INTERACTION OF TEMPERATURE AND DAY LENGTH IN FLOWER INITIATION

As far as time permits I should like to treat the, as I think, most important ones without, however, attaching equal importance to each individual effect. I shall therefore restrict treatment of vernalization to a minimum.

1) If the temperature is varied over a sufficient range in an experiment during the flower inducing period, a temperature maximum, a temperature minimum and an optimum temperature will be observed. These cardinal points vary from one species of plant to another and from one variety to another. With short-day plant these temperature optima normally lie around 20–25°C (for example (5)), with certain long-day plants the optima are apparently lower. The temperature-minimum for flower reaction seems to vary considerably among various species of plants.

On the basis of such experiments we are unable to say how photoperiodical and other partial processes depend on the temperature. For flower formation several partial processes are undoubtedly necessary which depend on the temperature, probably in various manners so that for example we can consider the optimum observed in an experiment to be the resultant of the effects of temperature on the various partial processes.

Since certain partial processes depend not alone on the temperature but also on other factors we can assume that on varying these factors, a shift for example of the temperature optimum will be observed, possibly on alteration of the light intensity or the nitrogen nutrition (fig.1). Little research has as yet been carried out on these interactions since they are relatively inaccessible to experimental methods. They also appear to be of little importance for understanding the physiology of flower-formation. However, they can be of help for understanding flowering reactions in practice.

Research has frequently been carried out on the effect of day and night temperatures and on whether a difference exists in the optima. In most cases no difference has been observed but with short-day plants it has been noticed that the temperatures at night are more important than the day temperatures (5, 27). This may to an extent result from the fact that in such experiments the various temperatures were allowed to act for a time corresponding to the light and dark periods so that the night

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