Directory UMM :Data Elmu:jurnal:S:Scientia Horticulturae:Vol84.Issue3-4.June2000:
Scientia Horticulturae 84 (2000) 245±254
Indole acetic acid and abscisic acid levels
in new shoots and fibrous roots of citrus
scion-rootstock combinations
Katsuji Nodaa,d, Hitoshi Okudab, Isao Iwagakic,*
a
The United Graduate School of Agricultural Science, Gifu University,
1-1 Yanagido, Gifu 501-1112, Japan
b
Department of Citriculture, National Institute of Fruit tree Science, Nakachyo,
Okitsu, Shimizu, Shizuoka-ken 424-0292, Japan
c
Faculty of Agriculture, Shizuoka University, Kariyado, Fujieda, Shizuoka-ken 426-0001, Japan
d
Faculty of Agriculture, Shizuoka University, 836 Ohya, Shizuoka 422-8529, Japan
Accepted 17 May 1999
Abstract
Citrus rootstocks are important for the growth of scion varieties, but it is not clear how they
regulate scion vigor. We studied three citrus rootstock cultivars; `Flying Dragon' (Poncirus
trifoliata var. monstrosa) a dwarfing rootstock, `Swingle' citrumelo (P. trifoliata Citrus paradisi)
a vigorous rootstock, and trifoliate orange (P. trifoliata) as the control. Tender buds from new shoots
of `Eureka' lemon were cleft grafted on etiolated rootstock seedlings.
Eighteen months after grafting, the dry matter of each part of the young grafts was measured. Top
weight was the greatest on `Swingle' citrumelo and smallest on `Flying Dragon', but there was no
distinct difference in root growth among the rootstocks. Endogenous indole acetic acid (IAA) and
abscisic acid (ABA) were measured in the new shoots and fibrous roots. The IAA level in the new
shoots was highest in `Swingle' citrumelo and lowest in `Flying Dragon'. The ABA level in the new
shoots was highest in `Flying Dragon' and lowest in `Swingle' citrumelo. Both the IAA and ABA
levels in the fibrous roots were highest in the strains of Poncirus trifoliata and lowest in `Swingle'
citrumelo. The vigorous rootstock `Swingle' citrumelo had the highest T±R and IAA±ABA ratios in
the new shoots. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Citrus; Rootstock; IAA; ABA
* Corresponding author. Tel.: +81-54-641-9500; fax: +81-54-644-4641.
E-mail address: iwagaki@po2.across.or.jp (I. Iwagaki).
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 0 8 0 - 1
246
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
1. Introduction
Trifoliate orange is the most common citrus rootstock, accounting for about
95% of all the citrus rootstocks used in Japan. Citrus trees grafted on trifoliate
orange have comparatively small tree canopies and produce consistent high
quality fruit. In spite of the merits of trifoliate orange, new types of rootstocks,
vigorous and/or dwarf, for new scion varieties and innovative culture systems are
needed.
Plant growth regulators have important functions in plant behavior. Lockard
and Schneider (1981), who summarized the existing information on the role
of the rootstock in the dwarfing of grafted apple trees, suggested that a
dwarfing mechanism is triggered by plant growth regulators. It is important to
clarify the relationship that exists between the top and root in scion-rootstock
combinations. The findings should contribute to the future development of new
rootstocks.
Auxin generally is considered as a plant growth promoter. Exogenously applied
Indole 3-acetic acid (IAA) strongly promoted stem elongation over a long period
in intact light-grown seedlings of both dwarf and tall peas (Yang et al., 1993), but
auxin generally inhibits root elongation (Pilet and Saugy, 1987; Tanimoto and
Watanabe, 1986). Abscisic acid (ABA) is regarded not only to be an inhibitor of
elongation (Bensen et al., 1988; Sakurai et al., 1985; Yadava and Dayton, 1972)
but a growth promoter (Bradford, 1983; Weston, 1976). A negative correlation
between root growth and endogenous ABA was reported (Pilet and Saugy, 1987),
and exogenous ABA inhibited the tip growth of excised roots (Gaither et al.,
1975). The roles of IAA and ABA in scion-rootstock interaction are not fully
known. We examined the levels of endogenous IAA and ABA in citrus scionrootstock combinations and here discuss the effect of the rootstock on scion
growth.
