The effect of in vitro culture conditions on the pattern of maximum photochemical efficiency of photosystem II during acclimatisation of Helleborus niger plantlets to ex vitro conditions

The influence of in vitro conditions including sucrose and nitrogen content in the culture medium as well as temperature on the pattern of chlorophyll a photochemical efficiency of photosystem II (PSII) of Helleborus niger L. plantlets upon transfer to ex vitro conditions were examined. In the experiment, the micro-shoots which were proliferated and rooted in vitro on the media with different levels of sucrose (10, 20, 30, 40, 50, 60, 70, and 80 g L-1) and nitrogen salts KNO3 and NH4NO3 (both 25, 50, 100 % according to MS medium), as well as temperature (15 and 20 A degrees C) were used. The multiplication of axillary shoots were performed on modified MS medium supplemented with cytokinins (2iP, BAP and kinetin, each at a concentration of 1.0 mg L-1) and GA(3) (2.5 mg L-1) added together. For the rooting of shoots, the medium was supplemented with IBA 1 mg L-1 and NAA 0.1 mg L-1 added together. The maximal photochemical efficiency of PSII immediately upon removal of microplants from in vitro culture ranged from 0.24 to 0.81 depending on the treatment. High concentration of sucrose (above 50 g L-1) and standard (100 %) or nitrogen salts decreased to 25 % in the MS medium as well as 20 A degrees C temperature decrease the photochemical efficiency of microplants at transplanting time to ex vitro conditions as well as hinder acclimatisation and further growth out of culture. During acclimatisation to ex vitro conditions, a gradual increase of the maximal photochemical efficiency of in vitro-derived leaves was observed in all treatments. The maximal photochemical efficiency of PSII ranged from 0.79 to 0.83 after 4 weeks and from 0.81 to 0.83 after 20 weeks, indicating that H. niger plantlets gradually adapted to the greenhouse conditions and recovered their photosynthetic performance.