New Forests 18: 301–314, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. Periodic chilling exposure during nursery culture Effects on growth, morphology and drought resistance of containerized white spruce seedlings J.J. ROBERTS and J.J. ZWIAZEK Department of Renewable Resources, 4-42 ESB, University of Alberta Edmonton, Alberta, Canada T6G 2E3 (E-mail: jroberts@NRCan.gc.ca) Received 15 March 1998; accepted 21 January 1999 Abstract. Five week old white spruce [Picea glauca (Moench) Voss] seedlings were placed within controlled-temperature growth chambers, and subjected to one of four periodic, chilling treatments for approximately 16 weeks. The treatments differed in the frequency of 24 hour exposure to 5 ◦ C the seedlings received, and ranged from no chilling (control), to once every two, three or four days. After 25 weeks of growth, the control seedlings were significantly taller than all chilling-exposed seedlings, but stem diameters were similar. Chilling did not affect either needle length or needle density compared to control seedlings. During a water deficit test, frequently chilling-exposed seedlings maintained significantly higher mid-day shoot water potentials under extreme water stress. Re-watering after the drought resulted in all chilling-treated seedlings recovering faster to pre-stress shoot water potential levels than the control seedlings. Measurements of new root growth after 21 days at 10 ◦ C indicated that previous chilling exposure delayed or inhibited new root production. Key words: chilling stress, drought stress recovery, field performance, growth, morphology, nursery culture, tree nursery Introduction During container seedling production, greenhouse air temperature is care- fully controlled, as it directly influences the growth and development of plants (Hellmers et al. 1970; Brix 1971; Landis et al. 1992; Odlum and Ng 1995). To promote rapid shoot elongation, needle production and root devel- opment during the initial 18–20 weeks of white spruce seedling growth, North American tree nurseries maintain greenhouse temperatures between 17 ◦ and 25 ◦ C (Landis et al. 1992; Wood 1995). However for northern tree nurseries, maintaining these warm temperatures to favor high growth rates may not be practical, as heating costs during the winter months become too high (Draper and Hawkins 1989). Reducing nursery heating costs by decreasing the greenhouse temperature has been shown to save up to 24% in nursery heating costs (Landis et al. 1992, 1994). In addition, Draper and Hawkins