The effect of additional far-red LED light and increasing sink strength on the yield of Solanum lycopersicum.

MSc-thesis abstract (submitted 9 May 2016):
The sun provides photons which are essential to drive the photo-chemical process of photosynthesis. However, global radiation gets added increasingly by substitutes like the traditional high pressure sodium (HPS) lamps or light emitting diodes (LED). These two light sources especially differ in their emission spectrum, since LED can emit a monochromatic light beam in a narrow wavelength band. Besides the use of photons for carbon fixation, light is perceived by photoreceptors affecting the plant physiologically and morphologically.

The effect of far-red (FR) light is investigated for more than 50 years and due to the development in LED this continues. From other studies it is known that these specific wavelengths affect elongation, rate of photosynthesis, rate of fruit development and fruit dry matter content. If more assimilates are formed, a yield increase can be expected. The partitioning of these extra assimilates depend on the vegetative and generative sink strength. Since only tomatoes are of value for a grower, increasing sink strength (less fruit pruning) can potentially positively influence yield. However, on the long term it is not recommend to partition all biomass to the fruits, assuming a high yield is desired, otherwise photosynthetic activities get limited due to insufficient light interception.

The aim of this experiment was to determine whether tomato plants respond to an FR enriched environment, and an increased sink strength to elucidate the value of adding FR LED light to a supplementary lighting system. Hypothesized was that FR caused a more uniform light distribution, higher assimilate production and an increased rate of fruit development and photosynthesis. Therefore, it was expected that yield would be positively influenced if plants were exposed to additional FR light and increased sink strength.
Grafted tomato plants (cv. Komeett) were grown in two greenhouse compartments, both supplied with 186 μmol m-2 s-1 red & blue LED light. One compartment was equipped with additional top FR LED light which was split into two sections: a low (31 μmol m-2 s-1) and high (55 μmol m-2 s-1) intensity FR. Furthermore, regular and increased fruit loads were applied at every light recipe, giving a total of six treatments.

The addition of FR light caused an increase in fruit production by 19% (high FR intensity) and 8% (low FR intensity). This was due to an increased rate of photosynthesis, increase in dry matter production and an enhanced fraction of dry matter partitioned into the fruits. Despite the elongation of the stem, no clear change was found in the light distribution and interception. Total fruit dry matter content increased and an increase in sink strength (allowing more fruits per truss) increased fruit yield, irrespective of whether or not additional FR light was used.

Further research is needed to investigate the effects of the spectral environment on plant processes and to assess viability in commercial practice.