Cold-Hardening Young 'Valencia' Orange Trees on Swingle

Proc. Fla. State Hort. Soc. 89:9-10. 1976
COLD-HARDENING YOUNG VALENCIA1 ORANGE TREES
ON 'SWINGLE' CITRUMELO (CPB-4475)
AND OTHER ROOTSTOCKS
G. Yelenosky
Agricultural Research Service,
U.S. Department of Agriculture,
Orlando, FL 32803
Additional index words. Freeze injury, citrus.
Abstract. <strong>Young</strong> 'Valencia7 orange trees (Citrus sinensis
(L) Osbeck) on 'Swingle' citrumelo (C. paradisi Macf. X
Poncirus trifoliata (L) Raf.) were cold-hardened and freezetested
in controlled-environment rooms. 'Valencia7 on
'Swingle' rootstock cold-hardened more than on rough lemon
(C. limon (L) Burm. f.), and compared favorably with 'Valen
cia' on sour orange (C. aurantium L.) rootstock. <strong>Cold</strong>-hardened
leaves were not injured at —3.89 C (25 F) for 3 hr, and stems
withstood -6.11 C (21 F) for 3 hr. Other rootstocks included
'Large Flower' trifoliate orange (Poncirus trifoliata (L.) Raf.),
and 'Carrizo' and 'Rusk' citranges (C. sinensis X P. trifoliata).
Agricultural Research Service, U.S. Department of Agri
culture, released 'Swingle' citrumelo rootstock to citrus
growers in 1974. 'Swingle,' formerly citrumelo CPB-4475,
has many favorable traits. 'Swingle' is associated with high
citrus yields (5), tolerates viruses, resists root-rot organisms,
withstands high salt content in soils, and seemingly lias cold
tolerance (4). Continued evaluations are warranted as
'Swingle' is phased into commercial plantings.
This study rates the cold-hardiness of young 'Valencia*
trees on 'Swingle' rootstock relative to other rootstocks.
Tissue tolerance of ice formation at different temperatures
for different durations is included for <strong>'Valencia'</strong> orange trees
on 'Swingle' rootstock.
Materials and Methods
Test trees were <strong>'Valencia'</strong> on 'Swingle/ sour orange,
rough lemon, 'Large Flower' trifoliate orange, and 'Carrizo'
and 'Rusk' citranges. Rootstock seed were germinated and
seedlings were grown in a glasshouse under normal daylight.
<strong>'Valencia'</strong> budwood was from a mature tree on the A. H.
Whitmore Foundation Farm near Leesburg, Florida.
Budded trees were maintained with routine glasshouse
procedures.
Controlled-environment tests included 9- to 18-month-old
uniform trees in groups of 10 to 30 single-tree replicates.
Potted trees were in 3-liter cans filled with a prepared soil
mix containing 1 part sand, 2 parts vermiculite, 7 parts
peat, and a trace of mixed fertilizers. One-half of the group
of test trees remained in the glasshouse as nonhardened
controls, and the others were cold-hardened in controlledenvironment
rooms. Both halves were freeze-tested simul
taneously.
<strong>Cold</strong>-hardening was moderate in controlled-environment
rooms as reported previously (7). Temperature-light periods,
except where noted, were 21.1 C (70 F), 12-hr light and 10 C
(50 F) dark for 14 days, followed by 14 days of 15.6 C (60 F)
light and 4.4 C (40 F) dark. During cold-hardening treat
ments, glasshouse temperatures ranged from 32.8 C (91 F)
days to 23.3 C (74 F) nights. These temperatures do not
cold-harden citrus. Along with previously reported coldhardening
procedures, similar freeze patterns were followed
with a low of -6.7 C (20 F) for 4 hr (7). In trials to de
termine tissue tolerance of external ice at different tempera-
Proc. Fla. State Hort. Soc. 89: 1976.
tures, trees were misted with precooled 1 C (33.8 F) distilled
water from a hand sprayer. Water froze on trees within 5
min after misting at different test temperatures. This in
duced H2O soaking and apparent freezing inside the leaves
and stems.
Plants were returned to a glasshouse after thawing at
about 24 C (75 F). Four weeks later, foliage kill and stem
dieback were rated.
