Effect of photoperiod and nitrogen supply on basal shoots development in Rhododendron catawbiense

In order to control young plant form by modifying culture conditions, plants of Rhododendron catawbiense from in vitro culture were grown in a greenhouse under different photoperiodic treatments (long or short days) combined or not with a several-week nitrogen starvation. After 12 weeks of culture under long days (16 h) with nitrogen supply, plants showed a rhythmic acrotonous development. When long days were combined with a six-week nitrogen starvation, the apical growth pause was extended leading to an increase of the number of acrotonous lateral ramifications. Short-day (8 h) treatment affected distal burst potential and moreover when a concomitant nitrogen starvation was applied. This lack of distal development allowed basal buds swelling, leading to basitonous plants. When plants were returned back to long days after 2, 4 or 6 weeks under short days, distal buds resumption competed with basal shoots development. Durable basitonous plants were obtained by a 12-week short clays treatment combined with a 6-week nitrogen starvation.