The reaction of ears of wheat (Triticum aestivum L.) to frost after their emergence from the flag leaf sheath was studied for a number of cultivars under controlled conditions of radiative and convective energy exchange. Frost injury was measured in terms of the fertility of the ear, viz. number of grains set per spikelet. Despite being covered with frost crystals, the ear was unaffected by freezing temperature until a threshold level, below which there was a steep reduction in grain set, approximating 100% per 1°C. A threshold of about -4°C and T50 of -4.7°C were observed for the hardiest cultivar, Florence. Crystallization can be initiated within a supercooled floret by spread of freezing from either the peduncle or the vegetative structures of the spikelet. Neither of these pathways was critical and it was concluded that any nucleation in the rachis was sufficient to result in the patterns of injury described. A mechanism of injury was discussed, based on the proposal that nodal regions within the rachis and rachilla provided a distributed system of bamers to the spread of crystallization. These could be effective in allowing supercooled reproductive organs to remain so and avoid lethal injury.

Internal and external factors which may influence the survival of emerged wheat ears during radiation frost were examined by a number of experiments in a freezing chamber. Hardening, stage of ear development, supercooling and cultivar were the internal factors studied in ears protected from external ice nucleation by enclosure within polyethylene bags. Neither hardening nor stage of ear development had any effect on freeze resistance. Spikelet survival was unaffected by supercooling. Variation in internal ear resistance was revealed among 16 cultivars frozen at –3.9°C. Removal of the cuticular wax coating over the unprotected ear reduced its ability to survive at –3°C in an atmosphere abundant in ice nuclei. This supports the proposal that hydrophobic wax coatings may be important in the field in protecting floral parts from external ice nucleation. The implications of these findings with respect to frost resistance in the field are discussed. _____________________ *Part III, Aust. J. Agric. Res., 17: 601 (1966).

The formation of ice crystals within leaves, stems, and ears of wheat plants (Triticum aestivum L.) was studied at temperatures of –3.0 to –5.0°C. The mean velocity of propagation of the ice front in leaves at –3°C was 120 ± 4.1 cm/min. In other plant parts it was slower and more variable. Values recorded for stem internodes ranged from 76 to 0.02 cm/min and for reproductive organs from 1.0 to 0.04 cm/min. Some stem and rachis nodes (in winter-grown plants only) arrested the spread of ice crystals for several hours. This appeared to be an important mechanism for protecting young ears from frost, as they remained super-cooled for long periods if inoculation from external sources was prevented.

Plant and air temperatures were measured during radiation frost within a crop of wheat (Triticum aestivum cv. Timgalen) grown in 1974 on the Liverpool Plains Field Station, Breeza, New South Wales. Observations were made at different heights within the canopy at several stages of crop development, and these were related to screen and grass minimum temperatures recorded in a meteorological enclosure adjacent to the crop. Coldest conditions were found near the surface of the crop where temperatures were as much as 2�C lower than those in middle regions of the canopy. The temperature just below the soil surface was between 4� and 6�C warmer than that of plant or air at 5 cm to 10 cm above its surface. Grass minimum temperature was linearly related to screen minimum and found to be 3�C lower. Either measurement was useful in estimating the lowest temperature within that canopy.

The duration of post-anthesis development was studied in eight spring wheat cultivars under field conditions. Variation in post-anthesis environment was achieved by varying the date of anthesis through the use of seed vernalization, extended photoperiod, and serial sowing.Multiple correlation and regression analysis of phase duration on temperatures and photoperiod was able to account for 75-97% of the variation in duration of the post-anthesis phase.Differences in the rate of post-anthesis development between cultivars were not statistically significant: nor was the influence of photoperiod significant. Air temperature was the principal factor influencing the rate of development in this phase.Observations of the duration of post-anthesis development taken from field crops in three successive years agreed well (R2 = 75%) with those predicted from the regression relationships previously established.The data are discussed in relation to the implication of the rate of post-anthesis development in yield variation.