Results and Discussion
<strong>'Valencia'</strong> trees cold-hardened to different degrees. Freezeinjury
ratings separated trees on different rootstocks based
on stem dieback, but not on foliage kill. Rootstock relation
ships were found in tests with both 9- and 18-month-old
trees. This is an improvement over field experience with
young citrus trees. Natural freezes often injure trees less
than 5 years old so uniformly that rootstock relationships
are lost. This is largely the result of inadequate coldhardening
prior to advective and radiation-type freezes.
Radiation-type freezes can be expected annually in Florida
and are characterized by rapid heat loss shortly after 4 p.m.
(6).
In this study, injury to 9-month-old <strong>'Valencia'</strong> trees was
lowest on 'Swingle,' followed by 'Rusk' citrange, then sour
orange, and was highest on rough lemon (Table 1). No
leaves survived —6.7 C (20 F) for 4 hr, regardless of rootstock.
However, <strong>'Valencia'</strong> stem survival differed among
rootstocks. <strong>Trees</strong> on 'Swingle' averaged 9% stem dieback.
Injury was three times greater on 'Rusk' citrange, more than
five times greater on sour orange, and more than nine times
greater on rough lemon.
Table 1. Mean freeze2 injury to 9-month-old Valencia orange trees on
different rootstocks.
Rootstock
Swingle
Rusk citrange
Sour orange
Rough lemon
C
100
100
100
100
Foliage killy
NC
100
100
100
100
Stem dieback
C
NC
9ax
27 ab
48 be
86 d
100
100
100
100
8-6.7 C (20 F) for 4 hr.
yC = cold-hardened for 14 days at 21.1 C (70 F), 12 hr light and 10 C
(50 F) dark followed by 14 days of 15.6 C (60 F) light and 4.4 C (40 F)
dark. NC = not cold-hardened (glasshouse).
xOdds are 19:1 that means are different if followed by different letters.
In trials with 18-month-old, cold-hardened <strong>'Valencia'</strong>
trees on 'Swingle/ foliage kill was severe, but less than
100%. Average leaf survival was best on trees on 'Swingle',
but this difference was not statistically significant (Table 2).
Differences in <strong>'Valencia'</strong> stem dieback were statistically dif
ferent between trees on 'Swingle' and on rough lemon. Stem
dieback was almost 4-10 times greater for trees on rough
lemon than for trees on the other rootstocks.
Differences between trials with 9- and 18-month-old
trees are attributed largely to differences in age and/or
tissue maturity. In general, the principle of "the older the
tree, the more tolerance of freezes" is expressed as less injury
to 18- than 9-month-old trees in this study. However, I
cannot account for greater differences among rootstock with

Table 2. Mean freeze* injury to 18-month-old Valencia orange trees on
different rootstocks.
Rootstock
Swingle
Rusk citrange
Carrizo citrange
Sour orange
Trifoliate orange
Rough lemon
Foliage kill*
C
72 ax
86 a
82 a
90 a
95 a
92 a
NC
100
100
100
100
100
100
f
% 0
Stem dieback
c
6a
7a
10 a
3a
6a
39 b
NC
100
100
100
100
100
100
z-6.7 C (20 F) for 4 hr.
yC = cold-hardened for 14 days at 21.1 C (70 F), 12-hr light and 10 C
(50 F) dark followed by 14 days of 15.6 C (60 F) light and 4.4 C (40 F)
dark. NC = not cold-hardened (glasshouse).
xOdds are 19:1 that means are different if followed by different letters.
9- than with 18-month-old trees. In the field, rootstock effects
tend to increase as trees age. <strong>Young</strong> trees are known to be
more cold susceptible than older trees because of size dif
ferences. Size differences among older trees sometimes re
quire adjusted freeze ratings in the field (3). Little is known
about the cold-hardening potential of young, budded citrus
trees. Controlled-environment screening will help provide
information.
<strong>Cold</strong>-hardening in 9-month-old <strong>'Valencia'</strong> trees on
'Swingle' was sufficient to prevent apparent injury at —3.9 C
(25 F) for 3 hr (Table 3). This was ice tolerance, and not
ice avoidance, as supercooling was prevented by precooled
Table 3. Freeze injury to 9-month-old Valencia orange trees on Swingle
rootstock at different temperatures for 1 and 3 hr.
