Effect of fall fertilizaton, taxonomic differences and light intensity on freeze resistance of azaleas

Abstract

Environmental factors such as light, fertilization and temperature interact to influence the freeze resistance of woody plants, but the combined effects of these factors are not well understood. Freeze resistance, growth, flower production and chlorophyll fluorescence of container-grown Rhododendron species were analyzed to investigate interactions between timing of fall fertilization, fertilizer application rate, species, and light intensity treatments. Influence of fertilization rate and timing on freeze resistance was examined in a study that exposed Rhododendron xkurume to five fertigation regimes. Fall fertigation increased leaf and stem dry weight (DW) without affecting freeze resistance compared to plants that received no additional fertilizer after Jul. 31. The high rate of N fertigation (125 mg.L-1 from Aug. 1 to Sept. 29 or Nov. 28) increased azalea leaf and stem DW, but reduced stem freeze resistance in Nov. and Mar. compared to plants that received N at 75 mg . L-1 from Aug. 1 to Sept. 29. Rhododendron canescens (Michx.) Sweet and R. xsatsuki ‘Wakaebisu’ were grown under three fall fertigation regimes to determine if extended fertilizer application in fall had divergent effects on freeze tolerance, nutrient uptake, growth and flower production of evergreen versus deciduous plants. There were no interactions of fertilizer and taxa on growth, freeze tolerance, nor timing of cold acclimation/deacclimation. When fertigation was extended in fall 120 versus 60 days, stem N concentration increased, but growth was not affected. To investigate the effects of fall fertilization and light intensity on photosynthesis and freeze resistance, an experiment with two fertilization treatments, and four light intensity treatments was initiated on Rhododendron xkurume ‘Pink Pearl’. Fertigation and light intensity treatments did not interact in their effects on freeze resistance or chlorophyll fluorescence (Fv/Fm). Plants grown in 50% photosynthetic photon flux (PPF) from May 1 through Sept.30 had greater DM compared to plants grown in 100% PPF during the same time period. The addition or removal of shade cloth beginning Oct.1 did not increase azalea stem freeze resistance compared to plants that were only exposed to 100%, or 50% PPF respectively. Fertigation and light intensity treatments affected Fv/Fm, but Fv/Fm was not correlated with stem freeze resistance.