Acacia divergensaveraged 11, 28, and 90% for no treatment, mechanical scarification, and boiling in wa-
ter for 30 sec, respectively. The coats of some seeds are impermeable to oxygen. In this case, scarification
allows oxygen to penetrate to the embryo. Tran and Cavanagh [32] reviewed the structural aspects of seed
dormancy, emphasizing seed coat impermeability and methods of increasing it. Microscopic examination
of seeds indicated [33] that treatment with boiling water or fire did not soften the seed coat but affected the
structure of the “lens” (strophiole) near the hilum, thereby allowing entry of water. In some species the
seed coat, although permitting entry of water and oxygen, is a mechanical barrier to germination; on its re-
moval the embryo germinates readily. The seed coat may also contain chemicals that inhibit germination.
Many factors can affect germination. Because of the many interactions possible, Karssen [34] cau-
tioned that “an absolute requirement for any stimulatory factor hardly occurs.” Therefore, one must be
cautious in discussing any one factor in isolation. Nevertheless, several factors, light and temperature in
particular, have pronounced effects.
C. Temperature and Seed Dormancy
Dormancy is often temperature dependent. In some cultivars of lettuce and celery, for example, germina-
tion occurs readily between 10 and 20°C but declines to nil as temperature increases to 30°C (Figure 1A).
164 DENNISTABLE 3 Optimum Conditions for Seed Germination in Selected Species
Conditions during germination
TemperaturePretreatment Light Low High Alternating Species Ref.
None Bean, tomato —
Birch (B. pubescens)10
Lettuce 10
Broadleafed dock 21
Lythrum salicaria, tobacco 22
(25°C) Pinus lambertiana 23
Dry storage Wild oats 24
Rice 25
Scarification Black locust 26
Chilling Apple, Pinus lambertiana 27, 23
Pinus strobus 28
Delphinium ambiguum 29
Scarificationchilling Redbud 30
Figure 1 Effect of temperature and light on germination of seeds of (A) lettuce and (B) Betula pubescens.
‘Grand Rapids’ lettuce seeds were tested immediately after harvest (“fresh”) or after storage at about 18°C for
18 months (“after-ripened”). Birch seeds were tested in darkness (), under a 20-hr photoperiod (), or were
exposed to red light for 15 min each day (). (From Ref. 10.)