Complete defoliation is essential for tree peony reflowering in autumn to trigger sucrose metabolism and adjust source-sink balance in buds and leaves

Abstract

Tree peony (Paeonia suffruticosa Andr.) has a short and concentrated natural fluorescence, and forcing culture is frequently used for its annual production. We previously found that defoliation is critical for tree peony to reflower in the autumn. In this study, we investigated how defoliation induced this process in more detail. We found that partial defoliation with only three top leaves remaining failed to trigger bud development or reflowering. Furthermore, complete defoliation, but not partial defoliation, decreased the contents of sucrose in buds, as well as increasing the contents of glucose and fructose in buds and leaves. Therefore, sucrose metabolism, particularly its hydrolysis, may function as a switch for tree peony reflowering, which is triggered by complete defoliation. Since the complete and partial defoliation treatments produced similar physical damage to plants, the induction of reflowering was unlikely to be related to a stress signal. In addition, the net photosynthetic rates of mature and newly grown young leaves in partial and complete defoliation groups, respectively, were lower than those of the control. Thus, photosynthesis itself may not be involved in this process. At the transcriptional level, defoliation treatment, but not partial defoliation, significantly induced PsSUT1 and PsSWEET12 expression. This was accompanied by the reduction of PsSWEET7 at 24 d in buds, which could facilitate the transfer of sucrose from the leaves to buds. PsCIN3, PsVIN1 and PsVIN2 were also substantially induced by defoliation treatment rather than partial defoliation, which should be mainly involved in sucrose metabolism. In short, complete defoliation is essential for tree peony reflowering in the autumn by triggering sucrose metabolism, as well as rebalancing the source-sink relationship in buds and leaves.