Quantification of additive response and stationarity of frost hardiness by photoperiod and temperature in Scots pine

Abstract

To present quantitative knowledge of photoperiod and temperature on frost hardiness for mathematical modelling, we carried out a growth chamber experiment for second-year Scots pine (Pinus sylvestris L.) seedlings. There were three treatments involved in the trial: a short photoperiod (7 h light : 17 h dark cycle) with a high temperature (15 °C) (SDHT), a short photoperiod (7 h light : 17 h dark cycle) with a low temperature (2 °C), and a long photo period (16 h light : 8 h dark cycle) with a low temperature (2 °C). The following variables were measured: (i) the frost hardiness of stems, needles, buds, and roots by controlled freezing tests, (ii) the electrical impedance spectroscopy (EIS) parameters of the stems, (iii) the chlorophyll fluorescence (Fv/Fm) of the needles, (iv) the dry mass/fresh mass ratio, and (v) the total soluble sugar and starch concentration of the stems, needles, and roots in the non-frost-exposed organs. There was a clear difference in hardiness among the organs by the end of the experiment. Stems by some EIS parameters, needles by Fv/Fm, and stems, needles, and roots by sugar concentration differed among treatments. A stationary level of frost hardiness was reached or asymptotically approached in all organs and treatments except in the SDHT treatment of buds. Very little support was found for the concept of additive effects by photoperiod and temperature. The results show that the additive model needs revision, since the hardening response is dependent on organ, and there is an interaction in the responses to photoperiod and temperature.