C. Parameters of Composite Lighting
Using the symbols given in Figure 2A for the general case of composite lighting, the two basic parame-
ters of composite lighting could be calculated as follows. The daily integrated PPF is given by
QPPF 1 P 1 PPF 2 P 2 (1)whereQis daily integrated PPF in mol m^2 day^1 , PPF 1 and PPF 2 are component instantaneous PPF in
mol m^2 sec^1 , and P 1 andP 2 are photoperiods in hr of the component instantaneous PPF.
The average instantaneous PPF is given by
PPFavePPF (^1)
P 1
P
1
P 2
PPF (^2)
P 1
P
2
P 2
(2)
where PPFaveis average instantaneous PPF in mol m^2 sec^1 , PPF 1 and PPF 2 are component instanta-
neous PPF in mol m^2 sec^1 , and P 1 andP 2 are photoperiods in hr of the component instantaneous PPF.
Note that, given Eq. (1), Eq. (2) may also be expressed as
PPFave
P 1 Q
P 2(3)
III. PHYSIOLOGICAL BASES
There are two major physiological bases for the premise that, for a given constant daily integrated PPF, a
given lighting profile can significantly affect crop growth or yield: (1) the duration of the dark period im-
plemented by the lighting profile, which affects the extent of the crop’s dark respiration, and (2) the
average instantaneous PPF implemented by the lighting profile, which determines in large measure the
crop’s light compensation point (LCP) at a given air temperature and CO 2 concentration.
A. For a Constant Daily Integrated PPF, a Longer Daily Photoperiod
Results in Lower Maintenance Respiration, Which Translates into
Greater GrowthStudies have shown that reduction in the maintenance component of dark respiration could improve a
crop’s carbon balance and ultimately its yield [7–11]. Indeed, there is a preponderance of evidence in the
literature showing that respiration rates and growth are negatively correlated. Heichel [12], working with
two varieties of maize seedlings (Zea mays) in a growth chamber environment, found that the leaf spe-
cific respiration rate was about 40% higher in the slower growing variety. And while the stem specific
respiration rate was about the same, the root specific respiration rate was also higher by about 35% in the
slower growing variety. Because the photosynthetic rates were significantly indistinguishable between
the two varieties, a slow rate of respiration resulted in greater carbon accumulation. Investigating tall
fescue (Festuca arundinaceae) genotypes, Volenec et al. [13] similarly found significant negative corre-
lations between dark respiration and yield per tiller and between dark respiration and specific leaf weight.
A reduction in respiration by 47% at 20°C resulted in an increase in specific leaf weight by 52% and a
rise in yield per tiller by 90%. Results of Winzeler et al. [14] showed that among genotypes of winter
wheat (Triticum aestivum), a reduction in respiration by 16% translated into an increase in area per leaf
by 22% and a rise in dry weight per leaf by 19%. And Wilson and Jones [15] observed a 10% increase in
the annual productivity of field-grown ryegrass swards (Lolium perenne) for a 20% reduction in mature
tissue respiration. Hence, minimizing respiratory carbon loss is one approach to increasing rates of dry
matter accumulation [14,16].
One avenue of manipulating the maintenance respiration of a given crop is through regulation of its
daily photoperiod. Logendra and Janes [17] investigated the influence of light duration on carbon parti-
tioning and translocation (references) in young tomato plants (Lycopersicon esculentum) growing under
similar daily integrated PPF. Using incandescent and fluorescent lamps as light sources, the plants were
grown inside controlled-environment growth chambers under daily photoperiods of 8 hr (and PPF of 300
mol m^2 sec^1 ) and 16 hr (and PPF of 150 mol m^2 sec^1 ) at a constant daily integrated PPF of 8.64
918 CUELLO