- Stage II (Active Metabolism and Hydrolysis)
Continuing water uptake activates stored enzymes and stimulates synthesis of new ones. These enzymes
hydrolyze and transform some of the stored reserves into energy and lower molecular weight, soluble
compounds, used for the production of more cells and tissues. These metabolic processes lower the wa-
ter potential of the embryo and surrounding tissues.
Stage II appears to be a lag phase between uptake and growth. During this stage, the rate of water ab-
sorption is governed by the internal osmotic potential. The duration of the lag phase may represent the
time required for weakening the restraints on embryo enlargement imposed by surrounding tissue to a de-
gree necessary to allow further water uptake by the embryo. During stage II, exogenously applied plant
hormones such as gibberellins or abscisic acid may assist with weakening or strengthening, respectively,
of the tissues surrounding the embryo [10]. The generally slow response to gibberellin treatment is prob-
ably related to the time lag for enzyme synthesis.
- Stage III (Visible Germination)
Rapid growth of the radicle and shoot defines the third stage. For this process to occur, the water poten-
tial of the external solution should not be lower than 0.2 to 0.3 mPa. Germinating solutions with wa-
ter potentials of 0.45 to 0.80 mPa noticeably slow radicle emergence, and solutions of 1.0 mPa or
lower severely restrict the expansion of radicle cells necessary for radicle protusion.
B. Temperature
When moisture is adequate, the next most important requirement for germination is suitable temperature.
Temperature affects the rate at which water is imbibed as well as the rate of metabolic processes such as
the translocation of nutrients and hormones, cell division and elongation, and other physiological and bio-
chemical processes.
According to Hartmann et al. [8], temperature is the single most important factor in the regulation of
the timing of germination, because of its role in dormancy control and/or release, or climate adaptation.
Generally, high temperatures induce or reinforce dormancy; low temperatures overcome dormancy.
Most seeds can tolerate prolonged hot weather if they are kept dry, and some can withstand even
greater extremes of hot or cold [8]. Seeds of some species, such as forest pines with a very hard seed coat,
germinate only after exposure to intense heat, such as that from a brush fire [8,11]. The heat shock from
the dry heat fractures the seed coat, allowing penetration of water or exchange of gases or freeing the em-
bryo from the physical constraint of the hard seed coat. Seeds are often placed in boiling water to control
disease or to soften the seed coat without affecting seed viability.
Seeds of different species have been categorized into suitable temperature groups: cool-temperature
tolerant, cool-temperature requiring, warm-temperature requiring, and alternating temperature [8,12].
The optimal temperature requirement for germination may be different from that for early seedling
growth. In the greenhouse, propagating nursery, or seed-germinating laboratory, the usual practice is to
shift the seedlings to a lower temperature regime, which makes them sturdy and more hardy for trans-
planting and growing [2,8].
C. Oxygen
Most seeds require an adequate supply of oxygen during germination. Oxygen is required for respiration
to oxidize starches, fats, and other food reserves, and its utilization is proportional to the amount of
metabolic activity [13]. Thus, a germinating medium or seedbed should be loose, friable, and well aer-
ated. Seeds sown in heavy soils may germinate poorly, especially during wet seasons, when the soil be-
comes saturated and often lacks sufficient oxygen. Deep planting is unfavorable to germination because
the oxygen supply may be restricted or seedlings may be unable to reach the surface, especially if the soil
or medium is hard or compacted.
D. Light
Provided moisture and temperature are adequate, most seeds germinate equally well in darkness or light,
particularly seeds of most agricultural food plants, which have been rigorously selected for ease of ger-
mination. Others are partially or completely inhibited by light or require it to germinate. Some species,
60 CHONG ET AL.