Horticulture Principles and Practices

TABLE 4–5
Plant
Bluegrass
Corn
Cucumber
Pea
English holly
Tomato
Kidney bean
Selected Day-Neutral Plants
Scientific Name
Poa annua
Zea mays
Cucumis sativus
Pisum sativum
Ilex aquifolium
Lycopersicon esculentum
Phaseolus vulgaris
FIGURE 4–7 Photoperiodic
response in flowering species.
Light interruption of darkness
affects short- and long-day plants
differently.
Photoperiod type
short-day
Continuous
long, dark period
Continuous
short, dark period
Interrupted
dark period
(long-night)
(a)
(b)
(c)
long-day
(short-night)
(d)
(e)
(f)
24-hour day cycle
Critical day length
106 Chapter 4 Plant Growth Environment
Interrupting the long night with such a short period of lighting is called flash lighting.
The responses of short-day plants and long-day plants to light interruption are opposite
in the two categories of plants.
The most sensitive part of the dark period regarding its response to light interruption
appears to be the middle of the period of exposure. The effect diminishes before or
after the midperiod. Further, the photoperiodic response can be very precise in that a
deviation of even less than 30 minutes from the critical value of required exposure can
cause failure to produce an induction of flowering. In henbane, for example, a photoperiod
of 10 hours, 20 minutes, induces flowering, whereas a photoperiod of 10 hours does
not. Further, environmental factors such as temperature can modify the photoperiodic
behavior of a plant. For example, flowering in henbane is induced by exposure to 11.5 hours
of light at 28.5°C, but it takes only 8.5 hours of exposure to light to induce flowering
at 15.5°C.
The photoperiodic response varies among species with respect to the number of
cycles of day-night treatment needed to induce flowering. Some species require only one
exposure to the appropriate photoperiod to be induced to flower, whereas others require
several days or even weeks (as in spinach) of exposure to the critical day-night cycle to
induce flowering. Further, the stage in development (age) affects the way the photoperiod
treatment is administered. Some plants respond as seedlings, but others need to
attain a certain age.
Growers manipulate the photoperiod requirements of certain seasonal and highincome
greenhouse plants to produce plants in a timely fashion. Short-day plants such
as poinsettia, chrysanthemum, and Christmas cactus are in high demand during specific