mol m^2 day^1. Although the plants in both treatments fixed statistically indistinguishable amounts of
carbohydrates per day, the plants in the 8-hr treatment respired a total amount of carbohydrates that was
184% greater than that for the plants in the 16-hr treatment. In addition, the amount of carbon translocated
during the light period in the 16-hr treatment was 157% greater than that in the 8-hr treatment. Over 24
hr, the amount of carbon translocated in the 16-hr treatment remained greater by 41% than that in the 8-
hr treatment. Thus, despite the plants in both treatments fixing comparable amounts of carbohydrates,
more carbon was partitioned into sucrose and translocated out of the leaves in the plants grown under the
16-hr treatment. These results agreed with the findings that plants grown under short light periods have
greater starch accumulation rates [18–22], starch content [20,21,23], and low sucrose content [24] in con-
trast with those grown under long light periods. The high starch accumulation rates observed under short
light periods are associated with low translocation rates [19,22] and decreased amounts of carbon translo-
cated [24]. Under short light periods, plants accumulate higher amounts of starch, most likely to satisfy
the carbohydrate requirements of the subsequent longer nights [18].
Similar results were obtained earlier by Jiao et al. [25,26], who investigated the influence of radia-
tion on whole-plant net CO 2 exchanges in rose plants (Rosa hybrida). They found that rose plants exposed
to a daily photoperiod of 24 hr (and PPF of 204 mol m^2 sec^1 ) retained 80% more carbon than those
exposed to a shorter daily photoperiod of 12 hr (and higher PPF of 410 mol m^2 sec^1 ), despite the two
treatments receiving the same daily integrated PPF of 17.6 mol m^2 day^1 as supplied by high-pressure
sodium lamps.
The results of Grange [24] on pepper plants (Capsicum annuum) demonstrated the same trends. Us-
ing controlled-environment growth chambers equipped with warm-white fluorescent and tungsten lamps
as light sources, three treatments with comparable daily integrated PPF values of 9.27, 9.94, and 8.94 mol
m^2 day^1 were given daily photoperiods of 14 hr (and a PPF of 184 mol m^2 sec^1 ), 10 hr (and a PPF
of 276 mol m^2 sec^1 ), and 6 hr (and a PPF of 414 mol m^2 sec^1 ), respectively. Indeed, the treat-
ment with the longest photoperiod of 14 hr (and lowest PPF of 184 mol m^2 sec^1 ) yielded the great-
est dry weight per plant of 20.3 g, and the treatment with the shortest photoperiod of 6 hr (and highest PPF
of 414 mol m^2 sec^1 ) produced the smallest dry weight per plant of 6.2 g. The treatment with the in-
termediate photoperiod of 10 hr (and intermediate PPF of 276 mol m^2 sec^1 ) yielded an intermediate
dry weight per plant of 16.5 g. Note that although the maximum daily integrated PPF level among the
treatments was greater than the minimum by only 11%, the resulting maximum dry weight per plant
among the treatments exceeded the resulting minimum by 227%.
Citing the results of Hurd and Thornly [27] for tomato plants as evidence, Moe [28] concluded that
if long photoperiods do not cause adverse effects, such as leaf damage or prevention of flowering in short-
day plants, prolonged low instantaneous PPF should be more effective than providing the same daily in-
tegrated PPF at a higher instantaneous PPF for a shorter period. But although Moe [28] and the foregoing
authors correctly established the significant correlations between protracted photoperiod, reduced main-
tenance respiration, and increased growth or yield, it should also be pointed out that the concomitant de-
cline in the instantaneous PPF when the photoperiod is prolonged—while keeping the daily integrated
PPF constant—contributes as well to the reduction in maintenance respiration. The latter constitutes the
second physiological basis for composite lighting.
B. For a Constant Daily Integrated PPF, a Lower Average
Instantaneous PPF Results in Lower LCP, Which in Turn Results
in Lower Maintenance Respiration, Which Translates into Greater
GrowthFonteno and McWilliams [29] found that a 15-week acclimatization of four tropical foliage species to 27
mol m^2 sec^1 , using cool-white fluorescent lamps as light source for 12 hr per day, resulted in a sig-
nificant reduction in LCP accompanied by a significant decline in dark respiration for each species. Light
compensation points decreased between week 1 and week 15 as follows: 33 to 7 mol m^2 sec^1 in
Philodendron scandenssubsp.oxycardium, 38 to 6 mol m^2 sec^1 inEpipremnum aureum, 14 to 4
mol m^2 sec^1 inBrassaia actinophylla, and 119 to 15 mol m^2 sec^1 inDracaena sanderana. Con-
comitantly, dark respiration decreased 63% in P. scandenssubsp.oxycardium, 71% in E. aureum, 53%
inB. actinophylla, and 64% in D. sanderanaduring the acclimatization period. Fonteno and McWilliams
COMPOSITE LIGHTING FOR PLANT FACTORIES 919