Factors affecting flower initiation and development in Dutch iris
Flower initiation and development to anthesis in Dutch iris were investigated using three cultivars, 'Wedgwood', 'Ideal' and 'Professor Blaauw'. In particular, environmental effects on flower initiation and on flower abortion were studied. With the aid of a thermal-time model, the rate of initiation (the reciprocal of time to initiation) was shown to be linearly related to the temperature used to store dry bulbs, provided that this was constant. When bulbs were transferred from one temperature to another, however, the rates of initiation differed from those expected. It was concluded that the sequence of temperatures was important and a possible mechanism for flower induction was discussed. Following the start of flower initiation per se, a different optimum temperature for flower-organ differentiation was observed. The effect of both pre-planting and post-planting temperatures on the growth and development of the flower were investigated. The proportion of bulbs with successful flower development to anthesis increased with mean flower and stem dry weight. The heaviest flower and stems were produced by those bulbs with the greatest leaf weight. This was attributed to the increased availability of current photosynthates in plants with the largest foliage frames. Flower development was affected by an interaction between time of planting, bulb size and glasshouse temperature for a given light integral. With later bulb plantings, between January and March, higher light integrals were required for the same flower development at moderately high temperatures (16-18°C). This higher light requirement was detected with the largest bulbs first and not until the last planting date with smaller bulbs. At higher temperatures (20°C) flower development was poor regardless of the light integral. At lower temperatures (14°C) the same flower development achieved regardless of the light integral within the range tested. Further investigation in controlled environments enabled the detection of a photoperiodic effect at the moderately high temperatures. Daughter-bulb growth was promoted by long photoperiods and high temperatures increasing sink strength there for assimilates with a corresponding decrease in flower development. The partitioning of assimilates under high and low light was examined by determining the distribution of 14C-labelled assimilates during growth in the glasshouse. Daughter bulbs under low light had a higher relative specific activity than those under high light, at the time when the flower was most prone to abortion. Application of cytokinin to the flower bud resulted in a reduced daughter bulb weight, but heavier flower buds. It was concluded that flower development was affected by the total current assimilate available and the partitioning of these assimilates between daughter bulbs and flower bud.