Chilling requirements of Paeonia cultivars

Dormant second year potted plants of Paconia 'Coral Sunset', 'Monsieur Jule s Elie', and 'Sarah Bernhardt' were placed into three chilling regimes cons tant 1, 4, or 7 degreesC) for different durations (3, 6, 9, or 12 weeks) to ascertain their chilling requirements for shoot and flower production. Chi lling was followed by forcing for up to 5 weeks at 18 degreesC, then plants were maintained in a controlled greenhouse until flowering had finished. M ean number of shoots and flowers per plant were recorded and the time taken for shoots to sprout was calculated. Control plants (forced immediately without chilling) produced no shoots or flowers. For all cultivars, the proportion of plants that sprouted, and the mean number of shoots and flowers increased as plants were subjected to co lder chilling temperatures, or longer chilling durations. However, there we re no significant within-cultivar differences between different treatments of 9 weeks or more. The time taken for sprouting to occur after the complet ion of each chilling treatment consistently decreased as the duration of th e chilling treatment increased. In most cases, lower chilling temperatures lead to more rapid sprouting once plants were placed in the 18 degreesC for cing conditions. When a simple model was fitted where the chilling temperature and duration of each treatment was described by a cumulative normal curve rising from ze ro to some maximum value (or potential) once adequate chilling had been rec eived, we found that temperatures of 4 and 7 degreesC provided only 83 and 59%, respectively, of the chilling accumulated per unit time at VC. 'Coral Sunset', an interspecific hybrid early flowering type, required the greates t amount of chilling to sprout consistently, while 'Sarah Bernhardt', a ver y late flowering type, required the least. Of the three cultivars, 'Sarah B ernhardt' also required the least amount of chilling to achieve its potenti al shoot and flower numbers, while 'Monsieur Jules Elie', a mid-season flow ering type, required the most chilling to achieve the same end for these tw o variables. This suggests that the response to spring temperatures as well as chilling influences the time of flowering. (C) 2001 Elsevier Science B. V. All rights reserved.