minated immediately on separation from the plant [41]. The lack of true seed dormancy severely limited
the spread of many tropical species when they were first discovered by early European explorers. Even
in modern times, dispersal of such crops as cocoa (Theobroma cacaoL.) has been difficult because the
seeds not only are adapted to immediate germination but also are highly susceptible to chilling injury (see
later) if held in cold storage.
A word on terminology: “dormancy” for seeds is used much as it is found in discussions of buds,
bulbs, and so on. Seeds that will not respond to usually effective treatments are said (most appropriately)
to be “recalcitrant.” Some authorities designate as recalcitrant only seeds that do not survive desiccation
[41]. Such distinctions are, however, beyond the terms of reference of this chapter, which is limited to the
effects, direct or indirect, of temperature. “Stratification” is used (not very logically) for chilling treat-
ments to break dormancy. Perhaps this comes from the old custom of filling a box with alternate layers
of sand and seeds from peaches (or other stone fruit) and setting it outside, exposed to the coldest possi-
ble weather.
For some seeds it has been demonstrated that dormancy is purely mechanical, being enforced as long
as the tough impermeable testa is intact [42]. In this regard, it used to be argued that hard freezing only
splits the peach pits, thus mechanically releasing the seed to germinate. Our pomology lecturer at the On-
tario Agricultural College, Guelph, settled this for us more than 50 years ago. At his direction, we com-
pared germination of “stratified” peach pits from the preceding year with that of fresh peach pits we had
carefully cracked. The result was quite fascinating. The seedlings from the stratified seeds were normal.
Those from the fresh, but mechanically cracked, seeds resembled tiny pineapple plants, producing leaves
with no internodes. Prolonged cold temperatures (most effectively between 2 and 6°C) are definitely es-
sential in such “stratification.”
Various treatments (such as presoaking) to encourage emergence of seeds used to be called “vernal-
ization,” presumably because it hastened the effects of spring. The term was brought into disrepute by
claims of permanent genetic changes by the Soviet charlatan Trofim Lysenko [43]. Today, “priming” is
appropriately used for seed treatments (involving temperature, solutes, etc.) in wet or dry media to accel-
erate germination. But if seeds have been primed, subsequent permissible holding temperatures may be
affected. Primed tomato seeds have been reported to retain viability at 4°C, but at 30°C they deteriorated
within 6 months [44]. Similarly, primed tomato seed was reported to retain viability at storage tempera-
tures as high as 20°C for 18 months. However, the seed degenerated at 30°C, particularly when primed
with potassium nitrate rather than with polyethylene glycol (PEG) [45].
Priming does not necessarily overcome adverse weather conditions, as shown by 3 years of unsuc-
cessful trials with primed sugar beet seed in cold Idaho spring weather [46]. Current research develop-
ments, however, promise to overcome these ill effects of too early sowing when they are due to a combi-
nation of moisture imbibition and too low temperature. A review article [47] reported success in such
circumstances when seeds of table beet (Beta vulgarisL.) were primed with PEG.
Imbibitional chilling injury is of particular concern for seeds of plants of tropical origin, such as cot-
ton, corn (maize, Zea mays), tomato, and many legumes, which are susceptible to chilling injury. For their
seeds, the onset of CI is related to rate of water uptake [47]. Treatment with materials (such as PEG) that
delay imbibition can be helpful but is not temperature specific. This problem appears to be surmountable
by use of temperature-sensitive polymeric seed coatings that become permeable to water at specifically
selected temperatures [47,48].
Too hot temperatures can also impede germination. Florida celery growers have been able to sur-
mount this problem by using high-temperature (30°C) priming in a solid matrix of calcined clay [49].
Recalcitrant seeds occur in all climates, and temperature can be a factor in achieving successful ger-
mination. Wild rice (Zizania palustris) is an excellent example. Deeply dormant at harvest, it will not ger-
minate without prolonged cold treatment [50]. It is thus perfectly adapted to self-propagation in the Min-
nesota wetlands and as a food staple for Native Americans, who have depended on it over the centuries.
Some of wild rice’s reputation as a “recalcitrant seed” involves a supposed desiccation intolerance, but
this misjudgment has been related to failure to understand the “novel relationship between seed viability,
temperature, and moisture content” [51].
An interesting form of recalcitrance in tropical seeds is that some, such as kola (Cola nitida), must
be aged for as long as 7–11 months, for which ambient temperatures are satisfactory [52]. This require-
ment accounts for how, for many centuries, the highly valued, but frail, caffeine-rich kola “nuts” (caffeine
being a stimulant not prohibited to Muslims) were traded all over West Africa, wrapped in damp leaves
TEMPERATURE IN THE PHYSIOLOGY OF CROP PLANTS 19