throughout the ages, as with pre-Columbian Andean potato growers. In recent years, the native peoples
of the Andean Altiplano have learned to revive the methods of their ancestors, growing potatoes on high
narrow beds at the foot of mountain slopes. On freezing nights, the cold air settles between the raised beds
without damaging the aerial parts of the plants, whose subterranean portions are protected by the latent
heat of the water accumulated in the troughs between the beds, an ancient example of sophisticated mi-
croclimate control.
Poinsettia is typical of an ornamental grown for a specific date; unless the plants are marketable at
Christmas, their value drops dramatically. Growth of the plants can be sharply reduced by too cool air
temperatures. Maintaining temperatures in a greenhouse in very cold weather is very expensive. How-
ever, it has been found that raising soil temperature to 23°C (which is much cheaper to do) could coun-
teract the adverse effects of air temperature as low as 11.5°C [77].
Sometimes the reverse modification is needed. Flowering of Alstroemeria(lily-of-the-Incas) was
stimulated by cooling the root zone with 10°C circulating water. There was also an interaction with light,
supplementary lighting being essential in winter but harmful in spring and summer [78]. A beneficial low-
ering of root zone temperature explained an anomalous result with azaleas pot-grown outside on either
clamshell mulch or black polyethylene. Placing the pots close together increased growth of azaleas in
black pots but not in white pots. The beneficial effect was traced to a decrease in root zone temperatures
by shading when the plants in black pots were placed close together [79].
Another unexpected root zone temperature effect was traced to the chilling effect of cold greenhouse
irrigation water in winter. The effect was noted with roses and chrysanthemums and was sufficient to af-
fect turgidity, stomate opening, and flowering. Such unforeseen deleterious temperature effects are par-
ticularly easy to overlook when they involve the temperatures of soil rather than air [80].
Root zone heating usually involves use of expensive fuel. This potential cost was halved in an inge-
nious system of pumping comparatively warm water from a well 100 m deep and circulating it through
buried pipes [81].
Temperature, of course, affects more than plant growth. It is sometimes necessary to tread a fine line
between temperatures optimum for growth and those that initiate or increase fungal attack. This can be a
problem for Florida foliage growers in warm weather, as shown in a study of aerial blight (Rhizoctonia
solani) infection of Boston fern (Nephrolepis exalta). Some plant quality had to be sacrificed if potting
medium and air temperatures were to be regulated to restrict development of the pathogen [82].
Given sufficient irrigation water, many deserts will blossom as the rose. But sometimes the desert
sun is too hot, with consequent potential for crop damage. An obvious remedy is to spray the crop with
an overhead irrigation system. The cooling effect of such sprinkling is sharply dependent on initial air
temperature. A California study [83] reported the following (the results have been converted from Fahren-
heit to Celsius):
Macroclimate temperature Lowered by
32°C 2–3°C
38°C 5°C
39°C 7.5°C
In addition to other benefits, the water spray at 39°C was reported as being successful in reducing exces-
sive “June drop” of small fruitlets. But such spraying of water in extremely hot weather can cause local-
ized injury due to the “lens effect” of standing drops of water on the leaves [84]. Lens effect injury can
be avoided, and better temperature reduction obtained, by using nozzles that emit a fine mist instead of
streams of water [85].*
Microclimate is being modified on a very large scale. Whole hectares are commonly covered with
plastic sheeting, which may be black, white, or transparent. Plastic covering may be spread over raised
beds, with the plants inserted through holes in the plastic; it may lie over individual rows secured along
the sides, with or without some form of framing [86]; or it may be used as “floating row covers,” sup-
TEMPERATURE IN THE PHYSIOLOGY OF CROP PLANTS 23
- A recent report indicates use of such “hot weather misting” to improve color of apples in Washington State.
