Effect of root restriction on flower bud formation of ‘Yuanmeng' peach trees

Abstract: 【Objective】Zhejiang province is a humid region with sunlight deficiency where peach trees grow vigorously because of abundant water supply. Large crown and poor flower bud formation lead to low yield. Root restriction can inhibit vegetative growth and promote reproductive growth, therefor, it is suitable for high-density orchard in which peach trees are trained to central leader form. However, the mechanisms underlying the effects of root restriction on flower bud formation are poorly understood.According to limited literature, change of carbon nitrogen ratio and disruption of hormonal balance might explain the case with grape. The study aimed at revealing the effects of root restriction on flower bud formation in peach.【Methods】Two-year-old‘Yuanmeng'[Prunus persica (L.) Batsch.] peach trees were used in this study. Trees for experimental group were planted in pots with volum of 60 cm ×60 cm. Trees for control group were planted in holes with the same volume in the field. 20 trees were used for each group. Leaves and buds were collected every two weeks from June 2 ndto August 25 thand frozen in liquid nitrogen for further experiments. Flower bud differentiation process was observed using Zeiss microscope. Leaves were dried and ground into powder for total carbon and nitrogen content determination. Leaves and buds were ground into a fine powder and stored in-80 ℃ for measurement of hormonal contents. Ethyl acetate was used to extract hormones from 200 mg of buds or 500 mg ofleaves. ZR and ABA content were determined by LC-MS.【Results】The differentiation of flower buds on lower part of primary long branch of the trees with the treatment of root restriction started around July 16 th, two weeks earlier than that in the control. Although the situation on upper part of primary long branch with the treatment of root restriction appeared four weeks later than that in the control, the differentiation process developed more rapidly of the trees with the treatment of root restriction. Buds on primary shoots started the differentiation earlier than secondary shoots and the process moved faster. Both treatments had shown a pattern that the closer to the base of the position of flower buds on long branch was, the earlier the flower buds started morphological differentiation, and the faster the processes moved along. As the flower bud differentiation proceeded, leaf total nitrogen content declined while C/N rised.Leaf total nitrogen content with the treatment of root restriction was lower than that in the control, while C/N was higher than that in the control. Moreover, leaves on lower part of long branch had lower total nitrogen content and higher level of C/N, consistent with earlier bud differentiation. These results showed that root restriction treatment sped up the bud differentiation process probably by adjusting nutrient partitioning. Finally, root restriction treatment significantly elevated flowering rate by 22%, resulting in a good first-year-harvest of 407 kg (Table 1) . Before June 30 th, leaf ZR content with the treatment of root restriction was higher than that in the control. After this date, bud ZR content gradually increased and then decreased. Buds started differentiating around June 30 thaccording to microscopic observation, indicating that high level of ZR at the ending of initial stage of flower bud primordium differentiation promote bud morphological differentiation. Peak value of bud ZR content with root restriction treatment was higher than that in the control, which might be the reason for more flower buds on the trees with root restriction treatment. Bud ABA content rose before the early July, then descended until the late August when bud ABA content ascended sharply and differentiation proceeded into calyx differentiation stage. Besides, ABA content in the buds was higher than that in the leaves on the treees with the treatment of root restriction. In spite of slightly lower value, bud ABA content on th etrees with the treatnet of root restriction fluctuated smoothly, indicating that moderately abundant and stable rather than excessive ABA in the buds was favorable for flower bud formation in peach.【Conclusion】Flower bud differentiation on upper part, middle part and lower part of primary long branch started at different times regardless of the root restriction treatment and the control. Early bud differentiation corresponded with relatively high level of leaf C/N. High level of bud ABA content and leaf ZR content at earlier time would result in the beginning of flower differentiation for the most buds from June 30 th. Peaking of bud ZR content probably led to the process moving from bud primordium differentiation to next stage on August 11 th. Bud differentiation developed faster with the root restriction treatment than the that in the control, which was consistent with steeply rising of C/N with the root restriction treatment. Overall, change of carbohydrates and nitrogen compounds ratio and disruption of hormonal balance would be part of the reason for the promotion of peach flower bud formation by root restriction. Moreover, under root restriction treatment, the transduction of hormonal signals and the effects of hormones on biosynthesis during flower bud formation need further researches.