Midseason cold tolerance screening for the NSW rice improvement program

The current rice varieties grown by Australian farmers are susceptible to low temperature events particularly during the reproductive stage of plant development. The best management practices of sowing within the recommended time period and maintaining deep water (20–25 cm) through the microspore development stage only offer limited protection. There is a need to develop more cold tolerant varieties and to do so requires the development of low-temperature screening capacity for the NSW rice breeding program. This study looked at the requirements of adapting a controlled-temperature glasshouse facility to enable screening for tolerance to low temperatures during the reproductive stage of rice development. The investigations were grouped into two areas; 1) the physical aspects of the low temperature facility including the location of plants within the facility and within the tubs used to grow the plants and whether these can influence the reliability of the screening and 2) the biological effects of nitrogen (N) concentration in the plant at panicle initiation (PI) and plant susceptibility to low temperatures, and whether growth stage of the plant relative to PI at the start of low temperature treatment influenced floret sterility. A series of nine experiments were conducted at the Deniliquin Agricultural Research and Advisory Station glasshouse facility using up to five rice varieties selected for their divergence in low-temperature tolerance. One other experiment was conducted in a different facility. The modified glasshouse facility in Deniliquin was effective in providing the targeted screening environment of 27°C day and 13°C night temperature regime. There was however a smaller than expected effect of the low temperature exposure in some of the experiments with sterility following low temperature ranging from 9.9% to 27.7%. There was also a higher than expected level of sterility in the controls (i.e. not exposed to low temperature) with sterility levels up to 58% recorded in one experiment. The causes of these overall effects are not known. Notwithstanding these overall effects there were a number of findings that are important for developing a reliable screening facility. The spatial arrangement of the plants within the low temperature facility effected the level of sterility highlighting the need for experimental design to consider spatial variation. The existence of edge effects was identified within the tubs used to maintain permanent water on the potted plants whereby the outer plants in the tubs were less damaged by the low-temperature treatment. The overall N level in the leaf tissue was low even at the equivalent rate of 250 kg N ha-1 and there was only a very modest and inconsistent response in N concentration at PI to N application rates ranging from 0 to 250 kg ha-1. However, the method of growing the plants in pots and of nitrogen fertiliser application did not alter the N concentration. The concentration was the same when N was added either, on the same day as permanent water application, or three days prior to permanent water application. Also restricting the direction of water movement through the pots and therefore the soil within the pots did not alter the amount of N in the plants at PI. The low plant N concentrations were consistent across two glasshouses in which soil from the same source was used suggesting a soil limitation. A soil test identified that the soil phosphorus (P) was at a level at which plants have responded to P application under field conditions, and the loamy texture of the soil had an associated low cation exchange capacity in comparison to medium to heavy clay soil types commonly associated with rice growing. These factors may have reduced the N retention and uptake and, in part, explain the low injury from the low temperature exposures. In the variety Millin, there was no significant effect of timing of the exposure on sterility until it commenced 12 to 15 days after PI. This is a significant finding for a breeding program that must expose lines of unknown phenological development. It means that even though there are small differences in the rates of development, there is no large effect of this on sterility. However, this response was not seen in the other varieties tested and thus requires further validation. It was difficult to induce repeatable levels of floret sterility in this series of experiments most likely due to the low N concentrations in part due to the properties of the soil used to grow the plants. This highlights the importance of standardising all cultural aspects in order to provide uniform and repeatable screening information. The spatial effects highlight importance of experimental design for effective exposure to low temperature treatments, incorporation of the capacity for spatial analysis in the statistical design, the use of standard variety checks for floret sterility after low temperature treatment, and the determination of N concentration in plant tissue prior to exposure