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Preconditioning ‘Mesa 659’ lettuce (Lactuca sativa L.) seeds with the moist solid carrier Micro-Cel E at 15C for 20 hours removed thermoinhibition and allowed the seeds to germinate at 35C. Participation of 1-aminocyclopropane-1-carboxylic acid (ACC) and its use in the removal of thermoinhibition were indicated from the following: a) the ACC level in the preconditioned seeds peaked before germination at 35C; b) addition of aminoethoxyvinylglycine (AVG), an inhibitor of ACC synthesis, reduced the ACC level and inhibited germination at 35C; c) response of preconditioned seeds to ethylene at 35C preceded that of ACC; d) exogenous ACC, (2-chloroethyl) phosphonic acid (ETH, ethephon), or ethylene reversed the inhibitory effect of AVG and restored the capability of the seed to germinate at 35C; e) Co, an inhibitor of the ACC-to-ethylene step, partially inhibited germination of preconditioned seeds at 35C, and the effect was reversed by ETH. The major free polyamines in dry seed were spermidine (Spd) and putrescine (Put). An increase in polyamine level occurred following transfer of conditioned seeds to 35C. Of the various polyamine biosynthesis inhibitors, difluoromethylarginine and cyclohexylammonium sulfate inhibited an increase in Spd and Put levels only after germination, whereas difluoromethyorinthine had no effect on polyamine biosynthesis. None of these inhibitors affected the relief of thermoinhibition. Thus, the increase in polyamine level during germination at 35C does not appear to be associated with the alleviation of thermoinhibition. Inhibition of lettuce seed germination at supraoptimal temperatures (thermoinhibition) leads to induction of a secondary dormancy (thermodormancy) (Khan, 1980/81). Preplant seed hydration with water, salt solution, or moist solid carriers, termed physiological conditioning, confers various degrees of thermotolerance to lettuce and other seeds, enabling them to germinate at high temperatures (Khan, 1992). A 20-h presoak in 1% K3PO4 at 15C was more effective than a similar soak in water for alleviating thermoinhibition in ‘Minetto’ lettuce seeds (Cantliffe et al., 1984). Moist solid carriers with high matric potential have been used recently to precondition (matricondition) lettuce and other seeds to improve germination at high temperatures (Khan, 1992). ‘Mesa 659’ lettuce seeds, presoaked on moist Micro-Cel E (a synthetic calcium silicate), consistently germinated above 80% at 35C (Khan et al., 1990). The biochemical basis for the relief of thermoinhibition (thermotolerance) remains unclear. Ethylene synthesis and action have been implicated in the alleviation of high temperature, osmotic restraint, and salinity on lettuce seed germination (Abeles, 1986; Khan and Huang, 1988; Saini et al., 1986). A synergistic or an additive effect between ACC and cytokinin in the alleviation of thermoinhibition and ethylene production has been demonstrated recently for publication 4 Nov. 1991. Accepted for publication 13 Apr. 1992. rch was supported in part by a grant form the Ferry Morse Seed Co., Calif. We gratefully acknowledge samples of difluoromethylornithine romethylarginine from Eckkehard H. Bohme, Merrell Dow Research Cincinnati; aminoethoxyvinylglycine from Hoffman-La Roche, NuMicro-Gel E from Manville Products Corp., Denver; (2-chloroethyl) c acid from Union Carbide, Modesto; and ‘Mesa 659’ lettuce seeds is-Moran Seed Co., Rochester, N.Y. The cost of publishing this paper yed in part by the payment of page charges. Under postal regulations, therefore must be hereby marked advertisement solely to indicate this Fellow, Dept. of Horticultural Sciences. Present address: Dept. of Zhongshan Univ., Guangzhou, People’s Republic of China. , Dept. of Horticultural Sciences. r. Soc. Hort. Sci. 117(5):841-845. 1992. in lettuce seeds (Khan and Prusinski, 1989), suggesting that enhanced use of ACC is possible during the alleviation. Active metabolism of polyamines has also been noted during germination of seeds (Scoccianti et al., 1990; Torrigiani et al., 1988). Since both ethylene and polyamines share S-adenosylmethionine (SAM) as a common precursor (Miyazaki and Yang, 1987), their biosyntheses are interrelated. Polyamine level and activities of polyamine biosynthetic enzymes may increase in response to plant stresses, including high-temperature stress (Das et al., 1987). Although the relationship between stress responses of seedlings or vegetative plant tissues and polyamine content has been extensively studied (Rabe, 1990), to our knowledge, nothing is known about changes in polyamines during early seed germination under stressful conditions. We investigated whether ACC and polyamines are involved in the removal of thermoinhibition in lettuce seeds preconditioned at 15C and attempted to determine by means of specific inhibitors which step(s), if any, in the ethylene and polyamine biosyntheses are involved in the prevention of thermoinhibition. Materials and Methods Seed preconditioning and germination. ‘Mesa 659’ lettuce (1988 harvest) seeds held at 7C and 28% relative humidity (RH) were preconditioned in batches of 2 g with the moist solid carrier Micro-Cel E (Khan et al., 1990). The seed : Micro-Cel E : water or test solution ratio (by weight) in the preconditioning mixture was 1:0.25:1.5. After mixing the contents of the mixture in 0.43-liter glass jars, the jars were loosely capped and held at 15C for 4 to 20 h under fluorescent light (16 μmol·m ·s). After this preconditioning, seeds were washed with running water to remove the carrier and blotted dry. Fifty Abbreviations: ACC, 1-aminocyclopropane-1-carboxylic acid; AVG, aminoethoxyvinylglycine; CHAS, cyclohexylammonium sulfate; DFMA, difluoromethylarginine; DFMO, difluoromethyornithine; ETH, (2-chloroethyl) phosphonic acid; Put, putrescine; Spd, spermidine; Spm, spermine.

Alleviation of Thermoinhibition in Preconditioned Lettuce Seeds Involves Ethylene, Not Polyamine Biosynthesis