Variation in suckering capacity among and within lateral roots of an aspen clone

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FOREST SERVICE
U.S. DEPARTMENT OF AGRICULTURE

507 — 25th Street, Ogden, Utah 84401

USDA Forest Service
Research Note INT-241 March 1978

VARIATION IN SUCKERING CAPACITY AMONG AND WITHIN LATERAL ROOTS OF AN ASPEN CLONE

George A. Schier!

ABSTRACT

Exetsed roots were used to determine vartatton tn suckering
capactty among and wtthtn lateral roots of an aspen (Populus
tremuloides Michx.) clone. Dtfferences among lateral roots were
stgntficant. Within segments of a lateral root sucker production
showed a high degree of polarity, tncreastng from the distal to
proxtmal ends. There was no evidence of a gradtent tn suckering
capactty tn a segmented root; t.e., distal segments were not
stgniftcantly different from proximal ones. This indicated that
aging was not a factor regulating suckering within lateral roots.
Sueker production was not affected by root length.

KEYWORDS: Populus tremulotdes, aspen, root suckers,
adventitious shoots, polarity.

Many investigators have found large interclonal differences in the relative capac-
ity of aspen (Populus tremulotdes Michx.) to produce root suckers (Farmer 1962; Maini
1967; Schier 1974; Steneker 1972; Tew 1970; Zufa 1971). However, only Steneker (1972)
has studied intraclonal variation in sucker production. When he propagated suckers
from root cuttings collected from various parts of a clone, he found significant differ-
ences in numbers of suckers produced by the ramets. He also observed a large variation
in numbers of suckers produced on cuttings from the same ramet and from the same lateral
root.

1Plant Physiologist, located at the Intermountain Station's Forestry Sciences
Laboratory, Logan, Utah.

INTERMOUNTAIN FOREST AND RANGE EXPERIMENT STATION

My objective was to examine within-clone variation in sucker production to deter-
mine (1) if there are significant differences between lateral roots in their ability to
produce root suckers, and (2) if there are gradients in sucker production along lateral
roots. Sucker production along a lateral root is strongly influenced by polarity (Maini
1967; Steneker and Walters 1971); that is, a larger number of suckers arise on the prox-
imal (end toward stem) than on the distal (end toward root apex) halves of root segments.
This gradient is believed to be caused by the polar movement of endogenous growth reg-
ulators. However, age-related changes may also cause polarity in roots, as they do in
stems. An aging gradient may exist in roots because the first-initiated, proximal
end, and chronologically oldest part of a root may be morphologically and physiologically
different from its more recently formed distal end. Generally, upper portions of stems
are difficult to root, while the lower portions retain the capacity to initiate roots
(Heuser 1976). In a similar manner, cuttings from sections of lateral roots near the
stem may produce more suckers than cuttings from sections near the root apex.

METHODS

In the summer of 1975, roots were excavated from an area of approximately 0.1 ha
within a single aspen clone in the Wasatch Mountains on the Cache National Forest east
of Logan, Utah. Single root sections of varying lengths and about 1 to 2 cm in diameter
were excised from 27 different lateral roots. Portions of the root sections free of
defects (cankers, scars, decay, etc.) were cut into 10 cm segments and washed free of
soil. The proximal ends of the segments were marked and their sequence within the lat-
eral root recorded. The segments were randomly distributed among six trays (6 by 26 by
54 cm) in which they were planted to an average depth of 1-1/2 cm in moistened vermicu-
lite. The trays were placed in a greenhouse where the diurnal air temperature varied
between 15° and 25° C, and were watered lightly each day. After 6 weeks, the cuttings
were removed from the trays and the number of suckers exceeding 5 mm in length on the
proximal and distal half of each root segment recorded. The height of the tallest
sucker on each segment was measured.

In the summer of 1976, a second root collection was from a different area within
the same aspen clone. This time single root sections 50 cm in length and 1 to 2 cm
in diameter, and free of defects, were severed from 60 different lateral roots. These
were randomly divided into three groups of 20 sections each and each group given one of
the following treatments: (1) uncut (1 x 50 cm); (2) cut into two 25-cm segments (2 by
25 cm); and (3) cut into five 10-cm segments (5 x 10 cm). Two sections (proximal ends
marked) from each treatment were planted in each of 10 trays. The segments from cut
sections were arranged in the sequence in which they were cut. Root sections were cul-
tivated by the same procedures as before. Six weeks after planting the number and dry
weight of suckers exceeding 5 mm in height were determined.

Differences in sucker production caused by either treatment or origin were tested
by analysis of variance. Significance of differences between ranked means was deter-
mined by Keuls' method (Snedecor 1956). A square root transformation was applied to
sucker numbers prior to analysis.

RESULTS
Long Root Sections

The length of the lateral root sections excavated ranged from 51 to 412 cm (mean,
182 cm). Diameters of the defect-free root segments (10 cm), cut from the root sections,
ranged from 8 to 24 mm (mean, 16.2 mm). There was not much taper within the long rope-
like lateral roots. The mean difference in diameter between the largest and smallest
segments within a lateral root was only 4.4 mn.

