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+2, 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.
Modulation of polyamine catabolism in pea seedlings by calcium during salinity stress
