Plant material
Fully developed, undamaged leaves of Prunus dulcis (Mill.) D.A. Webb and Pyrus communis L. were used for this study. Leaves of each species were selected from the central part of current-year sun-exposed shoots, of southeast facing branches in five mature trees growing at the University of California Davis campus (38.542154° N, 121.796485° W, Davis, USA). Branches were randomly collected in the early morning, enclosed in dark plastic bags containing damp paper towels and transferred to the laboratory. Homogeneous looking leaves were chosen from branches for further experimental treatments and analyses.