Effect of far-red light in a LED background and in combination with sunlight on photosynthetic properties of Solanum lycopersicon.

MSc-thesis abstract (submitted 8 October 2015):
In the northern hemisphere, supplementary lighting is used for cultivation of plants in greenhouse horticulture to increase productivity. LED lamps are becoming an attractive alternative compared to traditional high pressure sodium (HPS) lamps since electrical efficiency of LED lamps is still increasing and heat emission is reduced. The narrow emission spectrum of LED lamps makes it possible to create a relatively well controlled light spectrum. Plants use light for photosynthesis, but also perceive light via photoreceptors resulting in a host of physiological and morphological responses. Far-red (FR) light is perceived by phytochrome photoreceptors, but can also enhance photosynthesis in conjunction with red light. The combination of red and FR light results in higher photosynthesis rates compared to applying red and FR light independently. This phenomenon is called the Emerson enhancement effect.

The aim of this experiment was to investigate the effect of FR light on the photosynthetic properties of greenhouse grown young tomato plants. Two experiments were performed. In the first experiment, only LED (95% red, 5% blue, 150 µmol m-2 s-1) light was used as background light, whereas in the second experiment, a limited amount of sunlight was added to the LED light. FR light was added in doses of 60, 130 and 170 µmol m-2 s-1, in addition to an end of day (EOD) FR treatment. It was hypothesized that photosynthesis rates are higher when continuous FR light was added, including changes in chlorophyll content and ratio and light absorbance.

Increasing continuous FR light intensity decreased the fraction of PAR and FR light absorbed by the leaves in both experiments. In addition, total chlorophyll content and chlorophyll a:b ratio were reduced, but the latter only in the first experiment. Net photosynthesis increased with 15% for leaves treated with 60 µmol m-2 s-1 FR light in the second experiment. However, leaves growing under the highest FR intensity of 170 mol m-2 s-1 showed no increased net photosynthesis. A possible underlying reason could be the reduced conductance.

A strong relationship was found between chlorophyll content and absorbed PAR fraction. A reduction in chlorophyll content resulted in a reduction in PAR absorption. Contrary to the lower light absorption for leaves growing under FR light, net photosynthesis is higher. It can be concluded that FR is indeed driving photosynthesis, in addition to altering the absorption spectrum as a consequence of changed pigment composition.

Further research is suggested to investigate the effect of plant architecture on local photosynthesis rates, and to assess the viability of using FR light in a conventional greenhouse environment with normal sunlight intensities.