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Ranking order for leaf damage was determined in six varieties of the tropical grass setaria after frosting in a controlled environment and was found to be similar to known field response. A frost temperature of -2.5�_C provided good discrimination between populations subjected to three successive frost cycles of 4 hr under programmed conditions which closely simulated field conditions. Further experiments were carried out to investigate: (a) The relative frost tolerance of a range of varieties in the genera Setaria, Digitaria, Paspalum, and Lolium; (b) the effect of vapour pressure deficit (VPD) during frosting and rate of temperature change into the frost period on leaf damage; (c) the effect of temperatures preceding frosting and duration of frost on leaf damage; (d) the relationship of concentration of alcohol-soluble carbohydrates to susceptibility to frost. The most tolerant tropical grass tested was Paspalum dilatatum. Significant variation in frost tolerance in Setaria was related to altitude of origin, whereas in Paspalum plicatulum latitude of origin appeared to be the controlling factor. Frost tolerance was slightly increased by pre-hardening for 3 days or by growing at a lower night temperature. Frosting at a VPD of - 3 mb resulted in less damage than frosting at a VPD of zero, when visible frost formed on leaf surfaces. Extending the duration of the frost resulted in an increase in leaf damage. Ranking order for frost tolerance was not attributable to differences in concentration of alcohol soluble carbohydrates and was not affected by environmental conditions either preceding or during the frost period. |
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