Comparative methods of estimating freezing temperatures and freezing injury in leaves of chaparral shrubs

Exotherms and lethal low temperatures were examined in four species of chaparral shrubs that co-occur in the Santa Monica Mountains, southern California: Rhus laurina, Rhus ovata, Ceanothus megacarpus, and Ceanothus spinosus. For all but R. ovata, excised leaves froze at significantly lower temperatures than leaves on intact branches (range 4.3 degrees-6.9 degrees C lower), indicating that only intact shoots should be used for studies of leaf supercooling. Of the four species, only R. laurina showed no leaf supercooling on intact branches. On excised leaves artificially seeded with ice crystals to remove supercooling, the lethal temperature for 50% change in activity or cell death (LT50) was determined by measuring (1) electrical conductivity into a bathing solution (electrolyte leakage), (2) photosynthetic fluorescent capacity (Fv/Fm), (3) the percentage of palisade mesophyll cells stained by fluorescein diacetate (vital stain), and (4) visual score of leaf color (Munsell color chart). In every case, the estimate of LT50 by the electrical conductivity method was significantly higher (less negative) by 2 degrees-5 degrees C than the other three methods, and electrical conductivity results did not correspond with field observations of dieback. Based on the photosynthetic fluorescent technique, which is both reliable and nondestructive, the LT50 ranged from -5.6 degrees C for R. laurina to about -8.5 degrees C for the other three species.