Protected Crops - Horticulture: Plants for People and Places - page 329

in greenhouse tomato production. However, recently the reasons are to be seen
more and more in ensuring a year-round production and improved quality, which
meets market demands and a more regular labor requirement (Marcelis et al. 2002 ;
Paradiso et al. 2011 ). At present approximately 90 % of rose growers in The Neth-
erlands use SAL, while the use of this form of lighting for other cut flowers, orna-
mental and vegetable crops is increasing at about 1 % each year (Heuvelink et al.
2006 ; Marcelis et al. 2002 ).
Several studies reported better external and internal quality of horticultural
crops. For example, increasing photosynthetic photon flux (PPF) increased plant
quantity of Petunia × hybrida flower mass grown in climate chambers (Frantz and
Ling 2011 ), and increased the number of flowering shoots and inflorescence size
of Kalanchoe ( Carvalho et al. 2006 ). Dorais and Gosselin ( 2002 ) reported a higher
sugar content and ascorbic acid concentration in tomato, and Gaudreau et al. ( 1994 )
documented increased head firmness of lettuce as a result of supplemental light.
Applications of SAL from 18 October until 20 March in a glasshouse in The
Netherlands, improved the yield at a light intensity of 188 μmol m −2 s −1 in com-
parison to a 125 μmol m −2 s −1 , by increased fruit set and average fruit weight for
two cultivars of sweet pepper, when light was used between sun rise and sun set
(Heuvelink et al. 2006 ). These authors concluded however that the use of SAL is
not economically feasible for sweet pepper, tomato, and eggplant due to high energy
and production costs. The position of lamps is important as well. Usually SAL is
applied on the top of the canopy. Under such lighting systems, light is not uniformly
distributed along the leaf layers of some crops such as e.g. tomatoes, cucumbers,
and peppers that are usually vertically cultivated, or roses as well as other plants
that are usually grown under high plant densities. Particularly, it has been calculated
that, considering a crop with a leaf area index (LAI) of 3, even when the light inten-
sity at the top of the plant is 400 µmol m −2 s −1 , approximately 33 % of the leaves in
the lower and inner zone of the canopy receive less than 100 µmol m −2 s −1 because
of self-shading (Paradiso et al. 2011 ).
Both light absorbance and the vertical distribution of light in the canopy are
of great importance for crop photosynthesis. Heuvelink et al. ( 2006 ) reported that
leaves low in the canopy, received higher light levels every day because of inter-
lighting, performed at their maximum photosynthetic capacity, although leaf age
and the time of leaf-removing, a cultural practice of lowering high-wire crops,
needs to be taken into account. However Pettersen et al. (2010) found in an experi-
ment with horizontally grown cucumbers, that the leaves showed no sign of reduc-
tion in photosynthetic capacity rate although the oldest leaves were approximately
30 days older than leaves at the moment of removal in a high-wire cultivated cu-
cumber crop.
Many species demonstrate benefits from interlighting or inner canopy lighting.
Grodzinski et al. ( 1999 ) found an increased photosynthetic activity in sweet pepper
canopy when side lighting was used jointly with top lighting, whereas Hovi et al.
( 2004 ) stated a 9 % increase in annual cucumber yield in Southern Finland, when
24 % of the SAL was supplied between the plants instead of all light on top of the