The Effect of Nitrogen Application on Bud Take, Scion Growth, and the Level of Endogenous Cytokinins in Shoots of Trifoliate Orange Rootstocks

Experiments were conducted at the University of Florida Citrus Research and Education Center, to study the effect of nitrogen application on bud take and scion growth, and on the biosynthesis and translocation of endogenous free cytokinins in shoots of trifoliate orange rootstocks. Liner trees of citrus rootstock ‘Swingle’ citrumelo (Citrus paradisi ×Poncirus trifoliata) were grown in washed quartz sand under growth chamber conditions (28 °C day/21 °C night temperatures, 14 hours daylight). In the first experiment, the bud wood (‘Valencia’ sweet orange) and liner trees were subjected to two treatments consisting of no N application and 150 mL of 200 mg·L ^-1 N solution per tree per week for 12 weeks before budding. Budding was performed to get four total treatment combinations as follows: N sufficient budded on N sufficient (N+/N+), N deficient on N sufficient (N-/N+), N sufficient on N deficient (N+/N-) and N deficient on N deficient (N-/N-). The nitrogen application treatments were continued post-budding. In the second experiment, the trees were subjected to two treatments: to one set 150 mL of 200 mg·L^-1 N solution was applied daily for 8 days, while no N was applied to the second set. Trees were destructively harvested for 5 consecutive days from each treatment for xylem sap extraction. After 5 days, the trees under both treatments were further subdivided into two categories: half of trees from N+ were moved to N- and half remained in N+, and vice versa, resulting in four categories: N+ to N+, N+ trees moved to N-, N- to N- and N- trees moved to N+. The extracted xylem sap was analyzed for cytokinin content. The results show that N deprivation decreased leaf chlorophyll content by 26%, while N application increased it by 28.6% in respective treatments. The whole plant nitrogen content (% dry weight) was also significantly higher in N+ trees. As a result, the N sufficient trees also had significantly higher net photosynthetic rate than the N deprived trees. The bud survival rate, bud break, and scion growth all were positively influenced by N application. The N sufficient trees had higher endogenous cytokinin levels before budding, at the time of budding and at unwrapping, but not 6 weeks after unwrapping when the scions were growing. The second experiment showed no significant changes in endogenous cytokinin levels with N application over 5 days.