The microspore stage in rice is when the pollen cells start to form in the anthers. Low temperatures at this stage disrupt the movement of sugars into the microspores, reducing the number of viable pollen grains produced, leading to floret sterility.
Microspore occurs in the middle of the reproductive period, about 14-16 days after panicle initiation (PI) and 12-14 days before flowering.
At this time the developing panicles are approximately 12-13 cm long. It normally takes 2-3 days for a panicle to complete microspore but 7 days or more for a field, due to crop variability.
The timing of rice sowing is aimed at the crop reaching microspore between the 15th January and the 5th February, as this is when the warmest night temperatures occur. This is why there are different optimum sowing windows for each variety and sowing method.