Freezing injury in the olive leaf and effects of mefluidide treatment

Foliar application of mefluidide, a synthetic plant growth regulator, at 1000 ppm concentration on three year old potted olive plants cv. Frantoio, which is sensitive to frost, induced a hardening effect by increasing the freezing resistance without provoking phytotoxic effects. The main effects of mefluidide treatment seem to be related to induced water stress caused by a reduction in the relative water content (10.5%) and water potential (0.75 MPa) in the leaves and to an increase in the stability of biological membranes. At -18 degrees C, K+, Na+ and Ca++ concentration were lower in the effusate from mefluidide-treated plant leaves than in controls. Electrolyte leakage studies and differential thermal analysis revealed that both the lethal freezing temperature (LT(50)) and the threshold supercooling temperature were significantly lower in mefluidide-treated than untreated plants. Clear water-soaked areas and permanent browning of tissues were evident in the leaves of treated and untreated olive plants at temperatures below LT(50) The combined action of these physiological effects reduced the lethal temperature in the leaves of mefluidide treated plants about 3.8 K below that of control plants and shifted the leaf tissue LT(50) from -11.1 degrees C to -14.9 degrees C.