No abstract

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 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.

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.

Rotational effects of chickpea, an important N2-fixing pulse legume of the northern grains region, on subsequent wheat require quantification of the contribution of the legume to soil N and the N status of the wheat, and of suppression of soil and stubble-borne pathogens, such as crown rot (Fusarium graminearum Schwabe Group 1). Results from selected treatments of 10 experiments in northern New South Wales in which chickpea and wheat in one season were followed by wheat in following seasons indicated generally higher dry matter (DM) and grain yields of wheat after chickpea than after wheat. Responses to chickpea were -0·8 to 3·3 t/ha (shoot DM) and -3 to 39 kg N/ha (shoot N). Responses in wheat grain yields were -0·1 to 1·7 t/ha (mean 0·85 t/ha); grain N responses were -2 to 33 kg/ha (mean 19 kg/ha). Grain protein responses were small (0·6%) and variable. Although these productivity responses could be explained largely in terms of additional nitrate-N following chickpea, we measured reduced incidences of crown rot in wheat after chickpea (range 1-36%, mean of 12%), compared with wheat after wheat (range 5-52%, mean 30%). Modelling the incidence of crown rot indicated highly significant interactions between prior crop and total water (pre-plant soil water plus in-crop rainfall). When wheat followed chickpea, incidence of the disease declined sharply with increasing water. When wheat followed wheat, there was a marginal decline in disease incidence with increasing water. Our results support the strategy of using legumes in rotation with wheat in the northern grains region for enhanced soil-N supply and disease-break effects.