dener’s initials when immature will grow to maturity with the initials as prominent scars. Any such
wounds inflicted on detached fruits would cause decay.
C. Temperature Light Interactions
A factor that is easily overlooked in determining optimum temperature for a given response is light, which
may play either a positive or a negative role.
Modern apple orchards are often based on clonal rootstocks rather than on seedling roots. The root-
stocks must be rooted from cuttings, which is not always easy, and light can be a complicating factor.
Rooting of M-26 clonal rootstocks has been reported to be maximum at 25°C, but only in the absence of
light, which may inhibit rooting [69].
The prospect of establishing life support systems in space has led to the prospect of crop production
under controlled conditions not necessarily corresponding to those in terrestrial horticulture. One such
study with lettuce (Lactuca sativa) found that maintaining a constant day/night temperature at 25°C max-
imized growth, but only with intensified light during the “day” period [70].
Many plants are known to respond sharply to photoperiod (a misnomer: it is the period of unbroken
darkness, not of light, that is controlling). A study of the effect of photoperiod on the growth of West In-
dian mahogany (Swietenia mahagoni), grown in southern Florida as an ornamental, found that its typical
response to photoperiod was inhibited by low temperatures atypical of its native tropics [71]. Flowering
of the annual ornamental Rudbeckia fulgidainvolves a cold treatment photoperiod interaction [72], as
is also reported for six herbaceous perennials [73].
The relationship between temperature and photoperiod and flowering of traditional ornamentals such
asChrysanthemumis now well understood by both professional and amateur growers. (But salable flower
quality also depends on growing temperature [74].) With the increasingly common introduction of exotic
ornamentals, specific responses (to temperature, light, watering, etc.) must be established for the new ar-
rivals. One such exotic is “kangaroo paw” (Anigozanthos manglesii), for which very sharp interactions
between day and night temperatures and between temperature and day length control flowering and even
mortality [75].
Individual species within a genus may respond quite differently to interactions of temperature and
light. A Peperomiaspecies imported to Indiana from the Andean highlands was unable to adapt to the
double change, in summer, of temperature and photoperiod. Another Peperomiaspecies from the low-
lands of Ecuador made the transition successfully [76].
Temperature-light interactions are not limited to higher plants. For example, sporulation of some
fungi, such as the citrus pathogen Diplodia natalensis(Physalospora rhodina), needs not only optimum
temperature but also exposure to light of high intensity (GE Brown, personal communication).
A complicating role for light is always a possibility in the investigation of temperature relationships.
D. Temperature Control in Crop Production
- Microclimate
Greenhouse (British “glasshouse,” often a misnomer in this plastic age) production is the obvious exam-
ple of microclimate temperature control. But greenhouse production has its own considerable expertise
and literature. Thus, the examples of greenhouse research cited here are included only to illustrate spe-
cific situations in which individual control of air and soil temperatures is important.
Even outdoors, although climate (including temperature) is usually regarded as beyond the control
of man, localized temperature control is sometimes effective on a microclimate scale. Vancouver,
Canada, is a few miles north of the 49th parallel, about 60 miles farther north than Minot, North Dakota,
with its legendary harsh winter temperatures. But constant foehn winds off the Pacific Ocean make Van-
couver winters mild and wet, although sunshine is scant. When I had a garden there in the late 1940s, a
neighbor used to say that I “cheated God” to bring in my lettuce and tomatoes earlier than anyone else.
The bed in which the vegetables grew was banked toward the south at approximately 50 degrees, and the
area between the plants was covered with flat stones, gathered from the nearby beach, to maximize soil
heating from the weak late winter–early spring sun.
This management was, of course, an extreme example of microclimate modification for crop pro-
duction. Nevertheless, it was no more than ingenious growers have done to survive inhospitable climates
22 GRIERSON