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Science and the Garden
Table 6.5 Estimate of probable regeneration intervals* for seeds stored at - 2 0 ° C and 5% moisture content. From (Roberts, E.H. and Ellis, R.H. (1 977). Prediction of seed longevity at sub-zero temperatures and genetic resources conservation. Nature, 368, 431-3.) Plant
Barley Rice Wheat Broad bean Pea Onion Lettuce
Cultivar
Probable regeneration interval (years)
'Proctor' 'Norin' 'Atle' 'Claudia Superaquadulce' 'Meteor' 'White Portugal' 'Grand Rapids'
70 300 78 270 1090 28 11
* The regeneration interval is the predicted time for viability to fall to 95% of its initial value. Thus, it will take the barley cultivar 'Proctor' 70 years to fall from 99% viability to 94% viability, if stored under optimum conditions.
However, it is important that the container is hermetically sealed, otherwise the seeds will equilibrate with the moisture content of the refrigerator. This will hasten their deterioration even though they are being stored at a low temperature. Loss of seed viability
Surprisingly, scientists do not know what causes loss of viability and ultimately the death of seeds. Aseptic seeds are known to last longer than nonsterile seeds, but the former still lose viability. Loss of viability also occurs at relative humidities below 65% when storage fungi are inactive and so it is concluded that fungi accelerate the loss of viability in stored seeds, but do not cause it. Other suggestions include the accumulation of inhibitory chemicals, the depletion of essential food reserves, or the breakdown of proteins and nucleic acids, but so far there is little evidence to support them. However, it has been demonstrated that the loss of viability is accompanied by genetic deterioration. Gardeners who grow peas, particularly from home-saved seeds, will be familiar with the odd albino pea (without chlorophyll) which emerges. This is a direct result of a mutation which occurs during seed ageing. Background ionising radiation was once thought to be a cause of such genetic lesions, but natural levels are too low to bring about this type of damage. It is evident that irreversible damage to the genome
occurs as the seeds age, but it is not clear whether this is a cause of the loss of viability or an effect.
NEW DEVELOPMENTS IN SEED TECHNOLOGY
Terminator gene technology This refers to the technology of genetic manipulation which prevents the seeds of genetically modified crops from germinating. A promoter sequence (a DNA sequence which activates or switches on a specific gene) is attached to a gene that prevents germination and this is then inserted into the seed of the target species. The genetically modified seed is itself capable of germination but the plants that develop from the seed are sterile because the germination-inhibiting gene is activated during seed maturation. This technology has caused widespread concern because it undermines a common premise of food production, namely the ability to produce self-saved seed. However, this is also true of the well-established practice of producing Fl hybrid seeds, which are the product of crosses between inbred parental lines. Seed produced on these plants do not breed true and so self-saved seed is