A continuous flow system was used to study the interactions between carbon dioxide and ethylene in intact sunflower (Helanthus aNua L.)

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).

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 '

The relationship between the temperature at which germination of 50% of the seeds is inhibited in the light (GTso Light) and secondary dormancy was investigated in three cultivars of Lactuca sativa L. Seeds were incubated for varying periods under non-germinating conditions and subsequent germination in response to red light (R) was determined over a wide range of temperatures. Dark incubation at 32 C reduced the GT5o Light of cv. New York but did not affect germination at temperatures below 24 C. Dark, 32 C incubation had no effect on the GTso Light of cv. Great Lakes. In cv. Grand Rapids, dark incubation at 15, 24, 32, or 35 C initially reduced the GTso Light. However, longer incubations induced a secondary dormancy, i.e., the seeds became unable to germinate at all temperatures in response to R given after the high temperature incubation. A single exposure to R at the binning of a 32 C incubation slowed the induction of secondary dormancy. Repeated exposures to R prevented the induction of secondary dormancy, but did not prevent a decline in the GT50 Light. GA8 mimicked the effect of repeated R.The differences in the germination behavior of the three cultivars suggest that there may be qualitative differences in the germination mechanism of these cultivars. This research demonstrates the significance of monitoring germination at a range of temperatures to avoid misinterpretation of the data.osmotic pressure have been shown to be related to changes in one or both of these GT50 (21, 24).Incubation of fully imbibed seeds under conditions not suitable for germination is known to reduce their germination potential. High temperature incubations can induce a light requirement in dark germinating lettuce seeds (1, 2), i.e., the seeds become photodormant. This effect has been attributed to a decline in the GT50 Dark (1 1, 12). In the light sensitive cv. Grand Rapids, prolonged dark, or high temperature incubations may also decrease photosensitivity until the seeds become unresponsive to R, i.e. secondarily dormant (3,4,9,27,31, 34,35).In most previous studies of secondary dormancy, its onset has been monitored at only one temperature, thus, little is known about the changes in GT50 Light during the induction of secondary dormancy. In the present study, changes in germination have been monitored over a wide range of temperatures in the examination of the effects of interactions among light, temperature and growth regulators on the induction of secondary dormancy in three lettuce cultivars.In this paper, high-temperature incubation refers to the lowest temperature capable of suppressing germination to zero in both light and dark treatments; and secondary dormancy refers to the seeds that become incapable of germination in response to R at any temperature.Lettuce (Lactuca sativa L.) seed germination is strongly temperature dependent. As the temperature rises above the optimum (18), germination declines sharply, often falling from 100%o to near 0% with an increase of only 2 or 3 degrees C (23). The temperature at ...

A random sampling analysis of laboratory air and of air from commercially available cylinders indicated that they contain appreciable amounts of low molecular weight hydrocarbons, viz. methane, ethane, and ethylene, as contaminants. These impurities could lead to erroneous conclusions in studies of plant growth and metabolism. Different methods for removal of these contaminants were compared and evaluated in the present investigation for their suitability in plant studies. Most of the methods currently being used were found inadequate. The use of metal catalysts at high temperature, adapted from gas analysis techniques, provides an inexpensive and efficient method for removing hydrocarbons from air in both closed and continuous flow systems.