Photoperiod and Flowering

One of the most important environmental factors affecting flowering induction is Photoperiod. Photoperiod is defined as the time plants are exposed to light in a daily cycle.

Plants can be classified based on photoperiodic response in day-neutral, short-day and long-day plants. Day-neutral plants flower at the same time irrespective of the photoperiod. Short-day plants are plants that require a photoperiod shorter than a specific threshold (called ‘critical photoperiod’) to initiate flowers (qualitative short-day response) or plants that flower initiation is accelerated by shorter photoperiods (quantitative short-day response). Long-day plants are plants that require a photoperiod longer than a specific threshold to initiate flowers (qualitative long-day response) or plants that flower initiation is accelerated (quantitative long-day response) by longer photoperiods. 

Plants with photoperiodic response (short-day and long-day plants) are further classified into 2 groups based on the specific response to varied photoperiods: qualitative and quantitative response. Qualitative response in plants happens when a particular photoperiod is an absolute requirement for the occurrence of flowering initiation. In contrast, quantitative response is when plant flower initiation takes place in both short and long days but one condition accelerates flowering initiation. The degree of quantitative responses is typically cultivar-specific and cultivars with weaker quantitative responses may be mistakenly considered as day-neutral plants. 

But, how does photoperiod affect flowering?

Previous research has demonstrated that plants respond to the darkness period in each day cycle. For example, Short-day plants will not flower under a short dark period, which is linked to a long day. 

Based on the fact that plants respond to dark periods we can then learn that night interruption can break one long night in two short nights. In this case short nights are related to long day conditions meaning flowering induction or flowering inhibition (depending on the type of plant) can be controlled by night interruption. Why is this helpful? When trying to promote long-day conditions we usually turn on lights for longer periods. However by doing night interruption, less electric light is needed. Therefore money can be saved. 

Plants of course can have a benefit for longer days which can also promote photosynthesis, growth and development. But if your intention is only to control the photoperiod, then night interruption can be a good option.

Other important information when looking to induce a photoperiodic response is to learn that plants can have a response to photoperiod from very low light levels (Around 2 μmol m-2 s-1). This is why specific lamps are designed to induce photoperiodic response. Flowering lamps are usually low intensity. These lamps provide good characteristics in light quantity and quality to induce a photoperiodic response. We also need to learn flowering lamps are not designed to promote strong effects in photosynthesis or growth. They are designed to trigger just a photoperiodic response. 

Light quality in flowering induction

Short-day and long-day plants have different photoperiodic response mechanisms, involving components of photo-perception and biological timing. The roles of photoreceptors are essential in the signaling of the photoperiodic control mechanism. 

Plant photoreceptors are light-sensitive molecules conformed by a protein and a light sensible pigment capable of regulating flowering by altering the expression of a key flowering regulator. The main photoreceptors involved in flowering regulation are the red/far-red light receptors, called phytochromes. Phytochromes are proteins responsible for photoperiod sensitivity in plants that utilize a linear tetrapyrrole, bilin chromophore to sense red/far-red light, existing in two inter-convertible forms. 

One form of phytochrome is Pr, which is considered the inactive form of phytochrome and has a primary absorption peak at the wavelength of 660 nm (red) and a secondary absorption peak at wavelength around 380 nm (UV-A). The other form of phytochrome is Pfr, which is considered the active form of phytochrome with an absorption peak at 730 nm far-red wavelengths and a smaller peak at 408 nm blue wavelengths.

Pfr accumulation can inhibit flowering in short-day plants and have the exact opposite effect (Flower induction) in long-day plants. During the dark period Pfr changes to Pr form. This is why long periods of darkness in short-day plants promote flowering. If we do a brief exposure with red light during a dark period, Pr will convert again to Pfr form. This is why night interruption works! In summary, night interruption can be used to avoid flowering in short-day plants or promote flowering in long-day plants.

Research applications

There are other factors affecting flowering response. Some good examples are temperature and Daily Light Integral (DLI). This is why flowering lamps can be really useful for research purposes.

When trying to test photoperiodic treatments and response in plants, it is recommended to extend the period of light artificially under the same temperature and maintain the same DLI within the treatments. In order to maintain the same DLI, the extended period of light should be really low light intensity. By applying treatments correctly we will be able to measure only the effect of photoperiod in flowering response. 

Greenhouse application

We can use different lamps in greenhouse operations to promote or inhibit flowering. Regular lamps designed for greenhouse operation to promote higher DLI (e.g. L1000 by Current) can also be used to induce a photoperiodic response. By adding light using regular growing lamps we can increase DLI, which can be good for different crops and also be useful to create longer days when looking to promote flowering in long-day plants or inhibits flowering in short-day plants. If your objective is only to induce a photoperiodic response, then night interruption can be used.

In order to create a short day in a greenhouse, we usually use covers to eliminate natural light, when natural light is above the threshold for a short day plant. Other options include to move plants or transplants from the greenhouse to a growth chamber in darkness for specific periods of time in order to induce flowering in short-day plants.

There is always an advantage in learning about photoperiod when working with long and short-day plants. Remember the more you know the better you can manage not only your growing system. But also your investment of capital for your projects… and of course! If you need any help in the process Hort Americas will be on board to help you make the best decisions for your projects.