LED lighting in greenhouse horticulture

Published on
January 26, 2015

The HI-LED project is an EU project with the aim to develop advanced LED modules for optimal lighting solutions for different applications. One of these applications is horticulture, where the use of assimilation lighting is essential for year-round production of high quality products. The amount of sunlight in greenhouses in the winter is insufficient to maintain growth and production of horticultural crops so that artificial lighting is necessary to supplement the amount of sunlight.

Which part of the spectrum is important for plant growth?

Plants use light for the process of photosynthesis (growth) as well as for photomorphogenesis (plant development and cellular metabolism). Plants use photosynthetically active radiation (PAR), light wave lengths between 400 and 700 nm for photosynthesis as shown in Figure 1.

Figure 1. Plant response in Photosynthetically Active radiation range
Figure 1. Plant response in Photosynthetically Active radiation range

This can be either sunlight or artificial light. The other light-driven process is called photomorphogenesis. PAR light is composed of different colors or wavelengths and what our eyes see is different from what a plant perceives and responds as shown in Figure 2.

Figure 2. Human eye response. Spectral response of rods  and cones cells on the retina
Figure 2. Human eye response. Spectral response of rods and cones cells on the retina

While the human eye sees especially green, yellow and orange colours, plants respond more to blue and red. Thus, different parts of the visible light spectrum, and some wavelengths just outside of PAR (see figure), are readily perceived by plants and trigger plant responses, resulting in changes in plant morphology and physiology. Morphogenesis determines plant architecture, flower colour and complex processes like flowering. In general, a light spectrum similar to that of sunlight is thought to guarantee a normal plant development.

What are recent developments in the use of artificial lighting in horticulture?

During the past decade light emitting diodes (LEDs) have made their introduction into horticulture. Starting with red and blue LEDs, obviously due to their contributions to growth (red) and morphogenesis (blue), LED modules with other wavelengths have made their introduction as well. Although possible to use monochromatic light, LEDs usually emit a band width of 20-50 nm with, for example, a peak at 450 nm for blue light. LED modules are used as top lighting and inter lighting in the greenhouse as well closely layered in plant propagation units.

What kind of horticultural experiments are done in HI-LED?

Within the framework of HI-LED, Wageningen UR Greenhouse Horticulture investigates the effects of light spectrum on growth, development and underlying plant processes in young tomato and pepper plants. Under a background of natural light, plants are grown under LEDs supplied by Hortilux in the colours white, red, blue, amber, green and a combination of red and blue (see Figure 3).

Figure 3. HI-LED experiment for growing tomato and pepper plants
Figure 3. HI-LED experiment for growing tomato and pepper plants

How are the spectral effects on plant growth evaluated?

We started our experiments with young tomato and pepper plants. They are grown for 3-4 weeks under the 6 light colours. During this period we measure plant length, photosynthesis, plant temperature, stomatal opening and leaf angles. At the end of the experiments, the plants are harvested destructively. We divide the plants into leaves and stems, count the number of leaves, measure their leaf area and weight and the weight of the stems. By measuring these parameters, we can establish the effects of spectral composition on elongation, assimilate production and partitioning and plant morphology. Under blue and green light we found the most outspoken effects: plants grown under green light were tallest with the largest leaf area whereas plants grown under blue light were shortest, had their stomates widest open, had low leaf temperatures and the lowest total plant weight.

What is the perspective for LED engines in greenhouse horticulture?

On 10 December 2014, 130 greenhouse growers visited the HI-LED trials in Bleiswijk, The Netherlands. They were highly interested in the effects of spectral composition on plant growth and development. Their questions focused on the applications and perspectives of using LEDs as steering light for their crops. Overall, their opinion was that we still need to gain more knowledge on the specific advantages of LEDs to influence plant development, but that LEDs offer great perspectives for production and product quality in greenhouse horticulture.