The vegetative stage refers to a period from germination to panicle initiation and is characterised by active tillering, gradual increase in height, and leaf emergence at regular intervals. Germination starts when seed dormancy has been broken, the seed absorbs adequate water, and is exposed to a temperature ranging from about 10oC to 40oC. Temperature has a profound influence on germination by affecting the activation stage and post-germination growth. There are clear varietal differences in seed germination at low temperatures (Yoshida 1981). Low temperature can affect the developmental processes of rice plants. Low temperatures impair photosynthesis, which reduces growth and results in indirect yield loss due to less carbohydrate availability for grain production (Smillie et al. 1988). Low temperatures causing poor establishment and vegetative growth in rice is a common problem in countries such as Australia and Laos.
The average minimum temperature during establishment (November) in Yanco, NSW, Australia is 12.3oC, with the variation being higher within, rather than across, years. For the corresponding period in Laos (December), the minimum temperatures in Xieng Khouang and Champassak were 8.3°C and 18.5oC respectively. Variation for temperature within and across years was calculated based on daily maximum and minimum temperature data. The coefficient of variation (cv) of temperature across years was calculated from the standard deviation of the mean temperature for each year divided by mean temperature across all years. The cv of temperature within years was calculated as the mean of the cv of daily data for each year. All cv's are shown as a percentage (Table 2). Establishment of dry-season rice crops in provinces such as Xieng Khouang is difficult, because 10oC may be considered as the critical minimum temperature for elongation of shoots and roots (Yoshida 1981). The temperature variability was higher within years than across years in all six provinces. The extent of low temperature appears to be related to the altitude of provinces in Laos with Vientiane (171m) and Champassak (120 m) provinces not affected by low temperature during establishment.
Seedling vigour is an important foundation for efficient crop production. Vegetative vigour, the rapid attainment of plant biomass, depends on the initial size of seedlings and the rate at which they grow. A controlled-environment experiment was conducted at Yanco to explore differences in seedling vigour among 38 direct-sown cultivars from the International Cold Tolerance Nursery. Seedlings were grown at 25/15oC for two weeks prior to the imposition of temperature treatments. Seedling size at two weeks was considered the initial size. The temperature treatments (7/22oC, 10/25oC and 13/28oC minimum/maximum temperatures respectively), reflected the range of conditions likely to occur during establishment in Australia and Laos. There was a five-fold difference in average seedling biomass between the low and high temperature treatments. Cultivars exposed to the highest temperatures had the greatest biomass, averaging 600mg per seedling. Average biomass of seedlings at the intermediate and low temperature treatments were significantly lower, at 290mg and 120mg respectively. A selection of four cultivars is shown, which include those with the greatest and least response to temperature (Figure 2). The tropical cultivar IR36 had the smallest response to increasing temperature. Surprisingly, a temperate cultivar from California, L202, showed a modest response to temperature. HSC 55, a cultivar from Hungary (a temperate region) showed the greatest positive response to increasing temperature. There were significant differences between cultivars at each of the temperature treatments, with differences greater at the highest temperature. The NSW cultivars Amaroo (shown in Figure 2), Jarrah, Millin and Illabong (not shown) performed similarly and were intermediate in their response to temperature. At the lowest temperature treatment, there were no cultivars that had higher biomass than Amaroo, but at higher temperatures there was significant genotypic variation. In Laos, temperature conditions during establishment resemble the 10/25oC and 13/28oC temperature treatments. There are potential benefits available from increased biomass production, if the early vigour of cultivars such as HSC 55 could be incorporated into commercial cultivars.
Nitrogen uptake by the rice plant at panicle initiation (PI) is recognised as a key determinant of yield potential in the NSW rice industry. The amount of nitrogen in the above-ground tissue of the crop at PI is driven by temperature conditions during the establishment phase. The mean air temperature from November 1 to December 31 was calculated from 1989-1999 at Yanco Agricultural Institute. For each year the average nitrogen uptake for all Amaroo crops sown between October 1-7 was calculated across all rice growing areas. There was a significant correlation (r2=0.44) between average air temperature and nitrogen uptake at panicle initiation (Figure 3-A). The average temperature in 1994 was 20oC and the nitrogen uptake was 87kgN/ha. In 1995 the average temperature was 23oC and the nitrogen uptake was 125 kgN/ha. A significant correlation exists between nitrogen uptake and yield (r2=0.45), which suggests that good early growth resulting in higher PI nitrogen uptake is an important factor contributing to higher yields (Figure 3-B). A strong correlation between average temperature during establishment and grain yield (r2=0.73) highlights the importance of early growth in contributing to Australia's high yields (Figure 3-C).
