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

The development of beech leaves (Fagus sylvatica L.) was characterized by d etermination of the pigment and electrolyte concentrations as well as the a ccumulation of dry mass and specific leaf mass from bud break to senescence . To test the hypothesis that stress tolerance and responsiveness of defenc es show developmental and/or seasonal changes, leaf discs were either incub ated in the absence (control) or presence of paraquat to induce oxidative s tress. Controls displayed developmental changes in stress susceptibility ra nging 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 c onductivity within 24 h in all developmental stages. Paraquat accumulation was about two-fold lower in senescent as compared with immature leaves, whe reas stress susceptibility, as characterized by the kinetics of the increas e in relative leakage, was similar in these developmental stages with 50% o f 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, respecti vely. Stress-resistance of mature leaves was accompanied by a rapid, approx imately two-fold induction of superoxide dismutase activity. Stress-sensiti ve mature leaves initially contained high superoxide dismutase activities b ut showed a rapid, more than sixfold loss in activity in 24 h. Correlation of meteorological data with leakage rates suggested that high air temperatu res and low precipitation might have been predisposing for loss of resistan ce against oxidative stress in beech leaves.