Cold hardiness of rhododendron cultivars grown in different photoperiods and temperatures

Two rhododendron cuItivars, 'Pahjola's Daughter' and 'Helsinki University', were grown at + 15 and + 24 degrees C, each combined with a photoperiod of 14 h (short day, SD) or 20 h (long day, LD). After a 112-day growing seaso n, they were subjected to a hardening regime of fornightly decreasing tempe rature (+ 9, +5, + 1 and -2 degrees C) and a 12-h photoperiod, except that part of the plants grown in LD had LD also at +9 and +5 degrees C. At -2 de grees C, all plants were in darkness. Controlled freezing tests of the leav es mere performed before each change in temperature. The injury was eluated vavisually and by electrolyte leakage (EL) tests. The observations on the visual assessment were analysed with logit models, and the EL data with non -linear sigmoid functions. The visually scored 50% damage (VD50) correlated better with the EL tests than 10 or 90% damage. Photoperiod and temperatur e during the growing season affected the cord hardiness of both cultivars, but they differed in their responses. 'Pohjola's Daughter' benefited from S D as well as from high temperature, while 'Helsinki University' attained be tter hardiness at a cool growing season temperature and was less sensitive to photoperiod.