Protected Crops - Horticulture: Plants for People and Places - page 327

Fig. 10.3 The effect of photosynthetic daily light integral on ( a ) time to first flower, ( b ) in situ net
CO 2 assimilation rate, and ( c ) plant dry weight of Cyclamen persicum 'Metis Scarlet Red'. Plants
were grown under an 8-h short day ( SD ) or 16-h long day ( LD ). Error bars indicate SE. (Source:
Oh et al. 2009 )
In the literature, sometimes the term “daily light integral” (DLI) is used in order
to address the amount of photosynthetic light received each day per unit area.
Increasing DLI, generally, increases biomass accumulation, accelerates the de-
velopmental processes reduces the plant development phases, and improves final
plant quality of many protected crops. For instance, the days to flower of petu-
nia ( Petunia ) and cyclamen ( Cyclamen persicum cv 'Metis Scarlet Red') decrease
as DLI increases when grown at 20 °C (Kaczperski et al. 1991 ) (Fig. 10.3a ). In
addition, increasing the DLI increases growth rate by promoting photosynthesis
(Fig. 10.3b ) which improves the quality of this plant by increasing the number of
leaves and flowers, and dry weight (Fig. 10.3c ) (Oh et al. 2009 ).
The external and internal quality of vegetable products is also influenced by
light. Grierson and Kader ( 1986 ) reported that low radiation and temperature re-
duced tomato fruit dry matter content, due to insufficient sugar content and in the
pepper resulted in flower abscission (Aloni et al. 1996 ). Furthermore, Canadian
researchers Dorais et al. ( 2001 ) reported that misshapen tomato fruits, as well as
the formation of swollen and hollow fruits, due to low light intensity and inappro-
priate temperature regimes, were observed during the growing season in spring. In
addition, light could be the limiting factor influencing the nitrate concentrations in
green leafy vegetables, such as lettuce and spinach, under poor light conditions in
greenhouses during winter (Blom-Zandra and Lampe 1985 ; Steingröver et al. 1986 ).
In general, as the light intensity declines there is a reduction in the content of
ascorbic acid in plant tissues (Gruda 2005 ). This close relationship between the
light conditions and ascorbic acid content have been reported in vegetables, such as
spinach, tomato, lettuce, sweet pepper and strawberry. Gautier et al. ( 2009 ) stated
that for tomatoes leaf irradiance has an impact on photosynthesis and sugar transport
to the fruits, whereas fruit irradiance had an impact on ascorbic acid metabolism.
The effect of light and light intensity on carotenoid content in vegetable products
is at present being controversially discussed in the literature. McCollum ( 1954 ) has
shown that tomato fruits exposed to direct sunlight during their development had
higher carotene levels than shaded fruits while the rates of lycopene and carotene
synthesis can be increased by illuminating tomato plants during the ripening of
the fruit at favorable temperatures (22-25 °C). Keyhaninejad et al. ( 2012 ) work
was contrary to this where foliar carotenoid increased approximately twofold with