Prediction of seed longevity at sub-zero temperatures and genetic resources conservation

Abstract

THE International Board for Plant Genetic Resources (IBPGR) is encouraging the development of an international network of seed banks for the conservation of the genetic resources of the major crop species of the world and their wild relatives¹. Most of the species in priority lists show ‘orthodox’ seed characteristics², that is their longevity is prolonged in a defined manner by a reduction in moisture content (MC) (at least down to 5% MC, fresh weight basis) and temperature³. From a consideration of biological factors and the design and cost of seed storage facilities, IBPGR recommends that orthodox seeds are stored at −18 °C or less and at 5± 1% MC². It is convenient to aim at −20 °C as the nominal temperature. Because of the accumulation of mutations associated with loss of viability^4,5, IBPGR recommends regeneration of stocks (that is growing plants to provide new seeds from the old) when viability has fallen by 5% (ref. 2). To plan ancillary facilities and procedures, it is necessary to estimate regeneration intervals (time taken for viability to fall by 5%) for the major crop species. Such estimates pose three problems: there are genotypically controlled variations in longevity within a species; seed ‘quality’, determined by conditions before harvest and during processing, can affect subsequent longevity; and loss of viability at −20 °C is extremely slow, and so far it has been possible to predict regeneration interval only by questionable extrapolation from more favourable storage conditions^5,6. We have examined these problems in barley (Hordeum distichum, L.) and have developed an approach which should make possible more accurate predictions from experiments lasting only 2 yr—even though fresh seed of most species shows no detectable loss of viability during this time.