Control of Seed Germination by Abscisic Acid

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Abstract
The physical mechanism of seed germination and its inhibition by abscisic acid (ABA) in B. 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 (.PSI.i), turgor pressure (P) and minimum turgor for cell expansion (Y) of the intact embryo as experimental parameters, dV/dt, IIi and .PSI.i were measured directly, while m, P and Y were derived by calculation. Based on the general equation of hydraulic cell growth [dV/dt = Lm/(L + m) (.DELTA.II - Y), where .DELTA.II = IIi - II of the external medium], the terms (Lm/(L + m) and IIi - 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 II of osmotic test solutions (polyethylene glycol 6000). During the imbibition phase (0-12 h after sowing), kG remains very small while GP approaches a stable level of .apprx. 10 bar. During the subsequent growth phase of the embryo, kG increases .apprx. 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. IIi and .PSI.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 IIi or L. ABA inhibits water uptake by preventing cell wall loosening. By calculating Y and m from the growth equation, cell wall loosening during germination comprises both a decrease of Y from .apprx. 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.