Antagonistic Effects of High and Low Temperature Pretreatments on the Germination and Pregermination Ethylene Synthesis of Lettuce Seeds

Abstract

Red light-induced germination of Grand Rapids lettuce seeds (Lactuca sativa L.) incubated at 20 C was inhibited if the seeds were first imbibed at 30 C for 36 hours. This effect was counteracted by exogenous ethylene and associated with a reduction in the rate at which the seeds produced ethylene throughout the pregermination period. A chilling treatment reversed the effect of a prior imbibition at 30 C on both germination and ethylene production. The possibility that the pretreatments influence germination through their effects on ethylene production is discussed.Other evidence presented indicates that the inability of seeds to germinate at supraoptimal temperature is not due either to a rapid loss of far red-absorbing phytochrome or to an inadequate capacity for ethylene synthesis. It was also shown that a chilling treatment potentiated germination at high temperature without affecting the ethylene synthetic capacity of the seeds.A number of the effects of temperature on the germination of Grand Rapids lettuce seeds have been attributed to the temperature-sensitivity of Pfr decay (5,9,10,12). Among these is the inhibitory effect of a short imbibition at a temperature which is supraoptimal for germination on the capacity of the seeds to germinate when subsequently incubated at a lower temperature (5, 10). However, other inhibitory effects have been inferred to account for the dormancy of seeds subjected to longer pretreatments at high temperature (5, 10). In the present study it was found that dormancy induced in this way was broken by ethylene. The possibility that this and other effects of temperature on the germination of lettuce seeds are mediated by regulatory effects on endogenous ethylene synthesis was, therefore, examined. MATERIALS AND METHODSSeeds (Lactuca sativa L. cv. Grand Rapids) harvested in 1970 were purchased in December of that year and stored for 10 months in airtight containers at -20 C before use. For the 'The research was supported in part by Canadian National Research Council Grant A2908. experiments described, seeds were selected for freedom from damage and uniformity of color and size.The seed pretreatments referred to below were carried out as follows. Lots of 50 or 135 seeds were spread over 4.25-cm filter circles. The latter were placed on a glass plate covered with two layers of filter paper. In turn, the glass plate rested on 1.5 cm high supports standing in an enameled dish. The filter paper covering the plate was folded down at the edges so that it was in contact with the bottom of the dish. Imbibition of the seeds was started by adding water to the dish which was then covered with another glass plate and incubated in the dark at the temperature, and for the period, required.All the light treatments consisted of a 1-min exposure to a red or far red filtered light source with maximum energy at 660 or 731 nm respectively (half peak height bandwidth 10 nm) and an irradiance of 6 X 10-J cm-2 sec-1.To determine the effects of exogenous ethylene on the germination of pretreated...

Ethylene evolution and endo-beta-mannanase activity during lettuce seed germination at high temperature

“…Ethylene overcomes the inhibitory effect of high temperature on lettuce seed germination (Abeles, 1986;Abeles & Lonski, 1969;Burdett, 1972a;Dunlap & Morgan, 1977;Fu & Yang, 1983;Huang & Khan, 1992;Keys et al, 1975;Khan & Prusinski, 1989;Negm et al, 1972;Rao et al, 1975;Saini et al, 1986;. However, the mechanism of ethylene action during seed germination is not understood.…”

“…Several authors reported that ethylene synthesis or sensitivity to ethylene action in lettuce was decreased during seed imbibition at high temperature (Abeles, 1986;Burdett, 1972a;Dunlap & Morgan, 1977;Khan & Huang, 1988). High temperature stress may inhibit ethylene production and/or action and raise the threshold concentration of ethylene needed for lettuce seed germination.…”

Effects of 35 C Heat Treatments on Photosensitive Grand Rapids Lettuce Seed Germination

“…This phenomenon is typically attributed to a conversion of Pfr to Pr (10,16,19). Thermodormancy may be completely eliminated by R2 at 20 C or, in some cases, by a combination of CO2 + ethylene (7,14).…”

“…Heat treatments (30-37 C) have been performed on imbibed seeds (7,9,10,14,16,19) inducing a thermodormancy in which dark germination percentage is reduced to zero in both photodormant and nonphotodormant seeds. This phenomenon is typically attributed to a conversion of Pfr to Pr (10,16,19).…”

“…This phenomenon is typically attributed to a conversion of Pfr to Pr (10,16,19). Thermodormancy may be completely eliminated by R2 at 20 C or, in some cases, by a combination of CO2 + ethylene (7,14).Heat treatments are being used routinely to drive dark germination percentage to zero in many studies of photodormant seed germination. This report examines the changes in the germination potential during heat treatments, as well as the effective response to R and FR after heat treatments.…”