Dormancy, viability and longevity.

Abstract

There are two basic physiological prerequisites for seeds to survive in soil: germination must be avoided by dormancy or quiescence, while viability must be maintained. The remarkable longevity of some individual seeds is legendary. This chapter reviews some of these accounts, assessing these long-lived individuals in the context of the overall seed population.

Germination Ecophysiology

“…As seeds imbibe, metabolism increases, damage may be repaired and microbial attack may be actively resisted. The effect of increasing water potential is thus completely reversed so that seed longevity increases with increase in moisture availability, provided the seeds remain dormant and air is present to allow respiration (Ibrahim et al 1983;Murdoch and Ellis 2000). Imbibed tissues and hence seed longevity are, however, very vulnerable to high temperatures, and predicted periods for 50 % loss of viability of imbibed seeds of S. hermonthica, O. crenata and O. cumana were, respectively, 98, 60 and 30 h at 45 C but only 8, 7 and 6 min at 60 C (calculated from regression coefficients in Dawoud 1995).…”

“…At the lower temperatures, likely to characterise most moist soils, fully imbibed seeds of Orobanche and Phelipanche spp. have been shown to survive for at least 3 months at 30 C and to show negligible loss of viability over 7 months at temperatures of 10-20 C provided air is present (Kebreab and Murdoch 1999a, b;Murdoch and Ellis 2000). In comparison to drier seeds whose survival follows the viability equation (11.1), these responses are relatively poorly quantified, but seed survival curves do approximate to negative cumulative normal distributions (Dawoud 1995;Kebreab and Murdoch 1999a, b).…”

“…Seed storage behaviour is classified as orthodox if seed longevity increases in a quantifiable and predictable manner with decrease in temperature and decrease in seed water potential from about À20 MPa to about À350 MPa, whereas recalcitrant seeds cannot be dried without losing viability (Ellis and Roberts 1980;Murdoch and Ellis 2000).…”

“…Such factors may include depth-sensing factors such as light quality and quantity and the amplitude of diurnal temperature fluctuation and seasonal factors such as soil nitrogen status, temperature and soil atmospheric composition (Roberts 1973;Roberts et al 1987;Murdoch and Ellis 2000). As pointed out, however, by Joel et al (2006), "the most specialized and intriguing germination mechanisms belong to some parasitic angiosperms that do not germinate until they detect the presence of a host plant".…”

“…It is especially significant in annual parasitic plants such as Orobanche, Phelipanche, Alectra and Striga species, most of which disperse and regenerate exclusively by seed. Environmental influences during maturation on the mother plant and subsequently in the soil seed bank interact with genotype and are adapted to increase the probabilities that a seed germinates in "the right place and at the right time" and regenerates successfully, completing a full life cycle (Murdoch and Ellis 2000). The key question in this chapter is: what proportion of viable seeds will regenerate successfully?…”