trition experiments in which air temperature is ignored. This relationship was confirmed in a controlled
environment experiment with six varieties of spinach (Spinacia oleracea). The nitrate content of six cul-
tivars of spinach grown at temperatures from 5 to 25°C varied significantly, not with whether nitrogenous
fertilizer was applied but with the growing temperature [16].
Hazards from soil pathogens can depend directly on soil temperatures. All Florida citrus seemed to
be doomed by a mysterious “spreading decline” until it was found that the cause was a nematode
(Radopholus similis) that could be cultured only at subsoil temperatures. Because Florida laboratory tem-
peratures normally exceed those of the soil below about 30 cm, cultures from diseased roots processed at
ambient temperatures never indicated that R. similiswas the causal agent [17,18].
- Annual Versus Perennial Crops
Temperature limitations differ sharply for perennial and annual crops. For perennials (largely tree, vine,
and bush crops, various grasses, and other pasture crops), ecological limits are usually set by winter
temperatures. Few species are hardy enough to survive subarctic extremes of winter cold. In the trop-
ics, the need for a cool winter rest period limits the cultivation of pome (e.g., apple and pear) and most
drupe (e.g., peach, plum, cherry, apricot, almond, walnut, pecan olive) fruits. Coconut (which botani-
cally is a drupe with a desiccated mesocarp and liquid endosperm) is a conspicuous exception. Con-
versely, the lack of winter freeze hardiness limits the potential growing areas for purely tropical fruits
(banana, mango, avocado, durian, mangosteen, etc.), tropical ornamentals, and purely tropical grasses,
including sugarcane.
This set of limits is in sharp contrast to those applicable to purely annual crops such as almost all veg-
etables and grains, and annual flowers, for which summertemperatures are critical. All these annual crops
require is about 3–5 months of suitable growing weather. Vegetables grow luxuriantly in the warm, long
summer days in Alaska; the subarctic winters are of no consequence for them.
B. Various Interactions with Temperature
In the years immediately prior to World War II, the writer was a young graduate student in Canada work-
ing on storage and ripening of pears. At that time, it was customary for Canadian housewives to put fruits
on sunny windowsills to ripen them. Because it seemed illogical that light should hasten ripening, I de-
cided to put a row of unripe pears on the laboratory windowsill and cover half of them with a black cloth.
Fortunately, I checked pulp temperatures: those under the black cloth were several degrees warmer. Then
I tried shading with a white-painted board. Better, but still quite a difference. By the time the next year’s
pear crop came in, I was in uniform on the other side of the Atlantic. I never did return to the sunlight-
pear-ripening problem but have ever since been acutely aware that one way or another, temperature can
be an interactant, wanted or not, in a great deal of plant research.
- The “Day/Degrees” Concept
A very useful concept for expressing heat units is “total day degrees”: that is, the accumulated number of
days (or sometimes hours) above a certain base temperature. Another version is the accumulated sum of
diurnal maximum temperatures times the number of days. For the reverse (cold units), the usual figure is
the total number of hours below a given temperature, such as 40°F or 5°C. The usefulness of such meth-
ods is not helped by overreliance on statistical analysis of findings based on an initial arbitrary decision.
In the United States, for example, 40°F (4.4°C) and 45°F (7.2°C) have been common baseline tempera-
tures for determining chilling hours. As the Fahrenheit scale is abandoned in favor of Celsius, 5 and 7.5°C
are more likely to be used. With such baseline variations, apparent fine statistical differences can be
deceptive.
Peaches afford an excellent example of the use of such methods. Florida peach breeders have very
successfully extended the southern limits for commercial production of peaches by breeding “200 hour”
peaches and nectarines, in contrast to the 400, even 600, hour peaches grown in districts with cold win-
ters [19]. In more northern states, versions of this day/degrees concept are used to forecast blossom freeze
risks for varieties in a given area [20] and date of bloom in others [21]. Readers interested in a highly so-
phisticated discussion of the mathematics involved are referred to correspondence in a 1991 issue of
HortScience[22].
16 GRIERSON