Diurnal and Nocturnal Flowering

Individual flowers species bloom at different times. Hypochoeris radicata (False Dandelion) is a diurnal plant (blooms during the daytime) while Oenothera biennis (Common Evening Primrose) is nocturnal, and only blooms at night.

What causes or controls these different bloom periods?

Plants are highly evolved species. They are in tune with the environment and know how to respond based upon certain stimuli.These stimuli begin for a plant as seed. Seeds have dormancies programmed in that will only allow the seed to germinate under certain growing conditions. Lettuce seed, for instance, have three requirements for germination: Moisture, Temperature, and Correct wavelengths of light.

After germination, the plants continue to accept outside stimuli and respond. They initially decide to flower based upon a daylength (photoperiod), and a temperature. Flowering too early may limit vegetative growth, and accumulation of sufficient resources to produce viable seed. Flowering too late, may cause the seed to be produced under adverse conditions such as a frost that will kill a percentage of the seed.

After flowering, it needs an avenue to conserve resources to ensure adequate pollination by the most efficient pollinators for the type of flower. The opening of flowers during the day limits the availability of the nectar to only those pollinators active at that time. Whereas, flowers that bloom at night, allow their nectar and pollen to be gathered by moths and nocturnal bees. Closing up also limits the amount of nectar the plant has to produce. If it needs the nectar for particular solitary wasp. it doesn’t have to waste the resource on feeding a moth at night that may not give adequate pollination.

These periods are controlled by nyctinasty. Nyctinasty is a circadian rhythmic nastic movement.

That is a mouthful! So let’s break this down to have a better understanding.

Nastic movements are “non-directional responses  to stimuli (e.g. temperature, humidity, light, etc). In the terms we are using for flowers, it is called “photonastic”.

Ciracdian rhythms is a term describing molecular changes within an organism based upon solar time. Most plants and animals (even humans) have some form of circadian clock that regulates things like metabolic changes.

In a nutshell, the leaves of the plants have photoreceptors called phytochromes. These photoreceptors, based upon certain wavelengths of light (typically Red (R)/Far-Red(FR), signal the flower to open. The flowers opens and closes in either one of two ways.

The first is by new cell growth. To open, cell growth on the inside of the flower is initiated, and to close the growth of cells on the outside is initiated. The differential expansion of growing cells either “pulls” the flower head open, or “pushes” it closed.

The second is by expanding cells on the inside and outside of the flower by pumping water in and out of the cells (increasing or decreasing turgor pressure).

The latter is helpful when purchasing cut flowers. If you take the flowers that are not fully open, and cut the stem at an angle prior to putting it into a vase, it can absorb more water to use to open the flower, and will help your flower stay viable longer.

References:

  1. https://en.wikipedia.org/wiki/Nyctinasty
  2. https://en.wikipedia.org/wiki/Nastic_movements
  3. http://www.plantcell.org/content/18/4/792.full
  4. http://www.plantphysiol.org/content/121/1/9.full
  5. https://academic.oup.com/jxb/article/66/3/719/479209/Time-to-flower-interplay-between-photoperiod-and

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