similar brief exposure to far-red light (730 nm), the effect of the red light treatment is nullified (Table
5). Alternating red with far-red light leads to germination or dormancy, depending on the wavelength
of last exposure. This is a classic case of a phytochrome-controlled response. Cone and Kendrick [41]
provide a thorough review of the role of phytochrome in seed germination. Certain chemicals, espe-
cially gibberellic acid, can substitute for red light treatment (see later). In seeds of some species, shade
from a plant canopy can reduce germination, relative to seeds held in darkness, by reducing the ratio
of red to far-red light [42].
E. Shallow Versus Deep Dormancy
Seeds that will germinate in response to environmental cues (light, alternating temperatures) are consid-
ered to have a shallowdormancy; those that require prolonged exposure to certain conditions (e.g., moist
chilling) are considered to have a deepdormancy. Certain seeds will not germinate immediately after har-
vest but do so after several weeks or months of dry storage (“after-ripening”) at room temperature (Table
6). This characteristic provides a safeguard against premature germination. In genotypes that do not pos-
sess this characteristic, germination can even occur on the plant, provided that moisture is abundant or
rain occurs. This is an example of viviparity(Latinvivusalive, plus parereto give birth). The length
of the dormant period in rice seeds is shortened as storage temperature is raised from 27 to 57°C [43].
Plotting the log of mean dormancy period (y) versus storage temperature (x) gives a straight line with neg-
ative slope (Figure 4). The depth of dormancy declines even at very low temperatures (75°C) in seeds
of orchard grass (Dactylis glomerata), although the rate of change is extremely slow [44].
166 DENNISFigure 3 Germination of Veronica hederofolia(A) and Polygonum persicaria(B) seeds at alternating
temperatures following burial in the field for varying periods of time. Arrows indicate dates of burial. Veron-
ica seeds were held at 4/10°C for 16/8 hr per day in darkness; Polygonumseeds were held at 12/22°C for 12/12
hr per day and were exposed to light during 12 hr at 22°C. (Adapted from Ref. 10, based on data of Karssen
[38] and Roberts and Lockett [39].)
TABLE 4 Time of Illumination Required to Break Dormancy in
Seeds of Selected Species
Time required Species
Seconds or minutes ‘Grand Rapids’ lettuce (Lactuca sativa)
Several hours Lythrum salicaria
Days Kalenchoë blossfeldiana
Long photoperiods Begonia (Begonia evansiana)
Short photoperiods Hemlock (Tsuga canadensis)
Source: Adapted from Ref. 10.