2. Material and methods
2.1. Plant materials
Three citrus rootstock varieties were used: `Swingle' citrumelo (Poncirus
trifoliata Citrus paradisi) a vigorous rootstock, `Flying Dragon' (Poncirus
trifoliata var. monstrosa) a dwarfing rootstock, and trifoliate orange (Poncirus
trifoliata) as the standard rootstock. In May 1996, tender buds from new shoots of
`Eureka' Lemon (Citrus limon var `Eureka') were cleft grafted on etiolated
rootstock seedlings which had been seeded in 6 cm plastic pots and grown in a
dark incubator at 298C for 21 days. After grafting, three replications of 10 plants
of each scion-rootstock combination were grown in a glass house. In June 1996,
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
247
the plants were transplanted to 9 cm diameter plastic pots, and in May 1997
retransplanted to 15 cm diameter plastic pots.
Eighteen months after grafting (October 1997), plants no. 1, 3 and 5, in order of
growth performance, underwent destructive harvest and dried in an oven at 708C
to obtain the dry matter weight. Twenty-four months after grafting (April 1998)
30 mm tips of new shoots and twenty-five months after grafting (May 1998)
fibrous roots were collected from the remaining plants. The collected samples
were immediately lyophilized then stored in a freezer at ÿ308C until the analysis
of IAA and ABA.
2.2. IAA and ABA analysis
The lyophilized materials were homogenized in 90% acetone (with 100 mg/l
buthylated hydroxyl toluene) with polytron, after which 200 ng of 13 C6 -IAA and
D6-ABA was added as the internal standard. IAA and ABA were extracted three
times for 1 h at 48C, and the extracts filtered and dried. The dried samples were
treated three times with pH 8.5 K-phosphate buffer in an ultrasonic bath and then
filtered. Five-tenths of a gram of PVPP (polyvinylpolypyrrolidone) was added,
after which the samples were stirred and then filtered. The extracts were
concentrated to about 5 ml, adjusted to pH 2.5 with 1N HCl, then filtered through
a nylon disk filter (25 mm diameter, 45 mm pore size). The extracts were
partitioned three times with half volume of ethyl acetate. The ethyl acetate phase
was collected and dried, after which it was treated three times with 400 ml of 1 : 9
ethyl acetate, hexane in an ultra sonic bath and then dried. The dried sample was
dissolved in methanol and then methylated with diazomethane and dried. The
methylated sample was dissolved in 50 ml ethyl acetate, then injected into GC/MS
by the EI ionization method. This procedure was repeated three times for each
sample; once to obtain the total ion current (TIC) chromatogram and twice using
selected ion monitoring (SIM). The selected ion for IAA was m/z 130 and for
ABA m/z 190, their respective main peak ions.
3. Results
3.1. Plant growth
Grafts were grown in a glass house, and dry matter was measured 18 months
after grafting. The dry weight of the total above-ground part was greatest for
`Swingle' citrumelo and smallest for `Flying Dragon' rootstock. Dry weights of
the leaves and stems showed the same pattern as that of the total top weight
(Table 1). There were no significant differences among the rootstocks for root dry
matter in total plant dry matter. The top±root ratio (T±R ratio) order, however,
was highest for `Swingle' citrumelo and lowest for `Flying Dragon'.
248
Rootstock
Dry matter (g)
Above ground
Flying Dragon
Trifoliate orange
`Swingle' citrumelo
Leaf
Stem
Total
6.38 0.16a
6.90 0.31ab
7.07 0.20b
2.60 0.12a
2.90 0.17ab
3.26 0.19b
8.98 0.25a
9.80 0.46ab
10.33 0.38b
Root
Total
T±R ratio
8.20 0.33a
7.85 0.29a
7.52 0.30a
17.18 0.53a
17.65 0.70a
17.85 0.57a
1.10 0.04a
1.25 0.05b
1.39 0.06c
Data are means standard error (n9); different letters indicate significant difference (P < 0.05).
