Effects of Fall Fertilization, Taxonomic Differences and Light Intensity on Freeze Resistance of Azaleas

1. Effects of Fall Fertilization, Taxonomic Differences and Light Intensity on Freeze Resistance of Azaleas Frank Henning R4 EPA-LGU Liaison fhenning@uga.edu

2. Rhododendron economically important • Varieties with desired horticultural traits susceptible to freeze damage

3. Leaf Retention Fertilization Light Intensity -Multiple factors affect freeze resistance -Interactions not well understood

4. Study #1 Effects of Fall Fertilization on Freeze Resistance, Flowering and Growth of R. xkurume ‘Hinodegiri’

5. Fertilizer Application Rate & Timing + Increase growth & flower production + Increase nutrient reserves & future performance - Increase production costs & nutrients leached - Reduce freeze resistance & increase freeze damage How can fall fertilization application rate and timing be managed to maximize production & minimize nutrient losses & freeze damage?

6. - Factorial design • 5 fall fertigation treatments

7. Fertigation - Fertilizer application rate - Time of fertilizer application

8. Freeze Resistance - Plant’s ability to tolerate the stresses associated with exposure to freezing temperatures - LT50 – lethal temperature for 50% of population - 40, 5cm stem segments per experimental unit - 4 segments exposed to each of 9 successively colder temperatures (-3 to -30C)

9. Table 3.1. Effect of fertilization treatments on leaf, stem and total above ground DW of R. xkurume ‘Hinodegiri’. Z Each mean based on observations of five plants in each of three replicates. y Mean separation within columns by Tukey’s Studentized range test at P≤0.05.

10. Table 3.3. Effect of fertilization treatments on freeze resistance of stem tissue of R. xkurume ‘Hinodegiri’. ZEach mean based on observations of four stem samples at each of 9 different freezing temperature increments for each of 3 replicates. yMean separation within columns by Tukey’s Studentized range test at P≤0.05.

11. Table 3.4. Effect of fertilization treatments on the date 50% of R. xkurume ‘Hinodegiri’ flowers were in bloom (F50), and total number of flowers per plant. Z Each mean based on observations of five plants in each of three replicates. y Mean separation within columns by Tukey’s Studentized range test at P≤0.05.

12. Study #2 Effect of Fall Fertilization on Freeze Resistance of Deciduous Versus Evergreen Azaleas

13. Leaf Retention • Leaves supply the carbohydrates required for active nutrient uptake • Leaves produce transpiration pull responsible for the movement of inorganic ions in xylem Determine if evergreen & deciduous azaleas respond differently to fall fertilization

14. 2 x 3 factorial design • 2 taxa (R. canescens and R. xsatsuki) • 3 fertilzation regimes

15. Table 4.1. Effect of fertilization treatments on plant growth index (PGI z) pooled for R. canescens and R. xsatsuki ‘Wakaebisu’. zMaximum height (H), maximum width (W1) and width perpendicular to W1, W2 were used in the calculation GI = (H + (W1 + W2) / 2) / 2. yData for the two azalea taxa were pooled within each fall fertilization treatment because fall fertilization and taxa treatments did not interact in their effect on growth. xMean separation within columns by Tukey’s Studentized range test at P≤0.05.

16. Table 4.2. Effect of fertilization treatments on pooled freeze resistance (LT50z) of R. canescens and R. xsatsuki ‘Wakaebisu’ stem tissue. zTemperature at which 50% of stems were killed yData for the two azalea taxa were pooled within each fall fertilization treatment because fall fertilization and taxa treatments did not interact in their effect on freeze resistance. xMean separation within columns by Tukey’s Studentized range test at P≤0.05.

17. Table 4.5. Effect of fertilization treatments on N concentration of azalea stem tissue harvested Feb.17, 2004. zMean separation within columns by Tukey’s Studentized range test at P≤0.05.

18. Study #3 Influences of Photosynthetic Photon Flux and Fall Fertilization on Fluorescence, Freeze Resistance, and Growth of Rhododendron xkurume ‘Pink Pearl’

19. Low Temperatures & High Light • Low temperatures inhibit photosynthesis • High light - more energy • Increase demand for orderly energy dissipation Fluorescence • MiniPam Photosynthesis Yield Analyzer • Rapid, non-destructive indicator of PSII • Fv/Fm inversely related to photosynthetic stress

20. Fluorescence and Freeze Resistance • Linear correlation between Fv/Fm and freeze damage (P. menziesii) • Fv/Fm accurately predicted stem LT50 • (Birchler et al., 2001; Perks et al., 2004) • Perks et al., 2004 developed fluorescence-based determination of LT50 - F.LT50

21. Shade • Reduce photosynthetic stress (increased Fv/Fm) • Influence freeze resistance (F.LT50) • May interact with fall fertilization Investigate the effects of shade & fall fertilization on freeze resistance, growth & flower production

22. Split-plot design 2 fall fertilization treatments 4 PPF (shade) treatments

23. Table 5.3. Effects of light intensity (PPF) on stem tissue freeze resistance (LT50) of R. xkurume ‘Pink Pearl’. Bilderbeck and Bir (1986) zIn the absence of interaction, data from two fertilization treatments were pooled within each light intensity treatment, so that each mean is based on observations of 8 stem samples at each of 9 different freezing temperature increments for each of 3 replicates. yMean separation within columns by Tukey’s Studentized range test at P≤0.05.

24. Table 5.4. Effects of light intensity on chlorophyll fluorescence (Fv/Fm) of R. xkurume. zIn the absence of interaction, data from two fertilization treatments were pooled within each light intensity treatment, so that each mean is based on observations of 5 measurements from 10 plants in each of three replicates. yMean separation within columns by Tukey’s Studentized range

25. Table 5.8. Effect of light intensity treatments on leaf, stem and total above ground dry weight of R. xkurume ‘Pink Pearl’. Z In the absence of interaction, two fertilization treatments were pooled within each light intensity treatment, so that each mean is based on observations of 10 plants in each of three replicates. y Mean separation within columns by Tukey’s Studentized range test at P≤0.05.

26. Conclusions • High rates of fall fertilization reduced freeze resistance, even when application was terminated Sept. 29. • 2. Fall fertilization neither affected the freeze resistance, nor growth of evergreen versus deciduous azaleas differently. • 3. Moderate fertilization though Nov. 28 increased stem nutrient concentrations with little impact on freeze resistance • 4. Shade treatments did not have a biologically significant effect on freeze resistance • 5. Shade reduced photoinhibition and increased growth • 6. Freeze resistance and chlorophyll fluorescence were not related.

27. Applications to the Nursery Industry 1. Avoid the use of high fertilizer application rates 2. Maintain moderate fertilization through Sept. 3. Moderate rates of fall fertilization through Nov. may be useful to build nutrient reserves. 4. Different fertilization regimes are not needed for evergreen vs. deciduous azaleas. 5. Shade management has little impact on freeze resistance, but may provide protection from temperature fluxes and freeze desiccation. 6. Shade can be used to increase growth and reduce leaf damage associated with photosynthetic stress. 7. Chlorophyll fluorescence (Fv/Fm) is not a useful indicator of freeze resistance for azaleas