Dormancy development in micropropagated plantlets at the acclimatization stage and early growth ex vitro is undesirable as it lowers their survival rate and restricts the efficient year-round production of planting material. Thus far, little is known about the factors and mechanisms involved in the dormancy development of micropropagated herbaceous perennials, including rhubarb. This study determined physiological and molecular changes in the Rheum rhaponticum (culinary rhubarb) 'Raspberry' planting material in response to photoperiod and temperature. We found that the rhubarb plantlets that were grown under a 16-h photoperiod (LD) and a temperature within the normal growth range (17-23 °C) showed active growth of leaves and rhizomes and did not develop dormancy. Rapid growth cessation and dormancy development were observed in response to a 10-h photoperiod (SD) or elevated temperature under LD. These morphological changes were accompanied by enhanced abscisic acid (ABA) and starch levels and also the upregulation of various genes involved in carbohydrate synthesis and transport (SUS3, AMY3, BMY3, BGLU17) and ABA synthesis and signaling (ZEP and ABF2). We also found enhanced expression levels of heat shock transcription factors (HSFA2 and HSFA6B), heat shock proteins (HSP22, HSP70.1, HSP90.2 and HSP101) and antioxidant enzymes (PRX12, APX2 and GPX). This may suggest that dormancy induction in micropropagated rhubarb plantlets is a stress response to light deficiency and high temperatures and is endogenously coordinated by the ABA, carbohydrate and ROS pathways.
Keywords: ABA; antioxidant enzymes; heat stress; molecular analysis; rhubarb; starch; stress response.