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
Table 1
Dry leaf, stem and root matter of `Eureka' lemon, 18 months after grafting on three rootstocks
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
249
Fig. 1. IAA (A) and ABA (B) levels in new shoots of `Eureka' lemon, 24 months after grafting on
three citrus rootstocks. Each column shows the means (s.e., n6) of the measurements.
3.2. IAA and ABA levels in above-ground plant parts
Twenty-four months after grafting, IAA and ABA levels were measured in the
new shoots of the grafts (Fig. 1). Endogenous IAA levels were highest for
`Swingle' citrumelo and lowest for `Flying Dragon'. These IAA levels in the new
shoots were correlated with scion growth. The ABA level in the new shoots was
lower on the vigorous rootstock than on the dwarf rootstock. Assuming that IAA
is a growth promoter and ABA an inhibitor in the above-ground parts, we
calculated the IAA±ABA ratio (Fig. 2). It was highest in `Swingle' citrumelo and
lowest in `Flying Dragon'.
3.3. IAA and ABA levels in under-ground plant parts
Fibrous roots were collected 25 months after grafting, and the IAA and ABA
levels measured (Fig. 3). IAA levels in the fibrous roots were highest in `Flying
Dragon', followed by trifoliate orange then `Swingle' citrumelo. ABA levels in
`Flying Dragon' and trifoliate orange also were higher than in `Swingle'
citrumelo. Endogenous IAA and ABA levels in the fibrous roots of the rootstocks
were negatively correlated with shoot growth and the endogenous IAA levels in
shoots.
250
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
Fig. 2. IAA±ABA ratios in new shoots of `Eureka' lemon. Each column shows the means (s.e.,
n6) of the measurements.
4. Discussion
4.1. Plant growth
Wheaton et al. (1991) reported that seven-year-old `Hamlin' and `Valencia'
oranges, `Murcott' tangor and `Redblush' grapefruit trees grafted on `Swingle'
Fig. 3. IAA (A) and ABA (B) levels in fibrous roots of `Eureka' lemon, 24 months after grafting on
three citrus rootstocks. Each column shows the means (s.e., n6) of the measurements.
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
251
citrumelo made large tree canopies and those grafted on `Flying Dragon' made
the smallest canopies in their experiment on the effects of three rootstocks on tree
growth. We obtained similar results. The weight of the dry matter of `Eureka'
lemon was greatest on `Swingle' citrumelo and lowest on `Flying Dragon', which
agrees with other reports and practical experience. There were no distinctive
differences in the dry root matter and dry total plant matter of the three
rootstocks. Takahara et al. (1994) reported that in Iyo rootstocks which showed
poor top growth there were low T±R ratios. Our findings support this, the T±R
ratio of `Swingle' citrumelo being highest and that of `Flying Dragon' lowest. In
apple, the rootstock is reported not to affect the T±R ratio (Beakbane, 1956;
Rogers and Booth, 1959), but in citrus the T±R ratio may be affected by rootstock
traits.
4.2. IAA and ABA levels in above-ground plant parts
Auxin is important for controlling growth, the size of shoots, and possibly, the
size of roots (Lockard and Schneider, 1981). Yang et al. (1993) found that
exogenously applied IAA strongly promoted stem elongation over the long-term
in intact, light-grown seedlings of both dwarf and tall peas. Endogenous auxin
levels were reported as higher in the shoots and apices of seedlings of tall than
that of dwarf peas (Law and Davies, 1990). Inoue et al. (1982) showed that the
endogenous IAA contents in seedlings of 12 barley strains were correlated with
the growth rate of coleoptile length. Endogenous IAA typically is greatest in the
apex and actively growing regions (Heremans et al., 1986; Ortuno et al., 1990;
Law and Davies, 1990; Iino and Carr, 1982). In our experiment, the apical portion
of new shoots (30 mm) was used for the IAA and ABA measurements. Results for
`Eureka' lemon showed a positive correlation between the endogenous IAA
levels and the dry matter of the above-ground parts on the three different
rootstocks, indicative that endogenous IAA may have promoted the shoot growth
of `Eureka' lemon.
