Abstract. Cold hardiness is a key factor limiting the distribution and productivity of perennial horticultural plants in the temperate regions of both hemispheres. Frequent frosts occurring during winter and spring in Khorasan, Iran cause serious damage to apricot, peach, nectarine, fig, olive, pomegranate, and walnut, which necessitating the selection of resistant cultivars. Before introducing new genotypes, their capacity for adaptation to different conditions needs to be studied. Walnut production in Khorasan has been increasing through area expansion and the introduction of new genotypes. In this area, affective freeze occasionally damages walnut tree. The minimum temperature on winter of 2008 was about -22°C. The objective of this study was to evaluate cold hardiness in 12 walnut genotypes (K21B5/13, K21B5/15, K21B6/17, K21B6/18, K28B2/5, K28B2/6, K23B5/13, K23B5/14, K21B3/8, K21B9/25, K28B3/8, and K21B1/3) which planted in 1986 on seedlings at the genotype collection (36° 17′ N and 59° 36′ E and 1,012 m altitude) of Khorasan Agricultural and Natural Sources Researches Center, Mashhad, Iran. The ionic leakages were evaluated in the Department of Horticulture at Ferdowsi University of Mashhad, Iran. One- and 2-year-old shoots of about 30 cm long were randomly collected on February, 2008 from mature trees. K21B5/13, K21B5/15, K21B6/18, K21B3/8, and K28B3/8 genotypes had more damage than the other genotypes, but K23B5/13, K23B5/14, and especially K21B9/25 showed to be resistant to -22°C. Linear correlation between ionic leakage and visual observation was found. Minimum ionic leakage was observed in K21B9/25 and visual observation confirmed its cold hardiness. Flower buds in K21B5/15 and vegetative buds in K21B3/8 were damaged completely. Sexual buds were the most sensitive organs of the tree. Annual shoots were damaged more severely than biennial shoots.

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