2.3.5      The germination process and the involvement of air and water


Water and oxygen must be taken up before germination can occur.  The figures below show that the embryo is concealed inside the seed coat which acts as a barrier to both, but the basal pore does permit entry.  The pore is not usually open, but contains the remains of the vascular connections between the mother plant and the embryo.


Perry and Harrison (1974) almost completely prevented germination by blocking the pore with Vaseline.  Entry between seed cap and the seed coat does not occur as Heydecker et al (1969) had earlier described.



(i)   Monogerm seed of variety Monohill (Aura 1975).







(ii) Opening of the seed cap and radicle emergence after two days (Aura 1975).



(iii)        Diagrams showing external view and out-section of a true seed (Lakon & Bulat 1958 cited by Aura 1975).






(iv)    Diagram of a transverse section of a monogerm seed (Perry & Harrison 1974).



Severe inhibition has also been described in many papers including Chetram & Heydecker (1967) and Heydecker & Chetram (1971) by excess water in the test substrate.  Excess water tends to be taken up and held in the basal pore by capillary action.  Consequently, oxygen can only enter by diffusion, through the water at a very slow rate.


Perry and Harrison (1974) applied Fick's Law to the dimensions of the basal pore and estimated that, when air filled, the oxygen diffusion flux was 4.5 ml/h and when water filled only 8.5 x 10-3 μl/h.  An embryo requires about 0.8 μl/h to germinate and therefore the process is inhibited when the pore is full. 

The observed uptake of oxygen in a water filled pore was 0.14 μl/h.  The difference from the water filled pore estimate was due to microbial respiration.


As germination continues cell number increases (Longden 1971) then the radical elongates forcing the seed cap open thus making more oxygen available to the embryo facilitating faster elongation.  Coumans, Côme & Gaspar (1976) showed that in a wet medium removing the cap before germination resulted in more seed germinating if positioned "face up" but not if "face down".  Therefore the external film of water could also inhibit germination.  Peto (1964) cited by Heydecker and Chetram (1971) chipped the seed cap beforehand and improved germination by both allowing more oxygen in and reducing the mechanical effort required by the radical to remove the cap.  Heydecker and Chetram (1971) viewed germination as more than a physiological process, i.e. complex ecological and microbiological components are involved too.  When 8 ml of water was used in a laboratory test dish it was excessive, but adding aureomycin to inhibit bacteria leaves more oxygen for embryos to use.  Washing seeds upsets the ecological balance between bacteria, fungi and the inhibitors in the seed coat, and the depressed laboratory germination of seeds treated with fungicides may be explained by a similar change to the micro-environment.

As germination is improved by cap removal or inhibitor removal, whether water is excessive or not, 02 uptake and respiration must therefore precede germination and not be a consequence of it!  However, Heydecker et al (1971) noticed that in red beet seeds when the concentration of inhibitors was high, the oxygen uptake was high also.  Therefore, some process different from normal respiration was occurring.  There may be competition for oxygen between different metabolic pathways with imbibed seeds - inhibiting respiration at low concentrations and uncoupling, i.e. preventing access to the true seed, if at higher concentrations but an actual mechanism has not been found.

Coumans et al (1976) viewed the seed coat as a physiochemical barrier to oxygen.  It (a) restricted diffusion and (b) actually absorbed oxygen so that very little if any reached the embryo through the seed coat.

Chetram & Heydecker (1967) and Heydecker et al (1969) found that hydrogen peroxide in solution was an excellent way of supplying oxygen to the embryo to improve germination.


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[Introduction & Contents]     [Chapter One]     [Chapter Two]     [Chapter Three]     [Chapter Four]     [Chapter Five]     [Chapter Six]     [Chapter Seven]

[2.3]    [2.3.1]    [2.3.2]    [2.3.3]   [2.3.4]   [2.3.5]