damage while allowing crops to mature under more favourable climatic conditions, thus improving yields. The primary aim of this study was to evaluate potential sources of post head-emergence frost resistance in winter cereals that could be incorporated into breeding programs. Research and screening for resistance to spring radiant frost has been undertaken in northern Australia for several decades with limited success. In this study, earlier field screening methods were further developed and refined, allowing more accurate assessment of frost resistance while minimising false positives from frost escapes. The field trials were conducted over nine seasons at two sites in southern Queensland. Although field screening of wheat and barley has been undertaken over nine years, to date, no tested genotype has shown a consistent improvement in post heademergence frost resistance over existing elite varieties. A number of wheat genotypes with putative frost adaptation mechanisms, including CIMMYT synthetics, exhibited no detectable advantage over standard Australian cultivars. Five barley genotypes previously tested for reproductive frost resistance in southern Australia were tested, post head-emergence, in the northern grain belt of Australia and compared to the typical northern control cultivars, Gilbert and Kaputar. All tested genotypes suffered severe damage to whole heads and stems at plant minimum temperatures less than −8°C. In 2003, 2004 and 2005, frost events reaching a plant minimum temperature of approximately −6.5ºC did not result in the complete loss of grain yield. Rather, partial seed set was observed. The control genotype, Gilbert,exhibited seed set that was greater than or equal to that of any genotype in each year, as did Kaputar when tested in 2005. Thus, Gilbert and Kaputar were at least as resistant as any tested genotype. This contrasts with trial results from the southern grain region where Gilbert was reported to be less resistant than Franklin, Amagi Nijo and Haruna Nijo. Hence, rankings for post head-emergence frost damage in the northern grain region differ from those previously reported in the southern grain region. Our results indicate that Franklin, Amagi Nijo and Haruna Nijo are not likely to provide useful sources of frost resistance or markers to develop improved varieties for the northern grain region of Australia. In the absence of identifying winter cereals with improved post head-emergence frost resistance, a more fundamental approach was undertaken to better understand frost resistance, with a view to identify potential resistance mechanisms. Two model Poaceae species were identified with good levels of post head-emergence frost resistance. Having confirmed these Poaceae species have higher levels of resistance than heading winter cereals, a strategy was developed to contrast resistant and susceptible types with the aim of elucidating mechanisms of resistance. Collections were made from extremes of altitude and latitude within Australia in order to sample the range of climatic variation locally available. A subset of the collected accessions were screened under natural frosts, however no frost susceptible type was identified. Additional collection and screening to identify a susceptible type, or a new strategy, will be required to identify mechanisms of resistance in these Poaceae species. Infrared thermal imaging was used to better understand the phenomena of field radiant frost and frost damage in wheat post head-emergence. Using infrared thermal imaging, freezing was visualised by observing tissue exotherms under field radiant frost conditions. This study demonstrated the utility of infrared imaging to study radiant frost and is the first report of observations under field frost conditions. Exotherms of leaf and floret tissues were clearly seen and differed between tissues. Further work is needed. Observations of freezing in the stem and peduncle will be important to improve the understanding of frost damage. Also, studies contrasting frost damage of wheat and barley will be useful to understand the relative frost resistance advantage of in-head barley over wheat. Furthermore, a better understanding of field radiant frosts will be useful to improve controlled environment testing. Work is continuing, but to date, the numbers of winter cereal genotypes assessed remain low. As the number of genotypes screened with the current methodology increases, there is a real chance of identifying resistant genotypes. To maximise the rate of progress, a targeted selection of germplasm for screening is recommended.