J.A. Schipper, G.G.M. van der Valk
Before the introduction of the ethylene treatment of iris a serious loss of flowers through bud blast during the darkest period of the year (December-January) was normal. After the introduction of the C2H4 treatment this changed, and the phenomenon of colourless flowers during the same period of the year, the so-called 'pale flowers', was observed. Until recently, too low light intensity or too few hours of light were considered to be the cause of the phenomenon. However, the sudden appearance of this phenomenon at different holdings under different circumstances threw some doubt on the importance of the role of light. It was suggested that the transpiration of the plants could be of importance as well.

In experiments, set up to investigate this, iris bulbs were grown under plastic tents, some of which were ventilated for 8 hours per day with air of a low humidity (the air temperature remaining constant). In this way, during a certain period of the day the humidity in these tents was lowered and the transpiration of the plants was stimulated. Periods without ventilation and periods with ventilation could be alternated.

When an unventilated period was followed by a ventilated period the transpiration of the plants increased drastically. The mean transpiration per day measured over the whole forcing period was higher in this case than after a ventilated beginning, followed by an unventilated period. An increase in the percentage of pale flowers was observed after low transpiration rates at the end of the culture period. The longer the unventilated period at the end of the culture, the higher the percentage of pale flowers. However, unventilated conditions at the beginning of the culture could be maintained for 35 days before the 'pale flower' phenomenon was seen.

The calcium uptake rose when the mean transpiration per day increased. Nevertheless, the relation between calcium concentration in the plant and the appearance of 'pale flowers' was not quite clear.

Schipper, J.A. and van der Valk, G.G.M. (1990). PALE FLOWERS IN IRIS CV. 'IDEAL'. Acta Hortic. 266, 277-284
DOI: 10.17660/ActaHortic.1990.266.36

Acta Horticulturae