Early summer frost hardiness in Picea abies seedlings in response to photoperiod treatment

According to previous studies, short-day (SD) treatment may increase frost hardiness in Picea abies (L.) Karst. seedlings during shoot elongation the next year. The purpose of this study was to examine how timing of the SD treatment affects spring frost hardiness. The following four treatments were applied in the first growth period: natural photoperiod (Nat), or SD treatment (8 h light : 16 h dark) either 14-28 July (SD1), 28 July - 11 August (SD2), or 11-20 August (SD3). After 125 days in cold storage (October-January), the seedlings were transferred to forcing conditions (20-25 °C, 24 h light) followed by freezing tests at -3, -5, -7, and -9 °C when most seedlings had reached bud break stage 7 (Krutzsch index). Seedling height measurements and analyses of carbohydrate status, nitrogen concentration, and dry mass of needles after cold storage were done to examine the treatments' impact on seedling quality. SD treatment reduced visual freezing injury to current- and first-year shoots. Mortality occurred at-7 and -9 °C and was significantly higher in the Nat and SD1 treatments (43% in both) than in the SD2 (23%) and SD3 (15%) treatments. Seedlings from the late SD treatments also showed better height growth and developed more shoots from dormant buds after freezing to -3 and -5 °C. Collectively, these findings demonstrate the importance of proper timing of the SD treatment in relation to the seedlings' natural growth rhythm.