More and more tomato growers both in the Netherlands and abroad are opting to use diffuse glass in new greenhouses. A literature review was carried out to examine the effect of a diffuse greenhouse cover or a greenhouse cover with a different light spectrum on pollination and fruit setting in tomatoes. Information was also collected on the influence of the light spectrum on insects and bumblebees. This desk study was carried out on the request of Koppert Biological Systems. Unfortunately, the amount of information found on the subject proved to be limited.
The extent of diffusivity or light scattering that occurs is expressed in terms of haze or F-scatter. The amount of diffuse light that occurs as a proportion of total radiation depends greatly on the area of cultivation and the time of year. In temperate climates, greenhouse covers that provide a high degree of light transmission are important for production. Direct light can be diffused by using diffuse glass, plastic films, moveable screens, or by applying a coating on the covering. Whitewash is particularly used on greenhouses in Mediterranean regions to keep out thermal radiation and to diffuse the light.
Positive effects of diffuse light
Diffuse light penetrates deeper into the crop canopy, allows for a greater horizontal spread of light, increases photosynthesis, and – with high solar radiation – lowers the plant head temperature and hence also the risk of photo-inhibition.
Diffuse glass can increase the production of tomatoes by around 10%, provided the light transmission of the greenhouse cover remains at the same level. Fruits become clearly larger, and sometimes increase in quantity, which could be the result of an increase in the number of fruit set due to improved flower quality. It is not known, however, whether there is an increase in the number of seeds per fruit under diffuse glass.
The global radiation that penetrates the glass cover can be divided into ultraviolet radiation (UV), photosynthetic active radiation (PAR), and near-infrared radiation (NIR). UV radiation can, in turn, be divided into three bandwidths: UV-A (320-400 nm), UV-B (280-320 nm), and UV-C (100-280 nm). PAR radiation falls within the range of wavelengths between 400 nm and 700 nm and NIR radiation between 800 nm and 2500 nm.
The light spectrum in the greenhouse can be altered by using plastic films, screens, and gauze or nets. The colour of the material influences spectral light penetration by absorbing its complementary colour.
Photoselective greenhouse covering materials
In an Israeli study into the use of different coloured nets, red and pearl nets resulted in a higher production of tomatoes than black and blue nets. The most probable reasons for this are improved flower quality and setting, better fruit growth, and lower temperatures with large amounts of sunlight when compared to the more commonly used black nets.
Influence of photoselective materials on diseases and pests
Yellow and pearl nets reduce virus infections by a factor of between 2 and 10 when compared to red and black nets. Pearl nets repel insects through light reflection and yellow nets cause insects to remain on the nets for longer, which reduces the time they spend transmitting viruses to the plants.
Insects use light signals to find their prey or host plant. There is a positive link between the extent of UV absorption in the greenhouse cover and the level of protection against insects, such as whiteflies, thrips, and aphids. However, limiting the amount of UV in the greenhouse can also have a negative effect on the search behaviour of useful insects, such as predators and bumblebees. The pollination activities of bumblebees are particularly reduced at temperatures above 30oC.