Involvement of polyamines in the chilling tolerance of cucumber cultivars

The possible involvement of polyamines (PAs) in the chilling tolerance of c ucumber (Cucumis sativus L. cv Jinchun No. 3 and cv Suyo) was investigated. Plants with the first expanded leaves were exposed to 3 degrees C or 15 de grees C in the dark for 24 h (chilling), and then transferred to 28 degrees C/22 degrees C under a 12-h photoperiod for another 24 h (rewarming). Chil ling-tolerant cv Jinchun No. 3 showed a marked increase of free spermidine (Spd) in leaves, once during chilling and again during rewarming. Putrescin e increased significantly during rewarming, but the increase of spermine wa s slight. Any of these PAs did not increase in chilling-sensitive cv Suyo d uring either period. PA-biosynthetic enzyme activities appear to mediate th ese differences between cultivars. Pretreatment of Spd to cv Suyo prevented chill-induced increases in the contents of hydrogen peroxide in leaves and activities of NADPH oxidases and NADPH-dependent superoxide generation in microsomes and alleviated chilling injury. Pretreatment of methylglyoxal-bi s-(guanylhydrazone), a PA biosynthesis inhibitor, to chilled cv Jinchun No. 3 prevented Spd increase and enhanced microsomal NADPH oxidase activity an d chilling injury. The results suggest that Spd plays important roles in ch illing tolerance of cucumber, probably through prevention of chill-induced activation of NADPH oxidases in microsomes.