Predicting Stem Elongation and Leaf Expansion: Percent far-red is a better predictor than PPE

In this video Dr. Bruce Bugbee explains why the measurement of percent far-red photons can be a better predictor of stem elongation and leaf expansion than the phytochrome photo-equilibrium (PPE). Measuring and controlling this is critical in crops grown under electric grow lights like leafy greens and cannabis.

Phytochrome photo-equilbrium (PPE) has long been a standard measurement in photobiology research, but the phytochrome molecule is shielded by multiple layers of chlorophyll and thus responds to transmitted photons more than incident photons. Recent studies under diverse LED spectral quality have found that stem elongation is poorly predicted by the PPE. In these studies, a simple measurement of percent far-red photons better predicted stem elongation and leaf expansion.

VIDEO CONTENT

01:21 Definitions of Phytochrome Photo-Equilibrium and % Far-red.

4:00 How did we come to use Phytochrome Photo-Equilibrium (PPE) and rely on it so much?

5:59 Four researchers who have studied the PPE ratio: Butler, Gardner and Graceffo, Kelly and Lagarias, and Sager. Each researcher(s) came up with their own weighting factors.

10:00 The problem. The four researchers did their measurements without measuring chlorophyll.

10:37 An explanation of the filtering effect of chlorophyll.

12:13 A list of the research papers Dr. Bruce Bugbee references in his presentation.

12:48 Dr. Bruce Bugbee and Paul Kusuma used the past research to develop a study. They discovered you can't predict stem elongation from PPE because it varies with the background color.

14:55 What happens if you take as simpler measurement and only do Percent Far-red? It results in a better prediction of leaf area and stem elongation.

16:10 Apogee Instruments developed a sensor, the new PAR-FAR sensor, with two detectors that separately measures PAR and Far-red. It quantifies far-red photons and for many applications reduces the need for a more complex measurement with a spectroradiometer.

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