| Flowering at an early age is required to shorten the breeding cycles for the improvement of eastern cottonwood (Populus deltoides Bartr. ex Marsh. var. deltoides), a fast-growing, commercial hardwood species. Experiments were conducted to induce flowering in rooted cuttings from mature and juvenile cottonwood trees. A treatment of water stress plus root pruning plus paclobutrazol was applied to approximately three-month-old rooted cuttings from mature trees under root restriction and long days. The treatment induced early flowering in all treated plants. Untreated plants did not form flower buds after they were rooted. Cuttings from juvenile plants did not behave in the same manner. Treatments of (a) paclobutrazol, paclobutrazol plus water stress, and paclobutrazol plus root pruning under “continuous” and “periodic” growth with root restriction, and (b) paclobutrazol, girdling, and paclobutrazol plus girdling under “periodic” growth with no root restriction did not initiate floral bud formation on juvenile cuttings. The treated plants had reduced vegetative growth in internode length, however, which indicated reduced GA biosynthesis. Molecular pathways that regulate flowering need to be elucidated to understand the different effects of flower induction treatments on cuttings from mature and juvenile trees.; Annual shoots of mature cottonwood trees were observed to sequentially initiate early vegetative buds, floral buds, and late vegetative buds in a repetitive manner. This sequential pattern was used to study genes with increased expression during the floral bud formation. Southern blot analyses indicated that two members of the CONSTANS-LIKE (COL) gene family (PdCO1 and PdCO2) exist in the cottonwood genome. Although PdCO1 and PdCO2 deduced protein sequences are 85% identical, the PdCO1 sequence contains a putative signal peptide with a potential transmembrane helix at the N-terminal end. Patterns of mRNA expression for both genes were studied in the leaf and the subtended bud at the ninth node of shoots collected in early spring. Simultaneous morphogenetic changes in the subtended bud were investigated. Increased expressions of both genes in the leaf were noted during the three weeks from March 30 to April 21, 2000. Following this peak in gene expression, there were morphogenetic changes that occurred in the subtended axillary bud meristem. These changes included (a) the cessation of leaf-primordium formation, (b) the transition of the axillary shoot apical meristem to an inflorescence meristem, and (c) the subsequent formation of bracts and floral meristems in the axils of bracts flanking the inflorescence meristem. The sequential changes in gene expression and morphogenetic development indicate that PdCOs in leaves of cottonwood may be involved in the flower induction pathway. |
