Effects of Fall Fertilization, Taxonomic
Differences and Light Intensity on Freeze
Resistance of Azaleas
Frank Henning
R4 EPA-LGU Liaison
fhenning@uga.edu
• Rhododendron economically important
• Varieties with desired horticultural traits
susceptible to freeze damage
-Multiple factors affect freeze resistance
-Interactions not well understood
Leaf Retention
Fertilization
Light Intensity
Study #1
Effects of Fall Fertilization on Freeze
Resistance, Flowering and Growth
of R. xkurume ‘Hinodegiri’
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?
- Factorial design
- 5 fall fertigation treatments
Fertigation
- Fertilizer application rate
- Time of fertilizer application
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)
Table 3.1. Effect of fertilization treatments on leaf, stem and total above ground DW
of R. xkurume ‘Hinodegiri’.
N fertilization
Dry weight (g)
(mg N
. L-1)
Aug. 1
To
Sept. 29
Sept. 30
to
Nov. 28
0
Leaf
Stem
Total
0
9.2 dz,y
8.3 b
17.4 d
75
0
17.2 bc
12.4 a
29.6 c
125
0
20.0 ab
14.2 a
34.3 ab
75
75
16.4 c
14.2 a
30.6 bc
125
125
22.0 a
15.0 a
37.1 a
Z Each
y Mean
mean based on observations of five plants in each of three replicates.
separation within columns by Tukey’s Studentized range test at P≤0.05.
Table 3.3. Effect of fertilization treatments on freeze resistance of stem tissue
of R. xkurume ‘Hinodegiri’.
N fertilization
(mg . L-1)
Aug. 1
------------------------- LT50 (C, F) -------------------------
Sept. 30
Stem Harvest Dates
to
to
Sept. 29
Nov. 28
0
Nov.12
Dec. 11
Jan. 14
Feb. 18
Mar.19
0
-13.5 (7.7)bcz,y
-22.5 (-8.5)b
-22.25 (-8.1)ab
-20.25 (-4.5)a
-6.42 (20.4)b
75
0
-15.5 (22.1)c
-21.25 (-6.3)b
-24 (-11.2)b
-20.75 (-5.4)a
-6.75 (19.9)b
125
0
-11.5 (11.3)ab
-20.75 (-5.4)b
-21 (-5.8)ab
-18.5 (-1.3)a
-4.5 (23.9)a
75
75
-13.75 (7.3)bc
-19.5 (-3.1)b
-22.25 (-8.1)ab
-18.75 (-1.8)a
-5.92 (21.3)ab
125
125
-9.25 (15.4)a
-9.5 (14.9)a
-18.75 (-1.8)a
-18.5 (-1.3)a
-4 (24.8)a
ZEach
yMean
mean based on observations of four stem samples at each of 9 different freezing temperature increments for each of 3 replicates.
separation within columns by Tukey’s Studentized range test at P≤0.05.
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.
Fertilization treatment (mg . L-1)
Aug. 1 to
Sept. 29
Sept. 30 to
Nov. 28
0
0
85.4 (Mar. 26)az,y
205.6ab
75
0
85.2 (Mar. 26)a
245.3a
125
0
83.4 (Mar. 24)ab
201.9ab
75
75
81.9 (Mar. 22)ab
206.2ab
125
125
80.1 (Mar. 21)b
151.3b
Z Each
y Mean
F50
Numeric date
Total number
of flowers
mean based on observations of five plants in each of three replicates.
separation within columns by Tukey’s Studentized range test at P≤0.05.
Study #2
Effect of Fall Fertilization on Freeze
Resistance of Deciduous Versus
Evergreen Azaleas
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
- 2 x 3 factorial design
- 2 taxa (R. canescens and R. xsatsuki)
- 3 fertilzation regimes
Table 4.1. Effect of fertilization treatments on plant growth index (PGI z) pooled for
R. canescens and R. xsatsuki ‘Wakaebisu’.
N Fertilizer (mg N . L-1)
zMaximum
PGI
Aug. 1 to
Sept. 29
Sept. 30 to
Nov.28
Dec. 15,
2003
May 21,
2004
75 y
0
38.6 ax
46.5 a
75
75
39.5 a
47.8 a
125
125
36.7 a
43.3 a
height (H), maximum width (W1) and width perpendicular to W1, W2 were used in the calculation GI = (H + (W1 + W2) / 2) / 2.
