Handbook of Plant and Crop Physiology

In contrast, seeds of birch germinate better in darkness at high than at low temperature (Figure 1B), but

exposure to light can markedly affect response. Other seeds germinate best when temperature is alternated

on a daily cycle (Table 3). For example, when seeds of signal grass [Brachiaria humidicola(Rendle)

Schweickerdt] are germinated at constant temperatures ranging from 13 to 38°C, germination does not

exceed 2%, whereas daily alternation between 13 and 32°C results in 60% germination (Figure 2). Baskin

and Baskin [36] reported that freshly harvested seeds of curled dock (Rumex crispusL.) remained “non-

dormant” for 2 years when buried 7 cm deep in moist soil. However, the seeds germinated in the light at

alternating temperatures. Few seeds (1%) germinated while buried. Therefore, the seeds would proba-

bly have remained dormant had they been held in darkness at constant temperature.

Seeds of certain species require prolonged exposure to relatively high temperatures before germina-

tion can occur. Chickweed (Stellaria mediaL.) and other “winter annuals” remain vegetative in the win-

ter, then flower and produce seeds in the early summer. Such seeds remain dormant until fall, then ger-

minate and repeat the cycle. Experiments have demonstrated that the periods at warm temperatures break

dormancy, provided that the seeds are subsequently exposed to appropriate conditions, especially alter-

nating temperatures and light [37]; temperatures below 20°C are ineffective in breaking dormancy re-

gardless of subsequent treatment.

Exposure of such seeds to low soil temperatures in the autumn reintroduces dormancy (see Sec.

III.H), so that they once again become incapable of germination. A seasonal pattern thus develops, with

periods of high germinability in autumn alternating with periods of low germinability in the summer. The

behavior of such seeds contrasts with that of seeds of summer annuals, such as Polygonum persicaria

[38], in which chilling is essential for breaking secondary dormancy (see later) and which germinate read-

ily in the late winter and spring but poorly in the summer and fall (Figure 3). Chilling temperatures are

required for breaking dormancy in other seeds (see Sec. III.E).

D. Light and Seed Dormancy

Seed response to light has been studied intensively in ‘Grand Rapids’ lettuce. Seeds of this and a num-

ber of other cultivars of lettuce and celery germinate readily in the light at 25°C but fail to germinate

in the dark (Figure 1A). A brief exposure of moist seeds to white or red light (660 nm) induces subse-

quent germination in darkness. The time of exposure required varies with species (Table 4). A brief ex-

posure is effective only at high temperatures in birch, whereas a long exposure time is effective at all

temperatures from 10 to 25°C (Figure 1B). However, if the brief red light treatment is followed by a

DORMANCY: MANIFESTATIONS AND CAUSES 165

Figure 2 Effects of constant and alternating temperatures on germination of seeds of signal grass (Brachiaria
humidicola). Seeds were held at indicated temperatures for 40 days. Percentage germination is indicated at
points where lines intersect perimeter of square, and is proportional to density of stippling. (From Ref. 35.)