Control of Seed Germination by Abscisic Acid

Abstract

The physical mechanism of seed germination and its inhibition by abscisic acid (ABA) in Brassica napus L. was investigated, using volumetric growth (= water uptake) rate (dV/dt), water conductance (L), cell wall extensibility coefficient (m), osmotic pressure (IIi), water potential (*i), turgor pressure (P), and minimum turgor for cell expansion (Y) of the intact embryo as experimental parameters. dV/dt, Hi, and *i were measured directly, while m, P, and Y were derived by calculation. Based on the general equation of hydraulic cell growth IdV/dt = Lm/(L + m) (Al -Y), where All = Hi -II of the external medium; the terms (L m/(L + m) and II -Y were defined as growth coefficient (kG) and growth potential (GP), respectively. Both kG and GP were estimated from curves relating dV/dt (steady state) to I of osmotic test solutions (polyethylene glycol 6000).During the imbibition phase (0-12 hours after sowing), kG remains very small while GP approaches a stable level of about 10 bar. During the subsequent growth phase of the embryo, kG increases about 10-fold. ABA, added before the onset of the growth phase, prevents the rise of kG and lowers GP. These effects are rapidly abolished when germination is induced by removal of ABA. Neither L (as judged from the kinetics of osmotic water efflux) nor the amount of extractable solutes are affected by these changes. Ili and *i remain at a high level in the ABA-treated seed but drop upon induction of germination, and this adds up to a large decrease of P, indicating that water uptake of the germinating embryo is controlled by cell wall loosening rather than by changes of II; or L. ABA inhibits water uptake by preventing cell wall loosening. By calculating Y and m from the growth equation, it is further shown that cell wall loosening during germination comprises both a decrease of Y from about 10 to 0 bar and an at least 10-fold increase of m. ABA-mediated embryo dormancy is caused by a reversible inhibition of both of these changes in cell wall stability.In a previous paper (29), we have provided evidence based on kinetics of water uptake and a factorial analysis that exogenous ABA controls germination of rape seeds by limiting water uptake of the embryo rather than by inhibiting energy* metabolism, protein synthesis, or related processes. It was concluded that ABA and osmotic stress interact at a common point controlling the water relations of the embryo specifically during the growth phase of germination. This phase commences about 12 h after sowing and is characterized by a resumption of rapid water uptake due to active embryo enlargement. In contrast, water uptake during the preceding imbibition phase (O to ,umol 1-' (29). However, a 10-fold higher concentration is needed to maintain the dormant state for longer periods of time.In the present paper, we attempt to localize the site(s) of action of ABA within the physical parameters governing the water relations of the germinating seed. The inhibitory effect of ABA on water uptake has been shown to be both rapidly ind...

Water Relations of Seed Development and Germination in Muskmelon (Cucumis melo L.)

“…The effect of increasing ABA concentrations on germination of rape seed was very similar to that of decreasing if, resulting in a linear interaction between inhibition of germination by ABA and by reduced 4I (20). Subsequent work on rape seed showed that the effect of ABA was to increase the minimum 42p for growth and decrease the proportionality constant between growth rate and 4,p (extensibility coefficient) (21).…”

Endo-β-Mannanase Activity from Individual Tomato Endosperm Caps and Radicle Tips in Relation to Germination Rates

“…Groot and Karssen (1992) showed that ABA inhibits mechanical weakening of the tomato endosperm cap, but we found no inhibition of mannanase activity at a time when seeds that were allowed to imbibe in water would have completed radicle emergence. It is possible that endosperm weakening occurred but that embryo growth was inhibited by ABA (Schopfer and Plachy, 1985). However, initial radicle protrusion is not prevented by ABA in seeds from which the endosperm cap is removed Toorop et al, 1996), and other data also indicate that ABA action is primarily on the endosperm (Groot and Karssen, 1992;Hilhorst and Downie, 1996).…”

Abscisic Acid and the Developmental Regulation of Embryo Storage Proteins in Maize

“…In many plant tissues, ABA levels rise as a result of water deficit (for review, see 31) and this has been shown to modulate the levels of mRNAs encoding proteins associated with water stress (4,11,12,20). In young embryos, however, it is the rise in ABA level itself that inhibits water uptake, probably by restricting cell wall extensibility (29), and regulation of gene expression in young embryos may be a result of water limitation, rather than a direct effect of ABA (9).…”