Polyamines and methyl jasmonate in bulb formation of in vitro propagated tulips

Effects of polyamines (PAs), their precursors, ornithine (Orn) and arginine (Arg), as well as methyl jasmonate (MeJA) were studied on bulb formation in vitro with four tulip cultivars differing in bulbing ability. The endogenous PA profiles of the shoots and bulbing shoots were monitored during successive micropropagation stages: S1, sixth week of the standard multiplication subculture; S2, fifth week of the last multiplication subculture; S3, 5 weeks later, just before the cold treatment; S4, at the end of the 13-week-cold treatment; and S5, 6 weeks after the end of cold treatment (shoot base swelling/bulb formation phase). Putrescine (Put), spermidine (Spd) and spermine (Spm) (50 and 100 µM) as well as Arg and Orn (500 and 1000 µM) were added to the medium at various bulbing stages (S3, S4 and S5). Positive effects of these compounds on bulb formation were found for the cultivar ‘New Beet’ (NB), with the lowest bulbing ability. Both PAs and their precursors, when used at lower concentrations at the beginning of bulbing process (S3), together with standard MeJA treatment, significantly enhanced bulb numbers in this cultivar. Putrescine was the major endogenous polyamine at the stages of shoot multiplication (S1–S2). Spd levels were the highest in ‘Fringed Black’ (FB), the cultivar of extremely high bulbing ability, and this PA dominated over Put during the entire micropropagation process in this cultivar. Endogenous Spd contents manifested similar dynamics in all cultivars showing two peaks: at shoot multiplication (S2) and at the end of cooling (S4). The contents of all the PAs were the lowest at the final bulbing stage (S5) except the Spm level in cultivar NB in which it increased at this time. It is postulated that the high bulbing competence may be related to a transient increase in Spd content in response to low temperature treatment (S4), followed by its rapid decrease during bulb growth and maturation (S5, 6 weeks after the end of cooling) when developing bulbs begin to enter dormancy. The low levels of all the PAs at final bulbing stage as well as low Put/Spd ratio may also aid in the bulb formation of tulip in vitro.