Another plant growth regulator, ABA, generally is regarded as an inhibitor of
elongation. In our experiment the endogenous ABA content was negatively
correlated with shoot growth. It has been reported to be associated with stunted
growth of squash (Sakurai et al., 1985) and soybean (Bensen et al., 1988)
hypocotyls. Levels of ABA-like substances were found to be higher in the roots,
leaves and stems of dwarf apple seedlings than in vigorous seedlings (Yadava and
Dayton, 1972). Although there is some evidence that ABA is an inhibitor, in
contrast it has been reported to promote tomato shoot elongation (Weston, 1976).
Bradford (1983) also reported that ABA promoted stem elongation when applied
to ABA-deficient, mutant tomato plants. Moreover, its level was found to be
higher in young regions of asparagus from which buds would sprout (Kojima
et al., 1993). Although the role of ABA in scion growth is still controversial, our
252
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
findings for `Eureka' lemon support the hypothesis that a high ABA level may
depress shoot growth.
4.3. IAA and ABA levels in under-ground plant parts
High concentrations of exogenous IAA or ABA inhibited the growth of excised
pea root tips and the elongation of lettuce roots (Gaither et al., 1975; Tanimoto
and Watanabe, 1986). Pilet and Saugy (1987) reported a negative correlation
between root growth and the endogenous content of IAA or ABA in maize roots.
We also found a negative correlation between citrus scion growth and endogenous
IAA and ABA in the fibrous roots. According to Ohwaki and Tsurumi (1976),
basipetal transport of IAA occurs mainly in the outer part of the root, whereas
acropetal transport occurs mainly in the inner part. They suggested that
basipetally transported IAA, passes through the outer part of the intact root,
inhibiting its elongation. High concentrations of endogenous IAA in roots would
inhibit root elongation, and there is more ester-linked than free IAA in maize
roots (Saugy and Pilet, 1987). To counter inhibition of root elongation, IAA in
vigorous rootstock roots, such as `Swingle' citrumelo, may be converted to esters
or other conjugates and transported basipetally. Moreover, IAA metabolism may
be more active in roots of vigorous than dwarf rootstocks. In spite of the
differences among the three citrus rootstocks in the IAA and ABA levels in their
fibrous roots, there was no significant difference in root dry matter. The
comparatively short period of cultivation (about two years after grafting) may not
have been long enough to show a difference, and there may be limitations in
using a potted plant model to determine the actual relationship between plant
growth regulators and root growth.
4.4. Interrelationship between scion and rootstock
The `Eureka' lemon grafted on `Swingle' citrumelo had a large amount of IAA
in its new shoots and a small amount of IAA in the fibrous roots. The latter
suggests that there is active IAA metabolism or conversion to conjugates in the
roots. Esters and other conjugates of IAA may be transported to the scion and
reconverted to IAA. IAA esterification in the roots could have a protective role in
preventing peroxidative attacks (Cohen and Bandurski, 1987). Dwarf rootstock,
on the other hand, had a large amount of IAA in its fibrous roots, and its IAA
metabolite transport to the scion may be less than in vigorous rootstocks. As for
ABA, our results suggest that a high concentration of it may cause growth
inhibition in the above and under-ground parts of citrus plants.
The IAA±ABA ratio in new shoots of `Eureka' on vigorous `Swingle'
citrumelo rootstock was higher than that for the dwarf rootstock `Flying Dragon'.
Moreover, the vigorous rootstock `Swingle' citrumelo has a larger T±R ratio than
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
253
`Flying Dragon'. The large amount of photosynthetic assimilates produced by
citrus trees on vigorous rootstock with high IAA±ABA and T±R ratios supports
the vigor of the plant as a whole.
5. Conclusion
Scion growth of `Eureka' lemon was best on `Swingle' citrumelo, then on
trifoliate orange, followed by `Flying Dragon'. There were no distinctive
differences in root growth among the three rootstocks. The dwarf rootstock
`Flying Dragon' induced a low level of IAA in the new shoots and high levels of
IAA and ABA in the fibrous roots. The concentrations of IAA and ABA in the
fibrous roots were negatively correlated with scion growth. As compared with
dwarf rootstock, vigorous rootstock induces a higher IAA±ABA ratio in new
shoots and a higher T±R ratio.
