Comparative analysis of physiological and phenological traits of rice (Oryza sativa) under aerobic production systems in dry and wet tropics of Queensland, Australia

Aerobic rice is becoming a more promising rice cultivation system due to increasing water scarcity for irrigation and occurrence of drought, especially in Australia. Rice cultivation on aerobic soil under rainfed conditions has shown potential for successful rice cultivation in tropical climate. Strategic irrigation during the critical growth period can help reduce the water demand on farm. Central Queensland has an annual rainfall of ca. 800 mm, and about 600 mm occurs during the wet season from December to March; whereas parts of the wet tropical north Queensland receive ca. 3000 mm annual rainfall, and about 1893 mm during the wet season from December to March. The study was carried out at Alton Downs, central Queensland (dry tropics) and South Johnstone, north Queensland (wet tropics) to investigate the phenological, physiological and agronomical responses of 13 different rice varieties with a view to identifying suitable varieties for dry land cultivation. The objectives were to assess rice varieties under i) rainfed conditions in the wet and dry tropics, ii) rainfed conditions and strategic irrigation condition in the dry tropics, and to iii) identify the physiological, phenological and agronomical traits of rice adaptation under aerobic conditions in the dry and wet tropics. In the dry tropics, the strategic irrigation was provided by drip irrigation and was scheduled when the rice plants showed water deficit symptoms (corresponding to the refill point at 21 mm /100 mm soil water). The average yield of rice varieties under strategic irrigation was significantly higher and the variety best yield (AAT 4) produced up to 5.23 t/ha in the year 2015 under strategic irrigation. The average yield of varieties was increased from 1.5 times (AAT 4) to 16.8 times (AAT 15) with strategic irrigation, as compared to rainfed conditions. The average water productivity was increased by 100 % in 2014 and by 110.3 % in 2015 using strategic irrigation as compared to rainfed. The average water productivity was 0.24 t/ML (in 2014) and 0.61 t/ML (in 2015) under strategic irrigation, whereas it was 0.12 t/ML (in 2014) and 0.29 t/ML (in 2015) under rainfed conditions. The high yielding varieties were early flowering types, which escaped the terminal drought caused by lower rainfall during the flowering stage, whereas the late varieties such as AAT 10, AAT 11 and AAT 15 were among the highest yielders in the wet tropics under rainfed conditions. The greater yield was associated with greater panicle fertility, leaf area index , higher photosynthetic rate and water use efficiency during flowering, and one of the high yielding varieties (AAT 3) had the highest photosynthetic rate during the grain filling period in both strategic irrigation and rainfed conditions. Root dry weight and root weight density in the top soil layer at 0–15 cm were found to be related to yield under strategic irrigation, but the varietal characteristic of deep rooting was not correlated with yield. It is important to consider variations in flowering time, yield potential and drought patterns while developing varieties for aerobic conditions, as the drought reduced the panicle filling percentage to 1% under rainfed conditions. The variety with most stable and consistent yield at Alton Downs was AAT 6, and had the lowest coefficient of variation across the years whereas the variety AAT 13 was found to be more responsive with better growing conditions at Alton Downs under rainfed conditions. The varieties when sown late, late flowering varieties were subjected to cold and terminal drought reducing the yield. AAT 6 and AAT 13 are both early flowering varieties. In the wet tropical environment, the crop received rainfall until harvesting time. The favourable physiological characteristic of high yielding varieties such as AAT 4 and AAT 6 in the dry tropics was greater water use efficiency, and the agronomic characteristics were higher panicle fertility, higher effective tillers per plant and grains per panicle. In the wet tropics (South Johnstone), the high yielding variety AAT 10 was characterised by high harvest index, longest panicle length, higher effective tillers, higher panicle fertility and higher water use efficiency. In South Johnstone, the days to flowering did not have any effect on the yield of varieties. The varieties those producing least yield under rainfed conditions at Alton Downs were among the highest yielders in South Johnstone. The high yielding varieties maintained greater effective tillers per plant, heavier 1000 grain weight, greater harvest index and fertility. Reliable soil moisture favoured photosynthetic rate and water use efficiency and the associated larger flag leaf area contributed significantly to higher yields at wet tropical South Johnstone as compared to dry tropical Alton Downs. Strategic irrigation in dry tropical environments could allow plants to cope with water stress caused by less rainfall during the grain filling period. Similar yield was achieved under strategic irrigation for late flowering varieties as under rainfed conditions for early flowering varieties. The varieties responded with an average increase of 11.87 kg/ha and 15.80 kg/ha with each additional 1 mm water application in 2014 and 2015 respectively. This shows that there is great commercial scope for strategic irrigation during water deficit periods, created by little or no rainfall, during critical crop growth periods for rice in the dry tropical environment of central Queensland. In conclusion, this thesis increases the understanding the role of strategic irrigation and varietal characteristics for rice cultivation under the dry tropical agro‐ecological domain of central Queensland and the wet tropical conditions of north Queensland. Higher productivity of aerobic rice in dry tropical central Queensland is achieved with early flowering varieties, supported by strategic irrigation management during the water shortage periods, with higher water use efficiency, greater number of spikelets, higher panicle fertility. In the wet tropical environment of northern Queensland, yield variation between varieties was not significantly affected by the days to flowering. However, further study for selection of varieties from more diverse germplasm for plant water status and fertility, and different water management strategies under aerobic conditions needs, to be explored, to achieve the rice yield that can assure the commercial opportunity for rice production in the dry and wet tropical environments of Queensland, Australia.