Far-red (FR) light, situated at the extreme end of the visible light spectrum with wavelengths ranging from 700 to 780 nanometers (nm), plays a pivotal role in plant growth and yield. Despite its dim visibility to the human eye, recent research has unveiled its profound impact on photosynthesis, plant behaviour, and overall crop production. This exploration delves into the multifaceted aspects of far-red light and its implications for indoor plant cultivation, emphasizing the need for tailored lighting strategies.
The Emerson effect, discovered by Robert Emerson in 1957, sheds light on the cooperative interaction between shorter and longer
wavelength photons in driving efficient photosynthesis. This phenomenon involves two photosystems utilizing red and far-red photons
to pump up electrons, ultimately contributing to the synthesis of essential biochemicals, including sugars. The efficiency of this
system peaks when both photosystems work synergistically, akin to two pump mechanisms transporting energy.
Beyond its role in photosynthesis, far-red light significantly influences plant behaviour. Plants perceive an increase in far-red
photons as an environmental signal indicating shading. In response, plants undergo morphological changes to outcompete neighbouring plants,
such as elongating stems and leaf petioles. This response extends beyond mere physical growth; it influences branching, leaf area, and the
production of biochemical compounds, thereby shaping overall plant appearance and yield.
Phytochromes, a group of receptors sensitive to far-red light, act as molecular switches regulating various biological processes
in plants. Absorption of far-red light converts a phytochrome to the red-absorbing form (Pr), while absorption of red light converts it back
to a form that can absorb far-red light (Pfr). This dynamic interplay serves as a mechanism for plants to gauge their light environment and
adjust physiological processes accordingly. Phytochromes play a crucial role in regulating flowering time in response to day length, distinguishing between long-day and short-day plants.
By manipulating the red to far-red ratio in LED lights, the flowering time of photoperiod-sensitive plants can be controlled.
For instance, short-day plants can have their flowering inhibited by extending the day with light having a high red to far-red ratio. Conversely,
night interruption with far-red light alone does not inhibit flowering and, in some cases, can even enhance flower set.
The impact of far-red light extends to fruit yield, as demonstrated by experiments at Wageningen University. Tomatoes grown under a far-red-enriched
spectrum exhibited increased total fruit biomass, higher fruit number per plant, and greater average fruit fresh weight (Kalaitzoglou et al., 2019). However,
researchers noted that the enhanced yield might come at the cost of lower resistance to pathogens, highlighting the need for a balanced approach.
In the realm of indoor plant production, the proportion of far-red in the grow light spectrum emerges as a critical factor influencing three major determinants
of crop yield: photon conversion efficiency, radiation capture effectiveness, and dry matter partitioning. The inclusion of far-red had a positive impact on biomass and plant structure,
with no adverse effects on total plant biomass in some species.
However, the response to far-red is not universal across all plant species. The type of response can vary substantially,
necessitating a careful adjustment of grow light spectra and far-red ratios to achieve desired outcomes without compromising plant stability.
For instance, rosette leaves in lettuces responded positively to far-red, while taller herbs like basil exhibited enhanced height without direct effects on biomass.
In conclusion, far-red light emerges as a crucial factor in optimizing indoor plant production. Its influence on photosynthesis, plant behavior,
flowering, and fruit yield underscores the need for tailored lighting strategies. As we delve into the intricate interplay of light and plant physiology,
the importance of a nuanced approach becomes evident. Consulting specialists and customizing lighting strategies based on specific cultivation goals and plant
characteristics are essential steps towards harnessing the full potential of far-red light in indoor agriculture.