Requirement for Ethylene Synthesis and Action during Relief of Thermoinhibition of Lettuce Seed Germination by Combinations of Gibberellic Acid, Kinetin, and Carbon Dioxide

Abstract

Application of exogenous ethylene in combination with gibberellic acid (GA3), kinetin (KIN), and/or CO2 has been reported to induce germination of lettuce seeds at supraoptimal temperatures. However, it is not clear whether endogenous ethylene also plays a mediatory role when germination under these conditions is induced by treatment regimes that do not include ethylene. Therefore, possible involvement of endogenous ethylene during the relief of thermoinhibition of lettuce (Lactuca sativa L. cv Grand Rapids) seed germination at 32°C was investigated. Combinations of GA3 (0.5 millimolar), KIN (0.05 millimolar), and CO2 (10%) were used to induce germination. Little germination occurred in controls or upon treatment with ethylene, KIN, or CO2. Neither KIN nor CO2 affected the rate of ethylene production by seeds. Both germination and ethylene production were slightly promoted by GA3. Treatments with GA3 + CO2, GA3 + KIN, or GA3 + CO2 + KIN resulted in approximately 10-to 40-fold increases in ethylene production and 50 to 100% promotion of germination as compared to controls. Initial ethylene evolution from the treated seeds was greater than from the controls and a major surge in ethylene evolution occurred at the time of visible germination. Application of 1 millimolar 2-aminoethoxyvinyl glycine (AVG), an inhibitor of ethylene synthesis, in combination with any of above three treatments inhibited the ethylene production to below control levels. This was accompanied by a marked decline in germination percentage. Germination was also inhibited by 2,5-norbornadiene (0.25-2 milliliters per liter), a competitive inhibitor of ethylene action. Application of exogenous ethylene (1-100 microliters per liter) overcame the inhibitory effects of AVG and 2,5-norbornadiene on germination. The results demonstrate that endogenous ethylene synthesis and action are essential for the alleviation of thermoinhibition of lettuce seeds by combinations of GA3, KIN, and CO2. It also appears that these treatment combinations do not act exclusively via promotion of ethylene evolution as the application of exogenous ethylene alone did not promote germination.The optimum temperature for the germination oflettuce seeds is in the vicinity of 20°C, though differences are encountered among varieties and seed lots (12,20,23). Germination is inhibited at temperatures above the optimum (thermoinhibition), often falling sharply to reach zero within a narrow temperature range (12,16). The seeds that fail to germinate upon imbibition at these supraoptimal temperatures, eventually enter '

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

“…Ethylene evolution prior to radicle protrusion was low and not detected before the 9-12 h collection period in seeds imbibed in water or with ethylene inhibitors (Tables 2 and 3). Several authors (Fu & Yang, 1983;Saini et al, 1986;Khan, 1994) reported that little or no ethylene was produced before radicle protrusion. In the present study, little or no ethylene was detected in seeds at 16 and 12 h at 20 and 35°C, respectively, and relatively large amounts of ethylene were produced at or after radicle protrusion.…”

“…ACC, the immediate precursor of ethylene biosynthesis (Yang & Hoffman, 1984), has been implicated in the alleviation of high temperature on lettuce seed germination (Abeles, 1986;Saini et al, 1986;Khan & Prusinski, 1989;Huang & Khan, 1992). Dutta & Bradford (1994) reported that ACC (via conversion to ethylene) extended the high temperature limit for lettuce seed germination by acting in the embryo to maintain a lower water potential threshold for the initiation of growth as temperature was increased.…”

“…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.…”

Expression of9-cis-EPOXYCAROTENOID DIOXYGENASE4Is Essential for Thermoinhibition of Lettuce Seed Germination but Not for Seed Development or Stress Tolerance

“…Exogenous application of ethylene or its precursor ACC can alleviate thermoinhibition in lettuce seeds, whereas ethylene biosynthesis or action inhibitors can enhance thermoinhibition (Saini et al, 1986(Saini et al, , 1989Nascimento et al, 2000Nascimento et al, , 2004Kozarewa et al, 2006). Silencing of NCED4 in Sal seeds increased transcript levels of two ethylene biosynthetic genes, ACS1 and ACO2 ( Figures 3J and 3K), both of which also have high transcript levels in UC seeds imbibed at high temperature .…”

Control Processes in the Induction and Relief of Thermoinhibition of Lettuce Seed Germination

“…Using continuous red light to induce germination, the role of endogenous C2H4 in germination at 32°C was studied. Ethylene evolution from irradiated seeds began to increase 2 hours prior to radicle protrusion, whereas the dark-incubated (nongerminating) seeds produced a low, constant amount of C2H4 throughout the 24 hour incubation period. Inhibition of C2H4 synthesis with 2-aminoethoxyvinyl glycine and/or inhibition of C2H4 action with 2,5-norbomadiene blocked the promotive effect of light.…”