Physiological effects of temporary immersion on Hevea brasiliensis callus

In vitro culture by temporary immersion generates potentially stressful con ditions for explants that may differ from those associated with classic met hods. In order to evaluate the effects of these conditions on physiological changes in explants, different parameters of metabolic activity were inves tigated for a friable embryogenic callus of Hevea brasiliensis (Mull. Arg.) , in response to 1 min, and 1, 12 and 24 h per day of immersion, using semi -solid and agitated liquid media as controls. The relative growth rate of t he callus was not significantly different for the 1 min immersion treatment and the controls, but it decreased by about 60% for the 1-, 12- and 24-h i mmersion treatments. During the immersed stage, the rate of respiration of the callus was comparable for all the treatments. However, during the emers ed stage, the respiration rate increased by 140 and 164% for the 12- and 24 -h immersion treatments, respectively. Meanwhile, the total adenylate nucle otide concentration and the ratio of ATP/ADP remained almost constant, or e ven decreased. The adenylate energy charge was comparable for all the treat ments, averaging 0.88. The superoxide dismutase activity and the lipid pero xidation increased with the immersion duration, and were significantly high er for the 12- and 24-h immersion treatments than for controls. However, af ter 24 h in emersed stage, there was no lipid peroxidation, regardless of p revious immersion duration. It appears from these results that the immersed stage induced a substantial oxidative stress, which was not associated wit h the callus immersion per se.