Cold treatment affects microspore development and induces IAA production in pollen sacs in tulip

In horticulture, tulip bulbs are generally lifted in summer and then stored at room temperature. Before forced for flowers, tulip bulbs need a period of low temperature to break dormancy for induction of floral stalk elongation and flowering after a complete flower formed in the center. Microsporogenesis of anthers occurs during the process of dormancy breaking of tulip bulbs. The microspore mother cells (MMCs) developed sequentially into tetrad microspores, microspores, vacuolated pollens, engorging pollens and mature pollens in bulbs cooled at 4 °C. Harvestable flowers with fully developed anthers and viable mature pollen grains were attained in these cooled bulbs. The MMCs did not undergo development until after the 9th storage week in 20 °C bulbs. The MMCs developed into tetrad microspores in 20 °C bulbs as did in 4 °C bulbs at the end of storage. However, the tetrad microspores were severely deformed in 20 °C bulbs and the four microspores did not separate from each other at later developmental stages. Bulbs of six cultivars stored at 20 °C did not produce harvestable flowers and five cultivars failed to produce viable mature pollen grains. Indole-3-acetic acid (IAA) was immunolocalized in anther pollen sacs in bulbs stored at 4 °C but not in bulbs stored at 20 °C, which indicates that IAA was biosynthesized in the bulbs during storage at cold. Results obtained suggest that low temperature induced production of IAA in the anthers and the IAA probably plays a vital role in the development of microspores.