Prevention of hyperhydricity in micropropagated carnation shoots by bottom cooling: implications of oxidative stress

Carnation shoot cultures were micropropagated in two different agar concentrations (0.58 and 0.85%) and placed in a bottom cooling system or control conditions. During the culture period of 28 days, it was observed that relative humidity, hyperhydricity, dry weight, multiplication rate, and the activity of the antioxidant enzymatic system changed in relation to the agar concentration used and the application of bottom cooling. The percentage of hyperhydric shoots also showed a significant decrease under bottom cooling conditions for both agar concentrations. Lipid peroxidation was always lower in shoots cultured with bottom cooling. All the antioxidant enzymatic activities were lower in bottom cooling treatments compared to controls. These results show that the normalization of the environmental conditions in vitro via bottom cooling can prevent the onset of different simultaneous stress reactions concomitant with hyperhydricity. The present work provides for the first time, direct evidence of a reduced H2O2 generation in the tissues cultured in bottom cooling able to reduce oxidative stress.