Abstract
To clarify at what stage of development flower buds are subject to blasting by high temperature, a correlation between the length of perianths and the stage of pollen formation in a miniature Cymbidium orchid (C. × Sazanami 'Haru-no-umi') was investigated.
1. High correlations between the variables were obtained during the stages of sporogenous cells and pollen mother cells (PMC) regardless of the position of the flower buds. Flower buds exposed to high temperatures (30°C day/25°C night) at sporogenous cell stage became blasted without entering into the meiotic stage. Similarly, buds which were heated at premeiotic stage became blasted at various stages after chromosomes underwent meiosis.
2. Flower buds reaching the meiotic prophase of PMC never responded to the high temperature and bloomed normally.
3. The application of gibberellic acid (GA3) reduced the low temperature requirement for bud development. GA3-treated buds entered the meiotic cell differentiation stage and proceeded to develop normally with mature viable pollen, even when the GA3 spray was combined with the high temperature treatment.
4. The results indicate that buds at premeiotic stage are very sensitive to blasting by exposure to high temperature, and that temperatures below 20°C are required for cells to undergo meiosis, form mature pollen, and develop normal flowers.
5. Different patterns of microspore arrangement were observed even in the same anther.Most uninucleate microspores of blasted flower buds were diads, fewer progressed to tetrads.On the other hand, binucleate microspores of both blasted and normal flower buds were mainly in the tetrad stage. The patterns and kinds of tetrads were not changed by the GA3 treatment, whether the buds were blasted or normal.
