Protected Crops - Horticulture: Plants for People and Places - page 326

and leaves, as the intensity of rays passing through the droplet is increased as they
act like a lens. To avoid such condensation the addition of additives alters the sur-
face tension of the film (NN 2012a ).
Anti-Dust Dust particles tend to adhere to polyethylene films. Over a long period
of exposure considerable accumulation of dust may lead to significant reductions
in light transmission (Möller et al. 2010 ). This can influence radiation levels and
has a negative effect on plants resulting in lower yield and slower growth. Special
additives, which migrate to the surface of the film, can prevent dust accumulation
(NN 2012a ).
The Influence of Light Intensity and Duration on Plant Growth
and Product Quality of Horticultural Plants
The primary energy source for protected crops is through natural solar radiation.
This source is used throughout the photosynthetic processes that converts the light
energy into chemical energy and accumulates as useful biomass. In addition, light
plays an important role in controlling the different biological processes, such as
germination and flowering and determines plant morphogenesis.
From the total light reaching the plants in the protected cultivation area, only a
very small part is used for the photosynthetic process, the remainder is reflected
or absorbed and converted into heat. Generally, there is a very strong correlation
between the crop yield and the total amount of PAR intercepted by the plants. Apart
from the light intensity, the light duration and the spectral quality are of crucial
importance with these variables essential in plant growth, the production processes,
and ultimately product quality. In the following an overview of the effects of these
variables under the specifics of protected cultivation is given.
Low Light Intensity
It is commonly accepted that yield is roughly linearly proportional to radiation (up
to the saturation level). Marcelis et al. ( 2006 ) demonstrated that for many green-
house crops, a 1 % light increment can result in a 0.5-1 % increase in harvested
product, and the effect was larger in winter than in summer. On short and cloudy
days, and during the winter period in northern latitudes, low light intensity becomes
the most limiting climatic factor in greenhouses. The same holds true even for cool
season plants such as lettuce grown in winter in the higher northern European lati-
tudes.
Low yields are often associated with reduced product quality. Light intensity can
influence the plant architecture, apart from genotype, other climate factors, and cul-
tivation practices. For instance, at low light intensities plants are generally elongated
and have less and longer internodes (etiolation) than plants cultivated under higher
light intensities but otherwise similar conditions. This is important for ornamental
potted plants which, in order to meet a particular market, have to be compact.