Effect of photoperiod, water stress and nitrogen nutrition on bud push, scion growth and cytokinin concentration in container-grown citrus nursery trees.

摘要

In Florida, the slow growth of citrus nursery trees on rootstocks with trifoliate orange (Poncirus trifoliata) parentage especially during winter is a concern of economic importance. These observations started coming after the shift from open field nursery production to greenhouses occurred following a new legislation. We studied the effect of several factors on the bud push and scion growth of citrus nursery trees. The first objective was to determine the effect of photoperiod on growth of container grown trees of two trifoliate-type rootstocks, 'Carrizo' citrange and 'Swingle' citrumelo with and without non-trifoliate scions. All trees, budded or not, had reduced growth under short days (SD 10h), whereas short days-night interrupt (SD-NI 10h + 1h night-interrupt) trees grew similar to long days (LD 14h). Net CO2 assimilation was higher under SD and SD-NI treatments than LD, with no differences in whole-plant total nonstructural carbohydrates, indicating that the growth difference is a phytochrome-mediated response. The second objective was to study the effect of drought stress on cytokinin concentration in xylem sap. In the trees grown under three water stress treatments 100% ET (Control), 50% (Mild) and 20% (Severe stress) cytokinin concentration initially started to increase with increasing water stress but later decreased with severe stress. The foliar BA application did not have any significant effect. In the third experiment, buds taken from budwood trees were inserted into rootstock seedlings grown under same well watered/drought stress conditions and BA 500 ppm was applied to buds. BA significantly enhanced bud-break in well-watered and in trees moved to well-watered regime. In drought stressed, two BA applications resulted in 36 % total bud break indicating an interaction between BA and water stress. The fourth experiment shows that Nitrogen deprivation decreased leaf chlorophyll concentration and whole plant nitrogen content (% dry weight) resulting in lower photosynthetic rate. The bud survival, budbreak and scion growth, all were higher in trees under N application. The N sufficient trees had higher endogenous cytokinin levels before and after budding but not after unwrapping. The trees showed no significant changes in endogenous cytokinin levels with N application over 5 days.