What is Phytochrome Photostationary State (PSS)?

by | 2021/02/22 | blogs, Grow Lights, Lighting Metrics

PSS – Seedling to sprout to flowering plant

What is PSS?  Plants go from seedling to sprout,  to flowering plant during certain months – but how do they know when to transform?

For many plants, this seasonal transformation is encouraged by the position and the duration of the Sun as it traverses the sky.  We will look at a light metric called PSS to see how we can use Grow Lights to trigger the same change in indoor plants.

This has many practical ramifications for plant factories as they can simulate the seasons and regulate production throughout the year.  For instance, you can time the growth of your roses to go from seedling to sprout to flower, just in time for valentine’s day.

How does a plant know when to sprout?

“How does the position and the duration of the sun trigger photomorphogenesis (i.e. the change in form caused by light)?

There is a photoreceptor (protein) in seeds (and plants) called a phytochrome that absorbs red light – it’s original state or isoform is called Pr (the “r” standing for red light). When this phytochrome absorbs a red light photon, it changes its physical state from Pr to Pfr (“fr” means far red). When the Pfr absorbs a far red photon it changes back to Pr.


Phytochrome molecular red/far-red representation

Sunlight contains both red light and far red light, but during the day, sunlight contains more red light than far red light. It means that more Pr is transformed to Pfr than vice versa. At night, however, the Pfr phytochromes will naturally degrade back to Pr. In effect, during the day, Pfr concentration rises. During the night, Pr concentration rises.

It is an interplay of rising and falling Pr and Pfr phytochromes.  The ratio between these two will change because the days are longer in the summer. And the concentration of Pfr to Pr will eventually reach a threshold that triggers photomorphogenesis, the transformation from seed to sprout.

In the winter months, where the sun is lower on the horizons and the days are shorter, it’s the opposite. During the day, Pr phytochromes are still changing to Pfr phytochromes, and Pfr to Pr. However, more Pfr phytochromes are being degraded back to Pr because of longer nights. The lower concentration of Pfr to Pr prevents photomorphogenesis.

How can you get a plant to sprout with a Grow Light?

If you can control the ratio of red light to far red light, you can control the ratio of Pr to Pfr and subsequently regulate photomorphogenesis in seeds and in flowering of plants.

Phytochrome PSS or Phytochrome Photostationary State is simply the ratio of Pr phytochromes to the aggregate Pr + Pfr phychromes.

Of course you cannot directly count the Pfr and Pr phytochromes in a seedling to determine this ratio.  However you can measure how a light source will influence this ratio, because red light correlates with Pr and far red light correlates with Pfr.  As you can see, the PSS calculation for light has the same structure as the PSS calculated for phytochromes.

See Phytochrome and PSS by Ian Ashdown “All things Lighting” 20190215

A higher PSS will favor inhibition of photomorphogenesis.

A lower PSS will favor photomorphogenesis

PSS is basically a ratio of Pr to Pfr

LED lights and PSS

LEDs have had a big impact on agriculture, because the SPD can be tuned for wavelengths needed for photosynthesis.  Indoor agriculture lighting is now more efficient and energy-saving as ever.  And now these same technologies can be applied to photomorphogenesis and red/far red light.

However, LEDs are inherently imprecise (Binning and CCT) and it does present the question of hitting the wavelength targets accurately.

For a long time, precision light measuring instrumentation was not readily available for accurately measuring and implementing red and far red light.  This is where products like the PG200N  spectrometers are important, a portable device with precision measurement to help agriculturists obtain the best lights for PSS based grow strategies.

pg200n and red-blue LED

Flowering plants and PSS

Plants follow the same basic mechanisms for flowering at certain times of the year.  However different plants will flower at different seasons.  Some plants flower when the days are longer (summer) – we call them “long day plants”.  Others can flower when the days are shorter (sprint/winter) – they are called “short day plants”.  As you might have guessed the PSS ratio threshold to trigger flowering is opposite between long day and short day plants.

 Controlling the Seasons

If farmers can accurately implement PSS strategies and control seasonal plant activities, it has important ramifications for the indoor agriculture industry.   With the advent of new LED technologies and cost-effective spectrometers, the opportunities are more promising than ever. 

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