Physiological Studies On Floral Bud Development And Flowering Mechanism In Tree Peony

Tree peony (Paeonia Sect Moutan) is one of the most popular landscape flowering plants, not only because of its beautiful flowers with pleasant and splendid colours, but also for commercial, socio-economic, cultural and medicinal reasons in China. Flowering physiology which is closely related to horticultural production is very important as it is the first step of sexual reproduction and the primary factor affecting yield and quality. In tree peony, the duration of various floral formation stages and hormonal levels in apical bud eventually determines flowering time and flower quality which in turn largely determines the ornamental values of the plants. Many research efforts are directed to the production of tree peony under both field and controlled (forced) conditions to ameliorate the undesirable single and short flowering period of the plant annually. But improper cultivation practices always pose series of setbacks, simply because physiological information on floral bud development and flowering mechanism is largely lacking. This study has analyzed the levels of selected hormones and carbohydrates in the apical buds of various tree peony cultivars during floral bud formation, dormancy, flowering processes under field conditions and during abortion under controlled conditions. The main findings are as follow:1. Endogenous hormones (indole-3-acetic acid-IAA, abscisic acid-ABA and gibberellic acid-GA3) and carbohydrates (sucrose, reducing sugar & starch) levels greatly influenced autumn flowering in tree peony. Through their developmental stages, autumn flowering buds (AFB) not only accumulated IAA, GA3, sucrose and reducing sugar, but also degraded ABA and starch that probably contribute to induce flowering. IAA appears to be an important hormone that induced and promoted autumn flowering in AFB as it peaked at three critical stages of bud development (bud swelling, shoot elongation and flower bud opening stages) when compared with spring flowering buds (SFB). Therefore, 'Ao Shuang' probably blooms in autumn due to lack of dormancy, a phenomenon induced by low ABA, low sucrose and reducing sugar and high starch contents at the initial stages of bud growth.2. The results further show that different levels of endogenous hormones (GA3, GA4, IAA and CTK) during induction, initiation and differentiation stages of flower bud formation could largely influence flowering pattern in autumn flowering tree peony cultivars ('Ao Shuang'and'Cangzhi Hong)-(AFC) and-non-autumn flowering cultivars of tree peony cultivar ('Luoyang Hong')-(NAFC). Cytokinin (CTK) could be a critical hormone in autumn flowering because its differences in levels between AFC and NAFC remained significant across bud developmental stages. Although CTK, particularly, zeatin riboside (ZR) and N6-(â–³2-isopentenyl)-adenosine (iPA) were the key hormone across bud development, GA3, GA4 and IAA were also significant during bud induction and initiation stages. However, only GA3 had negative differences across bud developmental stages, indicating that GA3 levels were always higher in NAFC than in AFC cultivars. Abscisic acid (ABA) and carbohydrates (sucrose, reducing sugar and starch) appeared to have little (if any at all) effect on the flowering pattern of tree peonies. The regulation of GA3, IAA and CTK levels, particularly, at induction and initiation stages of bud development initiated autumn flowering in tree peonies most likely by enhancing the completion of floral formation cycle well in advance of bud dormancy period.3. Winter temperature accumulated ABA, starch, sucrose and reducing sugar in tree peony buds which most likely induced dormancy. Spring temperature, on the other hand, degraded ABA, starch, sucrose and, reducing sugar and accumulated GA3 that possibly release dormancy in tree peony buds indicating that the accumulation of ABA, GA3 and sugars in buds during dormancy or budbreak stage is directly related to the degree of temperature experienced at that period. It is likely that the reduction of ABA and carbohydrates played an important role in bud dormancy release or alteration in bud growth of tree peonies.4. Seasonal patterns of sucrose and starch were almost the same in buds of the tested tree peony cultivars which contradicts previous studies suggesting inverse relationships between sucrose and starch in terms of accumulation in winter. However, seasonal accumulation of endogenous compositions varies with cultivar type. Among the the investigated cultivars,'Louyang Hong'not only accumulated less ABA, GA3, starch, sucrose and reducing sugar but also released bud dormancy earlier than the'Zhao Fen'and'High Noon'.5. Bud abortion in forced'Luoyang Hong' is possibly caused by ABA elevation, which in turn inhibits the accumulation of sugars and hormones that restricted bud growth. However, un-potted chilling method combined with 200 mg/L GA3 application reduces abortion and increases flowering rate. Flowering rate in un-potted was 58% versus 21% in potted with 200 mg/L GA3 applications. Overall, chilling methods in combination with GA3 applications enhanced plants'growth and development, particularly, bud sprouting, growth and flowering.