WATER LOSS AND CHLOROPHYLL FLUORESCENCE DURING EX VITRO ACCLIMATIZATION IN MICROPROPAGATED BLACK MULBERRY (MORUS NIGRA L.)

Desiccation after ex vitro transfer is one of the main causes of mortality of tissue culture-derived plantlets. Water loss and chlorophyll a fluorescence parameters were determined in micropropagated regenerants; derived from a superior 80-year-old black mulberry (Morus nigra L.) tree in the course of a 56-day ex vitro acclimatization period. On days 0, 7 and 35 after transfer, leaves that were formed in vitro lost more than 80% of their water content within 90 min of air-drying. Poor control of water loss clearly demonstrated the inability of non-acclimatized plantlets to avoid desiccation. In contrast, ex vitro formed leaves sampled on day 56 showed slight witting and had a water loss rate of less than 29% after 90 min. The lowest rates of maximal photochemical efficiency of photosystem II (F(v)/F(m)), potential electron capacity ('area'), and maximum-to-initial fluorescence ratio (F(m)/F(0),) were found on day 0 at the end of the in vitro culture period. The recovery of these rates took place gradually during the acclimatization period with the most prominent changes found between the days 0 and 7 as well as 35 and 56. The acclimatization was achieved 8 weeks after transfer when new ex vitro formed leaves started to grow.