to the crop below. It is now known that the smoke particles are not in a size range suited to re-
flect infrared emissions. It is, however, possible to generate very fine water fog with droplets of
the appropriate size. An added benefit is that any fog droplets that freeze give off latent heat to
the surrounding atmosphere.- Freezes are classified as “convection freezes” or “advection freezes.” Convection freezes occur
with calm air and cloudless skies, conditions in which the earth is radiating heat to the sky, with
consequent rapid cooling of the air near the ground. For orchard crops, it is beneficial to have
bare ground to radiate ground heat to the trees. Weeds or cover crops trap such radiated heat at
the expense of the trees. Convective conditions commonly result in atmospheric inversions, in
which the lower air is colder than that at 10–30 m above the ground. In such conditions, “wind
machines” mounted on tall towers or pylons can be beneficial. Helicopters have sometimes been
used to achieve the same effect, particularly to prevent dangerously cold air from accumulating
in the “frost pocket” hollows.
In an advective freeze, a wind strong enough to disrupt normal convection patterns freezes crops on
the exposed higher ground, with much less freeze injury in the valleys and lowlands. Wind machines are
worse than useless in an advective freeze, but rows of heaters placed at right angles to the wind direction
can benefit crops for a considerable distance downwind.
A deadly interaction among temperature, humidity, and wind speed can occur in an advective freeze.
Tender leaves and shoots can be killed, not by freezing, but by desiccation, if wind speed is high enough
when the temperature approaches the freezing point of plant tissues under conditions of very low humid-
ity (which frequently occur).
For further information on methods and principles of freeze protection, readers are referred to an ex-
tensive chapter on freeze protection [95].
E. Incidental Effects of Temperature
Old Ecclesiastes said, “Of the making of many books there is no end,” and a number of them probably
could be written on the incidental effects of temperature. However, only a very few examples can be cited
here to indicate how often temperature is an unforeseen or unplanned-for variable.
Temperature can move in mysterious ways, its wonders to perform, through its subtle influence on
the activity of growth regulators. As noted earlier, fruit setting in tomato plants is inhibited by too high
temperatures. A role for growth regulators in this high-temperature inhibition is indicated by a report [96]
that relative levels of gibberellin and auxinlike growth regulators were sharply affected at high tempera-
tures.
On a purely physical basis, temperature can be expected to affect gas diffusion rates, hence rates of
photosynthesis and leaf respiration. However, not only can the physical effects of temperature be com-
plicated by the metabolic effects of temperature on rates of photosynthesis and respiration, but such gas
exchange is reported to be affected by an interaction between temperature and humidity [97]. Exact con-
trol of temperature is routine, but equivalent accuracy in control of humidity can be difficult, and exact
simultaneous control of temperature and humidity can be very challenging indeed.
Vegetable transplants usually benefit from hardening by controlled temperature and/or moisture
stress before being planted out in the field [98]. This does not appear to be the case for sweet potato trans-
plants, which are vine cuttings rather than seedlings. Transplants held at 13–18°C were reported to have
greatly increased vitality and ultimately higher yields compared with transplants held at an ambient tem-
perature of 26.7°C [99]. (That “26.7°C ambient” temperature is curiously exact and is possibly a transla-
tion from “ca. 80°F ambient.”)
A particularly intriguing example of an unexpected temperature effect is reported in a study of male
sterility in the common bean (Phaseolus vulgarisL.) [100]. When, in the course of an atypically cool sum-
mer, unexpected fertility was noted in supposedly male sterile plants, research was transferred to growth
chambers. A day/night temperature regime of 30/18°C for an average of 12 days was sufficient to cause
most unstable steriles to produce sterile buds. Day/night conditions of 18/7°C for an average of 14 days
were effective in converting sterile to partially sterile phenotypes. Both temperature-stable and tempera-
ture-unstable genotypes were identified; this is an excellent example of valuable research findings
achieved by following up on a temperature-related anomaly revealed in a field study.
TEMPERATURE IN THE PHYSIOLOGY OF CROP PLANTS 25