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Indole acetic acid and abscisic acid levels
in new shoots and fibrous roots of citrus
scion-rootstock combinations
Katsuji Nodaa,d, Hitoshi Okudab, Isao Iwagakic,*
a
The United Graduate School of Agricultural Science, Gifu University,
1-1 Yanagido, Gifu 501-1112, Japan
b
Department of Citriculture, National Institute of Fruit tree Science, Nakachyo,
Okitsu, Shimizu, Shizuoka-ken 424-0292, Japan
c
Faculty of Agriculture, Shizuoka University, Kariyado, Fujieda, Shizuoka-ken 426-0001, Japan
d
Faculty of Agriculture, Shizuoka University, 836 Ohya, Shizuoka 422-8529, Japan
Accepted 17 May 1999
Abstract
Citrus rootstocks are important for the growth of scion varieties, but it is not clear how they
regulate scion vigor. We studied three citrus rootstock cultivars; `Flying Dragon' (Poncirus
trifoliata var. monstrosa) a dwarfing rootstock, `Swingle' citrumelo (P. trifoliata Citrus paradisi)
a vigorous rootstock, and trifoliate orange (P. trifoliata) as the control. Tender buds from new shoots
of `Eureka' lemon were cleft grafted on etiolated rootstock seedlings.
Eighteen months after grafting, the dry matter of each part of the young grafts was measured. Top
weight was the greatest on `Swingle' citrumelo and smallest on `Flying Dragon', but there was no
distinct difference in root growth among the rootstocks. Endogenous indole acetic acid (IAA) and
abscisic acid (ABA) were measured in the new shoots and fibrous roots. The IAA level in the new
shoots was highest in `Swingle' citrumelo and lowest in `Flying Dragon'. The ABA level in the new
shoots was highest in `Flying Dragon' and lowest in `Swingle' citrumelo. Both the IAA and ABA
levels in the fibrous roots were highest in the strains of Poncirus trifoliata and lowest in `Swingle'
citrumelo. The vigorous rootstock `Swingle' citrumelo had the highest T±R and IAA±ABA ratios in
the new shoots. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Citrus; Rootstock; IAA; ABA
* Corresponding author. Tel.: +81-54-641-9500; fax: +81-54-644-4641.
E-mail address: iwagaki@po2.across.or.jp (I. Iwagaki).
0304-4238/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 3 0 4 - 4 2 3 8 ( 9 9 ) 0 0 0 8 0 - 1
246
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
1. Introduction
Trifoliate orange is the most common citrus rootstock, accounting for about
95% of all the citrus rootstocks used in Japan. Citrus trees grafted on trifoliate
orange have comparatively small tree canopies and produce consistent high
quality fruit. In spite of the merits of trifoliate orange, new types of rootstocks,
vigorous and/or dwarf, for new scion varieties and innovative culture systems are
needed.
Plant growth regulators have important functions in plant behavior. Lockard
and Schneider (1981), who summarized the existing information on the role
of the rootstock in the dwarfing of grafted apple trees, suggested that a
dwarfing mechanism is triggered by plant growth regulators. It is important to
clarify the relationship that exists between the top and root in scion-rootstock
combinations. The findings should contribute to the future development of new
rootstocks.
Auxin generally is considered as a plant growth promoter. Exogenously applied
Indole 3-acetic acid (IAA) strongly promoted stem elongation over a long period
in intact light-grown seedlings of both dwarf and tall peas (Yang et al., 1993), but
auxin generally inhibits root elongation (Pilet and Saugy, 1987; Tanimoto and
Watanabe, 1986). Abscisic acid (ABA) is regarded not only to be an inhibitor of
elongation (Bensen et al., 1988; Sakurai et al., 1985; Yadava and Dayton, 1972)
but a growth promoter (Bradford, 1983; Weston, 1976). A negative correlation
between root growth and endogenous ABA was reported (Pilet and Saugy, 1987),
and exogenous ABA inhibited the tip growth of excised roots (Gaither et al.,
1975). The roles of IAA and ABA in scion-rootstock interaction are not fully
known. We examined the levels of endogenous IAA and ABA in citrus scionrootstock combinations and here discuss the effect of the rootstock on scion
growth.
