DEEP FLOODWATER PROTECTS HIGH-NITROGEN RICE CROPS FROM LOW-TEMPERATURE DAMAGE

A field experiment was conducted to investigate the interaction of nitrogen (N) status, sowing date, and water depth on rice yield in the Murray Valley, New South Wales, where low minimum temperatures often lead to pollen sterility and low yield in high N status crops. The experiment combined 4 N rates (0, 75, 125, 250 kg N/ha) applied as urea just before permanent flood; 3 cultivars (Jarrah early maturing; Amaroo and Doongara, midseason); 2 sowing dates (26 September, 31 October 1991); and 2 water depths (5, 20 cm) at the microspore stage. The early-sown crops and the later sown Jarrah crop encountered minimum air temperatures of 19-degrees-C at the cold-sensitive microspore stage, compared with 15-degrees-C for the other 2 later sown crops. Total dry matter production was little affected by either water depth or sowing date, but increased from 16 to 22 t/ha with N application. Shallow water depth and delayed sowing date affected yield through reduced harvest index. Grain yield response to N fertiliser was dependent on sowing date, water depth, and variety. For the early-sown crops grown in deep water, yields of Amaroo and Jarrah increased from 7 to 13 t/ha with increasing N supply, while the yield response of Doongara plateaued at 9 t/ha. With shallow water, the yields of all varieties decreased from 7 to 3 t/ha with increasing N. For the later sowing date, Jarrah growing in deep water yielded up to 13 t/ha at high N, but yields of Amaroo and Doongara decreased from 7 to 2 t/ha with increasing N supply. For the later sowing date, yields of all varieties growing in shallow water decreased to <2 t/ha with applied N. In the deep water crops, developing microspores were submerged or partially submerged and so avoided low air minimum temperatures. In the early-sown crops, the microspore stage occurred during a period of relatively warm nights. Harvest index was successfully modelled using panicle temperature and N content at early pollen microspore. The experiment shows that high N applications can lead to high rice yields provided the microspores are protected from low temperatures by the use of deep water at the stage of microspore development. As a result, recommendations for N fertiliser application need to be adjusted for sowing date and expected water depth at early pollen microspore.