C. Definitions
- Temperature
Temperature per se does not need definition. Not all the work cited, however, deals with the temperature
of the actual plant tissues involved. Often “temperature” refers to that recorded for the immediate vicin-
ity of the plant or organ.
- Crop Plants
Crop plants are taken to be any grown for profit or pleasure, thus including ornamental plants grown for
either indoor decoration or outdoor landscaping. No attempt has been made to include nondomesticated
species.
II. ECOLOGICAL ROLE OF TEMPERATURE
Temperature obviously limits the geographical areas in which various crops can be grown. However,
temperature per se is often not the only determinant: the effects of temperature extremes are usually as-
sociated with other factors such as availability of water, prevalence of high winds, and the duration and
intensity of sunlight (insolation). An important aspect, as discussed in the following, is that limitations
imposed by extremes of temperature differ sharply for annual versus perennial crops.
A. Extremes of Temperature
- High-Temperature Limitations
The limiting effect of high temperatures on crop production takes two principal forms: limitation of veg-
etative growth such as for cereal grains [2] and peanuts [3] and adverse effects on fruit settings [4]. Veg-
etable crops subject to very high transpiration losses, such as asparagus, lettuce, and all the Brassica
species (cabbage, cauliflower, broccoli, brussels sprouts, etc.) are obviously limited by the excessive tran-
spiration concurrent with exposure to extremely high temperatures. Tomato (Lycopersicon esculentum
Mill.) is the quintessential example of a crop for which very high temperatures limit fruit setting. (In this
regard, the small-fruited “cherry tomatoes” are more tolerant than the usual commercial varieties.) Plant
breeders are having limited success in developing more heat-tolerant tomato varieties because heat toler-
ance and cold tolerance in fruit setting have only moderate heritability and such inheritance is complex
[5].
A further complication is that the upper limit for fruit set can be correlated with humidity levels [6].
Successful breeding of truly heat-resistant tomatoes may well turn out to depend on the physiologists and
biochemists more exactly defining the influence of temperature and humidity on the hormonal systems
controlling anthesis, pollen tube activity, ovule receptivity, and, in some instances [5], parthenocarpy. A
press account [7] reported that a major U.S. seed company has developed both a tomato and a zucchini
that set fruit in temperatures as high as 35.6°C (96°F). For commercial purposes, assuming that the report
is correct, this “high temperature fruit set” will have to be incorporated into varieties having commercially
acceptable yield and eating quality.
Very high temperatures can also limit fruit setting of citrus fruits. In this case, intensity of insolation
appears to be another limiting factor, because flowers within the leafy canopy, protected from direct ex-
posure to sunlight, will usually set some fruit [8]. A less subtle effect of extremely high temperatures on
fruit set of citrus is the “burning” or “scorching” of blossoms, particularly on young trees, that is occa-
sionally reported from desert areas such as southern California, Arizona, and the Negev of Israel. Even
without such drastic effects, fruit set of navel oranges is reported to be sharply affected by temperatures
during the bloom period [9].
A high-temperature effect causing no visible symptoms is a cessation of growth even though nutri-
ents and soil moisture are adequate, as reported for citrus trees during very hot weather in Arizona [10].
- Low-Temperature Limitations
The obvious limitation imposed by low temperature is killing of plant tissues by freezing. Most plant tis-
sues can be destroyed by freezing temperatures suddenly imposed during a period of rapid growth. Some
14 GRIERSON