Japanese morning glories (Ipomoea nil) are short-day plants. This means they will start flowering under day lengths shorter than a certain length.

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Morning glory growers can use this knowledge to their advantage and induce flowering at will instead of waiting for the seasonal daylight period to do their thing. This way we can enjoy the flowers much earlier in the season and ensure the seed will ripen before the end of summer in short-season climates.

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Critical Night Length

In case of Ipomoea nil particularly it is the length of the dark period which regulates the flowering phase. The minimum length of the dark period is called the critical dark period or critical night length and has been established as 9 hours for Ipomoea nil. Experiments have shown a single critical dark period of 9 hours has been found sufficient to induce flowering in Ipomoea nil.

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Night Break

To effectively regulate photoperiodic flowering the dark period must be constant. The flower-inducing effect of the dark period is cancelled when the night period is interrupted by light. This means a light period inserted in the middle of a long night will inhibit flowering of short-day plants. This effect of the inserted light period is called a night break. Even a short exposure, sometimes even only for a few minutes, is effective as a night break.

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Photoperiodic Perception

Flowering is induced in short-day plants when their leaves don't receive light for a period of time longer than the critical night length. In Ipomoea nil the cotyledons have an especially high sensitivity to the light period. Research has concluded that photoperiod-sensitive plants like Ipomoea nil can be induced to flower by exposing a single leaf to a suitable photoperiodic cycle. The minimum area of a leaf or a cotyledon sufficient for flower induction has been established as only 1 cm².

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Stress-Induced Flowering

Ipomoea nil can also be induced to flower under constant light when grown under high stress conditions. These include low nutrients, low temperatures or high light intensity conditions. The presence of cotyledons seems essential for stress-induced flowering. This suggests that the cotyledons exposed to stress produce flower-inducing compounds.