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.
yData
Table 4.2. Effect of fertilization treatments on pooled freeze resistance (LT50z)
of R. canescens and R. xsatsuki ‘Wakaebisu’ stem tissue.
N fertilization
-------------------- LT50 (C, F) -------------------(mg
N . L-1)
Aug.1
to
Sept.29
Sept.30
to
Nov.28
75
0y
75
125
z Temperature
Stem harvest dates
Nov. 12
Dec. 11
Jan. 14
Feb. 18
Mar. 19
-6.3 (20.7) bx
-20.4 (-4.7)c
-23.8 (-10.8) b
-24.8 (-12.6)b
-15.4 (4.3)a
75
-5.6 (21.9)b
-16.8 (1.8)b
-22.3 (-8.1) ab
-22.6 (-8.7)ab
-18.6 (-1.5)a
125
-3.4 (25.9)a
-13.6 (7.5)a
-19.9 (-3.8) a
-19.8 (-3.6)a
-19.6 (-3.3)a
at which 50% of stems were killed
Data 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.
y
Table 4.5. Effect of fertilization treatments on N concentration of azalea
stem tissue harvested Feb.17, 2004.
N fertilization (mg . L-1)
Taxa
R. canescens
R. xsatsuki
zMean
Aug.1 to
Sept.29
Oct.1 to
Nov.28
N
Concentration
( g . kg-1)
75
0
7.7
bz
75
75
13.5
a
125
125
14.6
a
75
0
7.9
c
75
75
10.7
b
125
125
13.5
a
separation within columns by Tukey’s Studentized range test at P≤0.05.
Study #3
Influences of Photosynthetic Photon Flux and
Fall Fertilization on Fluorescence,
Freeze Resistance, and Growth of
Rhododendron xkurume ‘Pink Pearl’
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
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 fluorescencebased determination of LT50 - F.LT50
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
Split-plot design
2 fall fertilization treatments
4 PPF (shade) treatments
Table 5.3. Effects of light intensity (PPF) on stem tissue freeze resistance (LT50) of R. xkurume ‘Pink Pearl’.
Light intensity (% PPF)
May 1 to
Oct.1 to
Sept. 30
May 1
-------------------- LT50 (oC) --------------------
Stem Harvest Dates
Nov. 22
Dec. 21
Jan. 19
Feb. 21
Mar. 22
50
50
-6.6
abzy
-14.1
a
-24.9
b
-15.8
a
-7.9
a
50
100
-5.1
a
-15.9
a
-23.0
ab
-17.1
a
-6.5
a
Bilderbeck and Bir (1986)
zIn
100
50
-8.8
ab
-17.0
a
-21.9
ab
-17.1
a
-6.9
a
100
100
-9.5
b
-17.9
a
-21.6
a
-18.8
a
-6.6
a
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.
Table 5.4. Effects of light intensity on chlorophyll fluorescence (Fv/Fm) of R. xkurume.
Light intensity (% PPF)
May 1 to
Sept. 30
z
Chlorophyll fluorescence (Fv/Fm)
Measurement Dates
Oct.1 to
May 1
Dec. 23
Jan. 21
Feb. 23
Mar. 24
50
50
0.70
azy
0.67 ab
0.67 ab
0.68 ab
50
100
0.64
b
0.59 b
0.60 b
0.61 b
100
50
0.74
a
0.72 a
0.72 a
0.72 a
100
100
0.64
b
0.63 ab
0.65 ab
0.65 ab
In 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
Table 5.8. Effect of light intensity treatments on leaf, stem and total above ground
dry weight of R. xkurume ‘Pink Pearl’.
Light Intensity (%PPF)
May 1 to
Sept. 30
Z In
Oct. 1 to
May 1
Dry weight (g)
Leaf
Stem
Total
50
50
21.3
az,y
18.4
a
39.7
a
50
100
19.1
a
17.1
a
36.3
a
100
50
10.0
b
4.8
b
14.5
b
100
100
9.7
b
4.3
b
14.4
b
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.
Conclusions
1. 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.
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