Table I.

The effect of photoperiod and temperature on freezing tolerance of birch leaves

Condition	Cont	1 d	2 d	4 d	7 d	14 d
	LT50
SD	−2.5 ± 0.21	−3.4 ± 0.25	−4.5 ± 0.31	−5.9 ± 0.28	−6.4 ± 0.35	−7.1 ± 0.6
LT	−2.5 ± 0.21	−4.7 ± 0.28	−6.8 ± 0.54	−10.5 ± 0.47	−11.6 ± 0.53
SD + LT	−6.4 ± 0.35	−9.6 ± 0.65	−12.0 ± 0.73	−14.8 ± 0.66	−15.8 ± 0.84

Birch seedlings were exposed to SD length (12 h light) for 7 d; LT in LD condition (4°C, 24 h light) for 7 d and SD for 7 d followed by LT (4°C) for 7 d. The freezing tolerance (LT₅₀) of leaves (means ± se) was measured by ion-leakage test (Sukumaran and Weiser, 1972).