Influence of ionic composition of the culture medium on de novo flower formation in tobacco thin cell layers

Floral and vegetative organogenesis in thin cell layers of Nicotiana tabacum cv. Samsun was studied in relation to the ionic composition of the medium. The control liquid medium was buffered to pH 5.5. NH4+, NO3−, K+, and Ca2+ were separately varied between 0 or 0.1 times and 2, 3 or 4 times their initial control concentration. Increasing the concentrations of NO3−, and Ca2+ above their control concentration promoted the formation of vegetative buds instead of flowers. When K+ at 80.0 mM, Na+ at 78.9 mM, and NH4+ as the sole nitrogen source used, neither flower formation nor morphogenesis were observed in 70, 100, and 100% of the explants, respectively. NH4+ as the sole nitrogen source acidified the medium to pH 4.2, whereas the other ionic conditions did not change the external pH. NO3− at 7.9 mM prevented this acidification and allowed flower formation in 75% of the explants. This acidification could increase the K+ and Na+ influxes via a possible lowering of the intracellular pH. This hypothesis corroborates the morphogenic effect of K+ and Na+. Key words: absence of morphogenesis, acidification of medium, floral and vegetative organogenesis, ionic composition of the medium, thin cell layers, Nicotiana tabacum cv. Samsun.