Low temperatures impose restrictions on rice (Oryza sativa L.) production at high latitudes. This study is concerned with low temperature damage which can arise mid-season during the panicle development phase. Low temperature damage at this phase is exacerbated by high application rates of nitrogen (N) fertilizer. The objectives of this study were: (1) to investigate the mechanisms of spikelet sterility caused by low temperature during reproductive development under different N conditions; and (2) to determine whether low temperature experienced by the root, panicle or foliage is responsible for increased spikelet sterility.
The problem was investigated under field and temperature-controlled glasshouse conditions. Four seasons of field experiments were conducted at Yanco Agricultural Institute (YAI) (34°37'S, 146°25'E; alt. 140 m), in a temperate rice-growing region in southern New South Wales (NSW). Treatments in the field and glasshouse experiments included N rates from zero to very high (i.e. 300 kg N ha-1) to achieve variation in N status of the crop. To increase the chance of low temperature occurring during panicle development, plants were sown 2 3 times in each field experiment. In the glasshouse experiments, 12 h periods of low (18/13°C) and high (28/23° C) day/night temperature were imposed over periods of 5 7 days during panicle development. In the glasshouse experiments, water depth and water and air temperatures were changed independently to investigate the effects of low temperature in the root, panicle and foliage on spikelet sterility. In all field and glasshouse experiments, the number of engorged pollen grains per anther, spikelet sterility and grain yield were measured.
In the field, low minimum air temperature during microspore development (i.e. 18 11 days before flowering) and at around flowering increased spikelet sterility. Minimum temperature below 20°C increased spikelet sterility; each PC decrease during microspore development and flowering increased sterility by 1.4 and 0.9%, respectively. Therefore, the microspore development period appeared to be more sensitive to low temperature than was the flowering period. Also in the field, an average minimum air temperature of 13.8°C during microspore development in shallow-watered (8 cm) fields resulted in 25% spikelet sterility. In the glasshouse experiments, an average minimum temperature of 13°C during microspore development caused 51% spikelet sterility.
The study of past temperature records at YAI has shown that, in 14% of the years, rice would be exposed to an average minimum temperature of < 13°C for 10 days during microspore development which occurs in late January. This would be expected even when the crop is sown early in the season (i.e. early October).
An increased number of engorged pollen grains per anther at heading resulted in more pollen grains being intercepted by the stigma at flowering. Consequently there was an increased number of germinated pollen grains on the stigma. Low temperature (18/13°C) imposed on the whole plant for 5 7 days during microspore development severely decreased the production of engorged pollen grains. This resulted in increased spikelet sterility.
While engorgement efficiency (the percentage of pollen grains that were engorged) was determined by both root and panicle temperature, the germination efficiency (the percentage of germinated pollen grains relative to the number of engorged pollen grains intercepted by the stigma) was determined only by root temperature. The interception efficiency (i.e. percentage of engorged pollen grains intercepted by the stigma), however, was not affected by either root or panicle temperature. Spikelet sterility can be expressed as a function of the number of total pollen grains per spikelet and the efficiencies with which these pollen grains become engorged, are intercepted by the stigma, germinate and are involved in fertilization. Of these, engorgement efficiency was the dominant component, explaining the largest variation in spikelet sterility.
Deep water (20 cm) in the field often protects the young panicle from being exposed to low air temperature. This practice, however, promoted culm elongation leading to the exposure of parts of the panicle with a consequent increase in sterility. Overcoming this effect requires increased water depth to achieve complete submersion of panicles. In glasshouse experiments, the detrimental effect of low air temperature was overcome by submerging panicles completely in water at 22°C even though the foliage was at low temperature. Low temperature damage resulted from exposure of the panicle, but low temperature in the root system also contributed to the damage. Low panicle and root temperatures appeared to have additive effects on spikelet sterility, but exposure of foliage to low temperature had no effect.
Application of N exacerbated low temperature damage. In the absence of applied N, an average minimum temperature of 14°C over 7 days during microspore development across all experiments resulted in 21% spikelet sterility. This was increased to 42% where > 150 kg N ha' had been applied at the pre-flood (PF) stage. A decrease of 1°C average minimum temperature below 20°C during microspore development increased sterility by 3.2% and 1.3% with and without N applied at PF, respectively.
The application of N decreased the number of engorged pollen grains per anther due, in part, to increased spikelet density. There was a significant combined effect of spikelet density and minimum temperature during microspore development on the number of engorged pollen grains per anther, resulting in variation in spikelet sterility.
The application of N also increased panicle height mainly because of increased culm length. Hence, panicles were only partially submerged during microspore development. This appeared to be a further reason for the susceptibility of rice to low temperature when high rates of N are applied.
This study demonstrated the usefulness of increased number of total pollen grains per spikelet to overcome the adverse effects of low temperature during microspore development, resulting in decreased engorgement efficiency. The mechanism involved at tissue or cell level in reducing the number of engorged pollen grains per anther when high rates of N are applied, however, remains unknown. Therefore evaluation of both quantitative and qualitative aspects of engorged pollen as affected by environmental variables is required further.