Light metering: Red / Far-Red

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Ignacio del Rey

Published on 3 February 2020

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The light is the engine of plant growth and the red/red-leaf ratio is a fundamental pillar of its development.

Their self-sufficiency in matters of nutrition still amazes us today. Few ingredients are needed for plant life to develop normally. But light is not only used for food. From light they also receive other really significant stimuli for development.

Influence of tree shade grows a fine, elongated lawn.

Plants develop on the basis of the spectrum of light they receive.

Plants absorb red light (660-680 nm) and reflect far-red light (720-740 nm). The photoreceptors that control the physiological developmentare called phytochromes. These phytochromes react to fluctuations in red and far-red light levels.

Red/far red spectrum light absorption meter

Powerful red - far red light meter.

Nanometres	Name	Effects
280-315	Ultra violet	Pequeña influencia en la morfología y en los procesos fisiológicos. Blanquea los colores, provoca quemaduras y fomenta la esporación de algunos mushrooms.
315-400	Ultra violet - Blue	Slight absorption by chlorophyll, influences photoperiodism, inhibits cell elongation, causes burns and promotes sporulation of some fungi.
400-520	Blue	High chlorophyll uptake, high photosynthetic influence.
520-610	Green	Low absorption by pigments.
610-750	Red	Low chlorophyll uptake. High influence on photosynthesis and photojournalism. Blocking it can slow down cell elongation.
750-1000	Far Red	Low uptake, strongly stimulates cell elongation, strong influence on germination and flowering.
+1000	Infra red	High impact on the incident surface temperature.

The far red causes weak growth and desperate movements of the plants to look for intense light.

The most significant responses to the red - far-red relationship are:

Seed germination
- Sowing, weeds...
Stem elongation
- When the far red levels are high it means that there is little direct light so the plant needs to stretch out in search of light intensity.
Flowering
- This ratio determines the time of flowering.
Gene expression
- Development of crop-specific characteristics
Desarrollo de más vegetation cover y de más concentración de cloroplastos.

The leaf tissues act as a filter by letting more far-red light through than red light. This changes the ratio of red to far-red light coming from the sun. This phenomenon occurs in plants with many leaves, causing those that do not receive direct light to have a low ratio of red to far-red or when the density of plants is high and they block each other.

Example of normal Red - Far Red ratios
Sunlight	1,2
Under the leaves	0,13
Under 5 mm of soil	0,88

Knowing the red-to-far-red ratio helps determine plant spacing and determines key times to apply growth regulators. By making precise applications of the regulators and having the right spacing, advantages such as more uniform, bushy growth or larger fruits are achieved.

PAR meter

PAR light sensor

The quality of the plant depends on the quality of the light (spectral composition). A light with too much far red is poor in terms of growth.

In the words of Peggy McMahon, associate professor in the department of horticulture and crop science at Ohio State University. Far-red light plays a critical role in plant growth. It is an important factor in promoting the shade avoidance response (elongation and stretching of stems) in plants. Far red cannot be detected with the naked eye, so having a meter that allows us to know how much red and far red light (antagonistic in their effects) the plants are receiving, especially that which hits the leaf beam is very useful. The meter can help the grower to space the plants or maintain a suitable height so that the plants have the space they need and at the same time do not waste space.