Regulation of flowering in Citrus limon by water-deficit stress and nitrogen compounds

In most citrus production areas, lemons (Citrus limon L. Burm. f.) flower in spring for winter harvest. However, by using the Italian forzatura process, where water is withheld for 6 to 8 weeks during summer, a second bloom can be induced that yields verdelli summer lemons. Since the drought controls floral induction, variability in soil moisture limits verdelli production. Field experiments with 'Frost Lisbon' lemon trees were conducted to evaluate the forzatura practice in California and to improve it with deficit irrigation and foliar application of low-biuret urea. The traditional forzatura practice imposed on the same trees for five consecutive years produced commercially acceptable quantities of summer lemons in only some years. Winter lemon yields for the same period were not affected by either the drought or the summer lemon yield. Three years of summer deficit irrigation demonstrated that the course and duration of the water-deficit were critical to flower induction. Flowering and yields were maximized by withholding irrigation until predawn xylem pressure potential (PDXPP) decreased to {dollar}-{dollar}2.4 MPa and then by maintaining a stress of {dollar}-{dollar}2.1 to {dollar}-{dollar}3.0 MPa PDXPP for 30 to 50 days using deficit irrigation. Foliar urea (0.5% N (w/v)) enhanced flowering in trees maintained by deficit irrigation under moderate drought ({dollar}-{dollar}2.0 MPa PDXPP). Experiments were also conducted with 3-year-old potted 'Frost Lisbon' lemon trees to more precisely investigate the role of foliar urea and tree ammonia status in the water-deficit stress induction of citrus flowering. A single urea spray enhanced flowering in deficit-irrigated trees under severe drought ({dollar}-{dollar}2.5 MPa average PDXPP), while weekly sprays were required to enhance flowering under less severe drought ({dollar}-{dollar}1.5 MPa average PDXPP). When urea failed to raise the ammonia status of water-deficit stressed trees over that of stressed trees not receiving urea, flowering was not enhanced. The growth pattern and morphological traits of these potted tree branches were diagramed to describe the relationships between branch length, nodes, thorns and flowers within a branch and between branches thereby providing a guide for pruning and tree management.; Verdelli summer lemon yield can be increased to a commercially acceptable level without reducing winter yield by monitoring tree PDXPP and leaf ammonia to guide deficit irrigation and foliar urea applications.