Handbook of Plant and Crop Physiology

where PPF 1 is the low instantaneous PPF, PPF 2 is the high instantaneous PPF, and mis a constant. Com-
bining Eqs. (2), (4), and (5) yields

PPFave
mkk 11 PPF 2 (6)

Because the desired value for the long photoperiod P 1 is set, the short photoperiod P 2 may be readily cal-
culated from Eq. (3) as

P 2


PP

Q

Fave

P (^1)  (7)
whereQand PPFaveare the known desired daily integrated PPF and average instantaneous PPF, respec-
tively. Hence, from Eq. (4), kis calculated as
k
P 1 /P 2 (8)
Letting

z
mkk 11  (9)

and understanding that zis the ratio of the average instantaneous PPF (PPFave) to the high instantaneous
PPF (PPF 2 ), a desired value for z, which must be less than 1.0, is assigned (e.g., 0.20). With the values of
kandznow known, mcan be calculated from Eq. (9) as

m
z(kk1)^1 (10)

Therefore, the high instantaneous PPF (PPF 2 ) may now be calculated as

PPF 2 
 (11)

Subsequently, the low PPF (PPF 1 ) is calculated as

PPF 1
mPPF 2 (12)
As an illustration, recall the second conventional-lighting case where the instantaneous PPF is set at
100 mol m^2 sec^1 and the daily photoperiod is set at 16 hr, yielding a daily integrated PPF of 5.76 mol
m^2. Assume that it is desired that the daily integrated PPF be increased by over 80% to 10.44 mol m^2
while keeping the daily photoperiod at 16 hr and the average instantaneous PPF at 100 mol m^2 sec^1.
Thus,Q
10.44 mol m^2 ,P 1
16 hr, and PPFave
100 mol m^2 sec^1. From Eq. (7), P 2 is calcu-
lated to be 13 hr and, from Eq. (8), kis calculated to be 1.23. Letting z
0.75,mis calculated from Eq.
(10) to be 0.55. Hence, PPF 2 , from Eq. (11) is calculated to be 133.3 mol m^2 sec^1 and PPF 1 , from Eq.
(12), is calculated to be 73.3 mol m^2 sec^1. Using the foregoing calculated values for Eqs. (1) and (2),
note that the resulting Qand PPFaveare 10.44 mol m^2 and 100 mol m^2 sec^1 , respectively, as desired.
Thus, the daily integrated PPF could be increased significantly without necessarily changing the values
of the average instantaneous PPF of 100 mol m^2 sec^1 and the effective photoperiod of 16 hr.

VI. CONCLUDING REMARKS

Composite lighting allows an increase in the daily integrated PPF delivered to a given crop to ensure that
the lighting requirement by the crop is properly met, while also providing a high degree of freedom in al-
lowing both the daily photoperiod and the average instantaneous PPF to be regulated as desired so as to
minimize maintenance respiration and improve crop growth or yield. It is desirable for the daily pho-
toperiod to be extended as much as possible. Whereas crops such as lettuce, sweet pepper, roses, and
chrysanthemum can tolerate long photoperiods, other crops such as tomato plants require 4–6 hr of dark-
ness [28]. Gislerod et al. [2] found that, when using the same daily integrated PPF with Begonia, Chrysan-
themum, Hedera, Kalanchoe, and Pelargonium, the growth was best when the plants were allowed to

PPFave



m

k

k





1

^1



922 CUELLO