MODELING FROST-RESISTANCE OF SCOTS PINE-SEEDLINGS USING TEMPERATURE, DAYLENGTH AND PH OF CELL EFFUSATE

The annual course of frost resistance (LT50) and the pH of the cell ef fusate in needles of two-year-old Scots pine seedlings were monitored in a field experiment in Oulu, Northern Finland (65 degrees N, 25 degr ees E) during 1995. The aim of the work was to to develop model to pre dict the annual variation in frost resistance by pH of the cell effusa te and meteorological data. The seedlings were covered with a fibre cl oth shelter which transmitted sufficient Light for them to experience the photoperiod, but prevented the accumulation of snow over them. The shelter above the seedlings was removed at the beginning of May and e rected again at the end of September. the seedlings were watered only for the time when the shelter was removed, and received fertilizer onl y during the previous summer (1994). Frost resistance was only -5 degr ees C during the growing season but more than -100 degrees C during th e winter rest period. It was about -10 degrees C at the end of August, increased to -55 degrees C in the next three weeks, and reached -100 degrees C at the beginning of October. The pH of the cell effusate was lowest during the growing season and highest in winter, the differenc e being about one and half pH unit. Needles exposed to -196 degrees C showed pH from 4.0 in summer to 5.5 in winter, while pH of the non-fro zen needles varied from 5.0 to 6.5, respectively. Seasonal variation i n frost resistance was explained by a regression model well (R-2 = 0.9 ) when day length, minimum air temperature and pH were entered as vari ables.