2. Material and methods
2.1. Plant materials
Three citrus rootstock varieties were used: `Swingle' citrumelo (Poncirus
trifoliata Citrus paradisi) a vigorous rootstock, `Flying Dragon' (Poncirus
trifoliata var. monstrosa) a dwarfing rootstock, and trifoliate orange (Poncirus
trifoliata) as the standard rootstock. In May 1996, tender buds from new shoots of
`Eureka' Lemon (Citrus limon var `Eureka') were cleft grafted on etiolated
rootstock seedlings which had been seeded in 6 cm plastic pots and grown in a
dark incubator at 298C for 21 days. After grafting, three replications of 10 plants
of each scion-rootstock combination were grown in a glass house. In June 1996,
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
247
the plants were transplanted to 9 cm diameter plastic pots, and in May 1997
retransplanted to 15 cm diameter plastic pots.
Eighteen months after grafting (October 1997), plants no. 1, 3 and 5, in order of
growth performance, underwent destructive harvest and dried in an oven at 708C
to obtain the dry matter weight. Twenty-four months after grafting (April 1998)
30 mm tips of new shoots and twenty-five months after grafting (May 1998)
fibrous roots were collected from the remaining plants. The collected samples
were immediately lyophilized then stored in a freezer at ÿ308C until the analysis
of IAA and ABA.
2.2. IAA and ABA analysis
The lyophilized materials were homogenized in 90% acetone (with 100 mg/l
buthylated hydroxyl toluene) with polytron, after which 200 ng of 13 C6 -IAA and
D6-ABA was added as the internal standard. IAA and ABA were extracted three
times for 1 h at 48C, and the extracts filtered and dried. The dried samples were
treated three times with pH 8.5 K-phosphate buffer in an ultrasonic bath and then
filtered. Five-tenths of a gram of PVPP (polyvinylpolypyrrolidone) was added,
after which the samples were stirred and then filtered. The extracts were
concentrated to about 5 ml, adjusted to pH 2.5 with 1N HCl, then filtered through
a nylon disk filter (25 mm diameter, 45 mm pore size). The extracts were
partitioned three times with half volume of ethyl acetate. The ethyl acetate phase
was collected and dried, after which it was treated three times with 400 ml of 1 : 9
ethyl acetate, hexane in an ultra sonic bath and then dried. The dried sample was
dissolved in methanol and then methylated with diazomethane and dried. The
methylated sample was dissolved in 50 ml ethyl acetate, then injected into GC/MS
by the EI ionization method. This procedure was repeated three times for each
sample; once to obtain the total ion current (TIC) chromatogram and twice using
selected ion monitoring (SIM). The selected ion for IAA was m/z 130 and for
ABA m/z 190, their respective main peak ions.
3. Results
3.1. Plant growth
Grafts were grown in a glass house, and dry matter was measured 18 months
after grafting. The dry weight of the total above-ground part was greatest for
`Swingle' citrumelo and smallest for `Flying Dragon' rootstock. Dry weights of
the leaves and stems showed the same pattern as that of the total top weight
(Table 1). There were no significant differences among the rootstocks for root dry
matter in total plant dry matter. The top±root ratio (T±R ratio) order, however,
was highest for `Swingle' citrumelo and lowest for `Flying Dragon'.
248
Rootstock
Dry matter (g)
Above ground
Flying Dragon
Trifoliate orange
`Swingle' citrumelo
Leaf
Stem
Total
6.38 0.16a
6.90 0.31ab
7.07 0.20b
2.60 0.12a
2.90 0.17ab
3.26 0.19b
8.98 0.25a
9.80 0.46ab
10.33 0.38b
Root
Total
T±R ratio
8.20 0.33a
7.85 0.29a
7.52 0.30a
17.18 0.53a
17.65 0.70a
17.85 0.57a
1.10 0.04a
1.25 0.05b
1.39 0.06c
Data are means standard error (n9); different letters indicate significant difference (P < 0.05).
