Freeze/thaw stress in Ceanothus of southern California chaparral.

Freeze/thaw stress was examined in chaparral shrubs of the genus Ceanothus to determine the interactive effects of freezing and drought and to consider which is the more vulnerable component, the living leaves (symplast) or the non-living water transport system (apoplast). We hypothesized that where Ceanothus species co-occurred, the more inland species C. crassifolius would be more tolerant of low temperatures than the coastal species C. spinosus, both in terms of leaf survival (LT(50), or the temperature at which there is 50% loss of function or viability) and in terms of resistance to freezing-induced embolism (measurements of percent loss hydraulic conductivity due to embolism following freeze/thaw). Cooling experiments on 2 m long winter-acclimated shoots resulted in LT(50) values of about -10 degrees C for C. spinosus versus -18 degrees C for C. crassifolius. Freeze-thaw cycles resulted in no change in embolism when the plants were well hydrated (-0.7 to -2.0 MPa). However, when plants were dehydrated to -5.0 MPa, C. spinosus became 96% embolized with freeze/thaw, versus only 61% embolism for C. crassifolius. Stems of C. crassifolius became 90% and 97% embolized at -6.6 and -8.0 MPa, respectively, meaning that even in this species, stems could be more vulnerable than leaves under conditions of extreme water stress combined with freeze/thaw events. The dominance of C. crassifolius at colder sites and the restriction of C. spinosus to warmer sites are consistent with both the relative tolerance of their symplasts to low temperatures and the relative tolerance of their apoplasts to freeze events in combination with drought stress.