Abstract: In order to study the effect of short-day on the spike development of spring and summer sown oat in the northern China，for which five photoperiod-sensitive oat cultivars and one photoperiod-insensitive oat cultivar strain were planted，and the growth and development of the six oat cultivars planted in natural short-day in Hainan Base were compared. Results showed that illumination time decreased from 15 h to 11 h under shading condition，the spike differentiation process of photoperiod-sensitive oat cultivars was abnormal，and in morphology the spike development degraded seriously，thus their development was not complete. The photoperiod-insensitive oat cultivars’ spikes differentiated normally，and the whole growth and development completed. The experiments of the natural short-day in Hainan indicated that photoperiod-insensitive oat cultivars of spring and summer sown oats under natural short-day eared and seeded，and their whole growth and development completed；while the photoperiod-sensitive cultivars did not ear during growth period，and there was only vegetative growth but no reproductive growth. In addition，there was no difference in the morphologies by scanning electron microscope of different naked oat cultivars in normal spike differentiation. Under the short day of 11 hours per day，the photoperiod-sensitive oat cultivars only grew in vegetative growth stage，even those in the shortest growth periods did not ear and flower，thus resulted in no yield. There was no correlation between growth period and photoperiod-sensitivity of oat varieties.

Key words: oat, shading, short-day, spike differentiation, growth stage