Water relations of lettuce seed thermoinhibition. II. Ethylene and endosperm effects on base water potential

A water relations (hydrotime) model was employed to characterize the responses of lettuce (Lactuca sativa L) seed germination to temperature, water potential (psi), endosperm disruption, and the ethylene precursor 1-aminocyclopropane-1-carboxyhc acid (ACC) The base water potentials (psi(b)), standard deviations of base water potential (sigma(psi b)) and hydrotime constants (theta(H)) estimated by the model were used to predict the germination rates at reduced psi and supraoptimal temperatures in the presence or absence of ACC The distribution of psi(b) values among seeds in the population (psi(b)(g)) increased (became more positive) as the imbibition temperature increased, indicating a greater sensitivity of seeds to reduced psi at thermoinhibitory temperatures Slitting the seeds at the cotyledonary end to disrupt the integrity of the endosperm extended the high temperature range for germination and maintained lower psi(b)(g) distributions compared with intact seeds as temperature increased ACC (10 mM) in the imbibition solution raised the upper temperature limit for germination by about 2 degrees C, and also lowered psi(b)(g) values by 0 1 to 0 2 MPa The effect of ACC on psi(b)(g) was more pronounced at higher imbibition temperatures and in slit seeds. ACC (via conversion to ethylene) extends the high temperature limit for lettuce seed germination by acting in the embryo to maintain a lower psi threshold for the initiation of growth as temperature increases Evidence from other species as well suggests that this may be a general mechanism for the action of ethylene in promoting seed germination.