[29] concluded that, by slowly lowering the conditioning irradiance in the acclimatization area, the LCPs
of these shade-tolerant species could be lowered. In general, leaves that have low LCP are such not be-
cause they photosynthesize better but because they respire less [30]. Consequently, they frequently
achieve more net photosynthesis because they respire less [30]. Thus, a lowered instantaneous PPF, which
goes hand in hand with a prolonged photoperiod for a given daily integrated PPF, by itself contributes to
the reduction in the crop’s maintenance respiration.
IV. INDUSTRY APPLICATIONS
A. Greenhouse Application
The term “composite lighting” was first used by Cuello et al. [5] in describing hybrid solar and artificial
lighting in a controlled-environment plant growth chamber. Although composite lighting had in practice
been used or demonstrated earlier in greenhouses, it had not been differentiated from what was merely
supplemental lighting and had not been recognized and adopted as a practical strategy for influencing crop
growth or yield through active regulation of both the crop’s photoperiod and light compensation point. In
supplemental lighting for greenhouses, the main interest had simply been in augmenting, using artificial-
light sources, whatever solar irradiance was available in order to increase the level of the daily integrated
PPF.
The results obtained by Gislerod et al. [2] in a greenhouse study, however, were quite telling. In-
vestigating the effects of photoperiod and instantaneous PPF on the growth of four greenhouse plants,
they determined how plants responded when they were exposed to the same level of daily integrated
PPF but at different average instantaneous PPF and photoperiod levels. Begonia hiemaliscultivar
‘Schwabeland’,Kalanchoe blossfeldianacultivar ‘Pollux’, Hedera helixcultivars ‘Svendborg’
and ‘Gloire de Marengo’, and Pelargonium hortorumcultivar ‘Alex’ were all supplied with the
same daily integrated PPF, using high-pressure sodium lamps, but under two different scenarios: (1) at
a PPF of 85 mol m^2 sec^1 and photoperiod of 16 hr and (2) at a lower PPF of 68 mol m^2 sec^1
and a longer photoperiod of 20 hr. The solar integrated PPF on average was approximately 39% of the
total integrated PPF, that is, of the combined solar and artificial-lighting components. The results
showed that, for all four species, the resulting dry weight per plant and percent dry matter were con-
sistently and significantly greater in the treatment with lower instantaneous PPF and longer photope-
riod than with the treatment with higher instantaneous PPF and shorter photoperiod. The dry weight per
plant in the first treatment exceeded that in the second treatment by 20% for Begonia, 35% for Kalan-
choe, 42% for Pelargonium, and 42% for Hedera. For percent dry matter, the first treatment exceeded
the second treatment by 14% for Begonia, 11% for Kalanchoe, 9% for Pelargonium, and 8% for
Hedera. These results made clear that the specific design of a composite-lighting profile could signifi-
cantly affect a crop’s growth performance even without changing the total integrated PPF delivered
to the crop. This is a restatement of the principal premise for composite lighting enunciated in the
Introduction.
B. Growth Chamber Application
The study conducted by Cuello et al. [5] on composite lighting appears to be the first performed in a
growth chamber environment. The hybrid solar and artificial lighting (HYSAL) system used in this
study consisted of a mirror-based optical waveguide (OW) solar lighting system as the solar component
and four 60-W xenon–metal halide illuminators as the artificial-light component. A reference (or con-
trol) system consisted of a conventional 250-W high-pressure sodium (HPS) lamp. Solar irradiance was
harnessed whenever available for the HYSAL treatment. During the course of the 30-day growth pe-
riod for lettuce (Lactuca sativa), the HYSAL’s instantaneous solar PPF varied with the natural fluctu-
ations of terrestrial solar irradiance, which changed dramatically within each day and between days.
When averaged over the entire growth period, the average instantaneous solar PPF for the HYSAL
treatment turned out to be 322 mol m^2 sec^1 for an average daily photoperiod of only 3.86 hr ow-
ing to numerous cloudy days.
Over the whole growth period, the xenon–metal halide lamps provided an average instantaneous PPF
of 30 mol m^2 sec^1 continuously for 24 hr each day. The resulting total moles of photons received with
920 CUELLO