Light quality influences the polyamine content of lettuce (Lactuca sativa L.) cotyledon explants during shoot production in vitro

Light quality has previously been shown to influence morphogenesis in lettuce cotyledon explants, with white or red light promoting adventitious shoot production, and blue light inhibiting it. Endogenous polyamine (PA) concentrations were compared between explants cultured under different light qualities. Explants cultured under white or red light accumulated PAs during shoot primordia production, with a 5.6-fold increase compared to initial concentrations under white light, and 6.7-fold increase under red light. These results suggest polyamines are involved in the formation of shoot primordia. After 18 days in culture PA concentrations decreased under white light, and to a lesser extent under red light, signaling a shift in polyamine metabolism that correlates with shoot expansion, which occurs more readily under white light. Explants cultured under blue light accumulated polyamines for the first 7 days, to a level 1.3 times greater than initial values, followed by a gradual decline during the remainder of the culture period. Explants cultured under blue light also contained a greater proportion of PCA-insoluble conjugated PAs, compared to explants under white or red light, which contained greater proportions of free or PCA-soluble conjugated polyamines. The ratio of putrescine to spermidine was also different with a lower Put:Spd ratio being associated with shoot production under white or red light, and higher Put:Spd ratio being associated with culture under blue light.