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

Abstract

Germination of lettuce seeds (Lactuca sativa L. cv Grand Rapids) in the dark was nearly 100% at 200C but was inhibited at 270C and higher temperatures (thermoinhibition). A single 5-minute exposure to red light completely overcame the inhibition at temperatures up to 280C, above which the effectiveness of single light exposures gradually declined to reach a negligible level at 320C. However, the promotive effect of light could be extended to 340C by repeated irradiations. At any one temperature, increased frequency of irradiations increased germination percentage, and with each degree increase in temperature, increasingly frequent irradiations were necessary to elicit maximal germination. Loss of the effectiveness of single irradiations with increase in temperature may result either from acceleration of the thermal reversion of the far red-absorbing form of phytochrome or decrease in seed sensitivity toward a given percentage of the far red-absorbing form of phytochrome. 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. Exogenous C2H4 overcame these blockages. The results showed that participation by endogenous C2H4 was essential for the light-induced relief of thermoinhibition of lettuce seed germination. However, light did not act exclusively via C2H4 since exogenous C2H4 alone in darkness did not promote germination.Germination of lettuce (Lactuca sativa L.) seeds is strongly influenced by temperature. Germination of lettuce seeds is nearly 100% in the dark at temperatures up to approximately 20°C, although the actual permissive temperature range varies among cultivars or even different seed lots ofthe same cultivar (4,16,27,29).

Germination of dormant seeds of Stylosanthes humilis as promoted by ethylene accumulation in closed environments

“…Moreover, when treated with inhibitors of ethylene biosynthesis, germination of non-dormant seeds is very low in Petri-dishes and relatively high in sealed flasks. This situation is identical to the one displayed by lettuce (Lactuca sativa) (Saini et al, 1989) and common lambsquarters D.M. RIBEIRO and R.S.…”

Ethylene Production during Development of Mustard (Brassica juncea) and Canola (Brassica napus) Seed

“…3) and then, below 40% seed moisture, it exhibited quite low rates of ethylene production more typical of the rates found in genninating seed (e.g. Saini et al, 1989). In canola, the transition from predesiccation to desiccation occurred at approximately 60% seed moisture.…”

“…The evolution of ethylene from isolated seed, intact detached siliques, and the isolated silique walls was measured in an open, continuous flow system adapted from those used previously in this laboratory (Eastwell and Spencer, 1982;Bassi and Spencer, 1989;Saini et al, 1989). Compressed air containing 350 PL L-' COz (Linde Union Carbide, Edmonton, Alberta, Canada) was passed through a platinum-catalyzed thermal oxidizer to remove hydrocarbon contaminants and then through a humidifying bottle to prevent desiccation of the plant material.…”

“…With this system (Bassi and Spencer, 1989) it is possible to prevent the accumulation of volatiles in the cu-' Financia1 assistance from the Natural Sciences and Engineering…”

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 .…”