DEHYDRATION OF GERMINATING PERENNIAL RYEGRASS SEEDS CAN ALTER RATE OF SUBSEQUENT RADICLE EMERGENCE

Perennial ryegrass (Lolium perenne L.) seeds (caryopses) germinate at or near the soil surface, where water potential can fluctuate widely. This study examined germination of 'Del Ray' perennial ryegrass seeds when imbibition was interrupted by dehydration prior to radicle emergence. Seeds were hydrated for 0 to 40 h (26 degrees C), dehydrated at atmospheric water potentials of -4, -40, -100 and/or -150 MPa for 4-168 h, then rehydrated. Germination (radicle elongation greater than or equal to 1 mm), seedling growth, solute leakage, and endogenous abscisic acid (ABA) levels were measured. Treatment differences in final germination percentage, seedling growth, and solute leakage were generally not significant. However, the onset of radicle emergence was delayed and the rate of germination slowed when dehydration at -150 MPa was initiated after 36 or 40 h hydration. Slowed germination rates were not observed when dehydration was initiated before 36 h, when dehydration occurred at -4 MPa, or when dehydration at -150 MPa was preceded by dehydration at -4 MPa for 24 h. Exogenous abscisic acid (ABA) concentrations above 10(-6) M inhibited germination. However, endogenous whole seed ABA levels declined during imbibition due to leaching, and did not increase during dehydration treatments that delayed germination. These results illustrate that rate of late-occurring dehydration treatments is critical in determining subsequent germination response. We propose that seed response to late-occurring dehydration may be of ecological significance in timing radicle emergence to coincide with adequate soil moisture for seedling establishment.