Treatment
C
NC
C
NC
C
NC
Tem
perature
C(F)
-3.9(25)
-5.0(23)
-6.1(21)
Leaf
kill
0
100
4±3
100
12±9
100
1 hr
Stem
dieback
0
11±9
0
57±5
0
92±6
i
Leaf
kill
0
100
8±5
100
44±11
100
3h
Stem
dieback
0
22 ±18
0
89±4
0
100
yC = cold-hardened for 14 days at 21.1 C (70 F), 12-hr light and 10 C
(50 F) dark followed by 14 days of 15.6 C (60 F) light and 4.4 C (40 F)
dark. NC = not cold-hardened (glasshouse).
a.
10
H2O mist sprays at test temperatures. <strong>Cold</strong>-hardened
<strong>'Valencia'</strong> stems showed no injury, even after 3 hr at —6.1 C
(21 F). Both lower temperatures and longer freezes in
creased leaf kill. After 1 hr at -5 C (23 F), leaf kill averaged
4%. This percentage doubled after 3 hr. A similar pattern,
but greater injury, resulted after the —6.1 C (21 F) test.
This test was essentially lethal to nonconditioned <strong>'Valencia'</strong>
on 'Swingle' rootstock. All treatments killed 100% of the
leaves of nonconditioned trees.
In this study, 'Swingle' compared favorably with mod
erately cold-tolerant citrus rootstocks. Observations on much
older trees in field plantings by other researchers indicated
similar results. Gardner and Horanic (3) rated 'Swingle'
close to sour orange in cold-hardiness ratings in a 10-yr-old
<strong>'Valencia'</strong> planting in Weirsdale, Florida. As a rootstock
for grapefruit in Texas, Cooper, et al. (1) rated 'Swingle'
moderately cold-hardy and equal to that of sour orange.
<strong>Young</strong> and Olson (9) reported that most grapefruit trees on
'Swingle' survived -11.1 C (12 F). During the 1951 Texas
freeze, 100% of the grapefruit trees observed on 'Swingle'
survived, whereas only 58% survived on 'Rusk' citrange (2).
As seedlings, 'Swingle' compared favorably with cold-hardy
trifoliate orange during freezes in Texas' (%) and survived
-16.1 C (3 F) in Georgia (7).
<strong>'Valencia'</strong> orange on 'Swingle' rootstock is considered
relatively cold-hardy based on available information.
Literature Cited
1. Cooper, W. C, E. O. Olson, N. Maxwell, and G. Otey. 1956. Review
of studies on adaptability of citrus varieties as rootstocks for grape
fruit in Texas. /. Rio Grande Valley Hort. Soc. 10:6-19.
2. , , , and A. Shull. 1957. Orchard per
formance of young trees of red grapefruit on various rootstocks in
Texas. Proc. Amer. Soc. Hort. Sci. 70:213-222.
3. Gardner, F. E., and G. E. Horanic. 1958. Influence of various root
stocks on the cold resistance of the scion variety. Proc. Fla. State
Hort. Soc. 71:81-86.
4. Hutchison, D. J. 1974. Swingle citrumelo—a promising rootstock
hybrid. Proc. Fla. State Hort. Soc. 87:89-91.
5. Wutscher, H. K., and A. V. Shull. 1972. Performance of 13 citrus
cultivars as rootstocks for grapefruit. /. Amer. Soc. Hort Sci 97*
778-781.
6. Yelenosky, G. 1972. Nocturnal radiation in a citrus planting Proc
Fla. State Hort. Soc. 85:71-74.
7- , R. T. Brown, and C. J. Hearn. 1973. Tolerance of tri
foliate orange selections and hybrids to freezes and flooding Proc
Fla. State Hort. Soc. 86:99-104.
8. <strong>Young</strong>, R. 1963. Freeze injury to young seedlings of citrus cultivars
and related species in the lower Rio Grande Valley. T. Rio Grande
Valley Hort. Soc. 17:37-42.
9- > and E. O. Olson. 1963. Freeze injury to citrus trees on
various rootstocks in the lower Rio Grande Valley of Texas Proc
Amer. Soc. Hort. Sci. 83:337-343.
Proc. Fla. State Hort. Soc. 89: 1976.