Developmental and seasonal changes of stress responsiveness in beech leaves (Fagus sylvatica L.)

The development of beech leaves (Fagus sylvatica L.) was characterized by determination of the pigment and electrolyte concentrations as well as the accumulation of dry mass and specific leaf mass from bud break to senescence. To test the hypothesis that stress tolerance and responsiveness of defences show developmental and/or seasonal changes, leaf discs were either incubated in the absence (control) or presence of paraquat to induce oxidative stress. Controls displayed developmental changes in stress susceptibility ranging from less than 15% of maximum electrolyte leakage in mature leaves to more than 20% leakage in senescent and 36-46% in immature leaves. Paraquat concentrations were chosen to result in about 95% of maximum electrolyte conductivity within 24 h in all developmental stages. Paraquat accumulation was about two-fold lower in senescent as compared with immature leaves, whereas stress susceptibility, as characterized by the kinetics of the increase in relative leakage, was similar in these developmental stages with 50% of maximum electrolyte conductivity (EC50) = 6.5 h in immature and 7.5 h in senescent leaves. In mature leaves with intermediate paraquat accumulation rates, two classes of stress-sensitivity were distinguished, namely stress-resistant and stress-susceptible leaves with EC50 = 9.5 and 5.2 h, respectively. Stress-resistance of mature leaves was accompanied by a rapid, approximately two-fold induction of superoxide dismutase activity. Stress-sensitive mature leaves initially contained high superoxide dismutase activities but showed a rapid, more than sixfold loss in activity in 24 h. Correlation of meteorological data with leakage rates suggested that high air temperatures and low precipitation might have been predisposing for loss of resistance against oxidative stress in beech leaves.