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
Table 1
Dry leaf, stem and root matter of `Eureka' lemon, 18 months after grafting on three rootstocks
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
249
Fig. 1. IAA (A) and ABA (B) levels in new shoots of `Eureka' lemon, 24 months after grafting on
three citrus rootstocks. Each column shows the means (s.e., n6) of the measurements.
3.2. IAA and ABA levels in above-ground plant parts
Twenty-four months after grafting, IAA and ABA levels were measured in the
new shoots of the grafts (Fig. 1). Endogenous IAA levels were highest for
`Swingle' citrumelo and lowest for `Flying Dragon'. These IAA levels in the new
shoots were correlated with scion growth. The ABA level in the new shoots was
lower on the vigorous rootstock than on the dwarf rootstock. Assuming that IAA
is a growth promoter and ABA an inhibitor in the above-ground parts, we
calculated the IAA±ABA ratio (Fig. 2). It was highest in `Swingle' citrumelo and
lowest in `Flying Dragon'.
3.3. IAA and ABA levels in under-ground plant parts
Fibrous roots were collected 25 months after grafting, and the IAA and ABA
levels measured (Fig. 3). IAA levels in the fibrous roots were highest in `Flying
Dragon', followed by trifoliate orange then `Swingle' citrumelo. ABA levels in
`Flying Dragon' and trifoliate orange also were higher than in `Swingle'
citrumelo. Endogenous IAA and ABA levels in the fibrous roots of the rootstocks
were negatively correlated with shoot growth and the endogenous IAA levels in
shoots.
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K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
Fig. 2. IAA±ABA ratios in new shoots of `Eureka' lemon. Each column shows the means (s.e.,
n6) of the measurements.
4. Discussion
4.1. Plant growth
Wheaton et al. (1991) reported that seven-year-old `Hamlin' and `Valencia'
oranges, `Murcott' tangor and `Redblush' grapefruit trees grafted on `Swingle'
Fig. 3. IAA (A) and ABA (B) levels in fibrous roots of `Eureka' lemon, 24 months after grafting on
three citrus rootstocks. Each column shows the means (s.e., n6) of the measurements.
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
251
citrumelo made large tree canopies and those grafted on `Flying Dragon' made
the smallest canopies in their experiment on the effects of three rootstocks on tree
growth. We obtained similar results. The weight of the dry matter of `Eureka'
lemon was greatest on `Swingle' citrumelo and lowest on `Flying Dragon', which
agrees with other reports and practical experience. There were no distinctive
differences in the dry root matter and dry total plant matter of the three
rootstocks. Takahara et al. (1994) reported that in Iyo rootstocks which showed
poor top growth there were low T±R ratios. Our findings support this, the T±R
ratio of `Swingle' citrumelo being highest and that of `Flying Dragon' lowest. In
apple, the rootstock is reported not to affect the T±R ratio (Beakbane, 1956;
Rogers and Booth, 1959), but in citrus the T±R ratio may be affected by rootstock
traits.
4.2. IAA and ABA levels in above-ground plant parts
Auxin is important for controlling growth, the size of shoots, and possibly, the
size of roots (Lockard and Schneider, 1981). Yang et al. (1993) found that
exogenously applied IAA strongly promoted stem elongation over the long-term
in intact, light-grown seedlings of both dwarf and tall peas. Endogenous auxin
levels were reported as higher in the shoots and apices of seedlings of tall than
that of dwarf peas (Law and Davies, 1990). Inoue et al. (1982) showed that the
endogenous IAA contents in seedlings of 12 barley strains were correlated with
the growth rate of coleoptile length. Endogenous IAA typically is greatest in the
apex and actively growing regions (Heremans et al., 1986; Ortuno et al., 1990;
Law and Davies, 1990; Iino and Carr, 1982). In our experiment, the apical portion
of new shoots (30 mm) was used for the IAA and ABA measurements. Results for
`Eureka' lemon showed a positive correlation between the endogenous IAA
levels and the dry matter of the above-ground parts on the three different
rootstocks, indicative that endogenous IAA may have promoted the shoot growth
of `Eureka' lemon.
Another plant growth regulator, ABA, generally is regarded as an inhibitor of
elongation. In our experiment the endogenous ABA content was negatively
correlated with shoot growth. It has been reported to be associated with stunted
growth of squash (Sakurai et al., 1985) and soybean (Bensen et al., 1988)
hypocotyls. Levels of ABA-like substances were found to be higher in the roots,
leaves and stems of dwarf apple seedlings than in vigorous seedlings (Yadava and
Dayton, 1972). Although there is some evidence that ABA is an inhibitor, in
contrast it has been reported to promote tomato shoot elongation (Weston, 1976).
Bradford (1983) also reported that ABA promoted stem elongation when applied
to ABA-deficient, mutant tomato plants. Moreover, its level was found to be
higher in young regions of asparagus from which buds would sprout (Kojima
et al., 1993). Although the role of ABA in scion growth is still controversial, our
252
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
findings for `Eureka' lemon support the hypothesis that a high ABA level may
depress shoot growth.
4.3. IAA and ABA levels in under-ground plant parts
High concentrations of exogenous IAA or ABA inhibited the growth of excised
pea root tips and the elongation of lettuce roots (Gaither et al., 1975; Tanimoto
and Watanabe, 1986). Pilet and Saugy (1987) reported a negative correlation
between root growth and the endogenous content of IAA or ABA in maize roots.
We also found a negative correlation between citrus scion growth and endogenous
IAA and ABA in the fibrous roots. According to Ohwaki and Tsurumi (1976),
basipetal transport of IAA occurs mainly in the outer part of the root, whereas
acropetal transport occurs mainly in the inner part. They suggested that
basipetally transported IAA, passes through the outer part of the intact root,
inhibiting its elongation. High concentrations of endogenous IAA in roots would
inhibit root elongation, and there is more ester-linked than free IAA in maize
roots (Saugy and Pilet, 1987). To counter inhibition of root elongation, IAA in
vigorous rootstock roots, such as `Swingle' citrumelo, may be converted to esters
or other conjugates and transported basipetally. Moreover, IAA metabolism may
be more active in roots of vigorous than dwarf rootstocks. In spite of the
differences among the three citrus rootstocks in the IAA and ABA levels in their
fibrous roots, there was no significant difference in root dry matter. The
comparatively short period of cultivation (about two years after grafting) may not
have been long enough to show a difference, and there may be limitations in
using a potted plant model to determine the actual relationship between plant
growth regulators and root growth.
4.4. Interrelationship between scion and rootstock
The `Eureka' lemon grafted on `Swingle' citrumelo had a large amount of IAA
in its new shoots and a small amount of IAA in the fibrous roots. The latter
suggests that there is active IAA metabolism or conversion to conjugates in the
roots. Esters and other conjugates of IAA may be transported to the scion and
reconverted to IAA. IAA esterification in the roots could have a protective role in
preventing peroxidative attacks (Cohen and Bandurski, 1987). Dwarf rootstock,
on the other hand, had a large amount of IAA in its fibrous roots, and its IAA
metabolite transport to the scion may be less than in vigorous rootstocks. As for
ABA, our results suggest that a high concentration of it may cause growth
inhibition in the above and under-ground parts of citrus plants.
The IAA±ABA ratio in new shoots of `Eureka' on vigorous `Swingle'
citrumelo rootstock was higher than that for the dwarf rootstock `Flying Dragon'.
Moreover, the vigorous rootstock `Swingle' citrumelo has a larger T±R ratio than
K. Noda et al. / Scientia Horticulturae 84 (2000) 245±254
253
`Flying Dragon'. The large amount of photosynthetic assimilates produced by
citrus trees on vigorous rootstock with high IAA±ABA and T±R ratios supports
the vigor of the plant as a whole.
5. Conclusion
Scion growth of `Eureka' lemon was best on `Swingle' citrumelo, then on
trifoliate orange, followed by `Flying Dragon'. There were no distinctive
differences in root growth among the three rootstocks. The dwarf rootstock
`Flying Dragon' induced a low level of IAA in the new shoots and high levels of
IAA and ABA in the fibrous roots. The concentrations of IAA and ABA in the
fibrous roots were negatively correlated with scion growth. As compared with
dwarf rootstock, vigorous rootstock induces a higher IAA±ABA ratio in new
shoots and a higher T±R ratio.
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