Table 1.--Variatton among lateral roots within an aspen clone in the mean
number of suckers per segment (10 em) and mean height of tallest
suckers per segment

Suckers : Height of =» ssuckers 0) = Height of

Lateral ° per : tallest Lateral : per : tallest

root : segment : sucker root > segment : sucker
mn mm
1 6.3 38 15 Gel 42
2 Sieh 48 16 4.8 37
3 US Sil 17 S55 42
4 8.6 41 18 eG 51
5 Teas: 45 3) 0.8 aly
6 4.0 35 20 one 45
i Deg iP 4] 2h ONG TZ
8 135-0 42 Up Hie 38
9 10.2 30 23 LOM 7, 51
10 1 eae 40 24 5.4 48
al 15e9 49 25 1235 35
12 4.9 42 26 6.8 49
£3 5ieZ 43 Zi (apa 44

14 4.1 SY,
MEAN 8.56 42.0

The difference among lateral roots in both number and height of suckers (table 1)
was highly significant (1 percent level). Suckering was not related to root diameter.
At" test for paired replicates showed that the proximal half of root segments produced
significantly more suckers than the distal halves. Sixty-four percent of all suckers
arose from the proximal ends and only 36 percent from the distal ends. There was no
evidence of a gradient in suckering capacity along the lengths of lateral roots. Mean
number of suckers produced from segments in the proximal and distal halves of root sec-
tions were: proximal, 8.36; distal, 8.75. One would expect less variation in suckering
capacity between adjacent segments than between distant segments, but this was not found.

Effect of Root Length

Effect of root length on number and dry weight of suckers produced was as follows:

Suckers per Dry wetght

Treatment section per sucker
(mg)
1 x 50 cm Sho 0) 14.4
2 x 25 em 55.9 12.9
S26 0) is S9ORF 1329

Cutting 50-cm lateral root sections into segments (10 or 25 cm) did not signifi-
cantly affect number or dry weight of suckers produced by the sections. Within each of
the three treatments (1 x 50 cm, 2 x 25 cm, or 5 x 10 cm), differences among lateral

Table 2.--Distributiton of suckers along 50-cm secttons of cut and uneut aspen roots
(proximal end at 0, distal end at 50 em)

2 > 25.-em : Sane aOnem
Sample 95 ip SS Ro
condition : 0 25 50 : 0 10 20 30 40 50
------------ - Pereent -------------------
Uncut 74.8 25:2 133.5 ZOSS mm NOES 22 Qe,
Cut SSia2 46.8 21.8 TOSS) LSS 18.8 21.4

1Underlined percentages are not significantly different from each other.

root sections were highly significant (1 percent level). Sucker numbers did not differ
Significantly between proximal and distal root segments (table 2). However, within un-
cut sections there was a distinct gradient in suckering capacity; the number of suckers
increased along the root from the distal to the proximal end.

DISCUSSION

Steneker and Walters (1971) also found that length of root cuttings usually did not
significantly affect sucker production. However, they found that cutting 36-inch
(91.4-cm) root sections into 6-inch (15.2-cm) segments significantly reduced mean sucker
heights. They also found more suckers on the proximal three segments than on the distal
three segments of cut 36-inch root sections, although polarity was not as evident as in
uncut roots.

The polarity of sucker formation on excised aspen roots appears to be caused by
physiological factors unrelated to aging. If aging had affected suckering, then gra-
dients in suckering capacity would have occurred along segmented lateral roots.
Polarity in roots is usually attributed to the transport of auxin toward root tips
(Batra and others 1975; Robinson and Schwabe 1977). Auxin, which suppresses suckering
in roots of intact plants, breaks down after the roots are excised (Eliasson 1971;
Schier 1973c, 1975). Suckers are then able to develop. Polar movement of the residual
auxin in segments probably causes higher concentrations in the distal than in the prox-
imal halves, so the distal halves produce fewer suckers. Cytokinins, which stimulate
shoot formation, may also influence the polarity shown in sucker development because
these hormones move in a proximal direction (El-Saidi 1971; Wareing and Phillips 1970).

Most suckers that arise on aspen roots appear to develop from suppressed shoot
primordia (Schier 1973b). Therefore, variation in the capacity of lateral roots to
sucker is probably caused primarily by differences in numbers of primordia. Some
roots may have many more primordia than others because they are exposed to injuries
that stimulate primordia formation.

Generally, roots vary in sensitivity to stimuli that initiate primordia because
of differences in hormone levels and ratios, water content, and concentration of nutri-
ents. Two factors that may affect the physiological condition of lateral roots are
microclimate and position in the clonal root system. Temperature, an important micro-
-climatic variable, varies with soil depth and exposure to radiation. The position of a
lateral root in the root system will determine its location with respect to ramets of

various ages and vigor. This will determine the quantity of carbohydrates and auxins
and other growth translocated to the root. Some clones have roots with few primordia;

a major portion of the suckers are initiated after excision (Schier 1973a). Root
cuttings from these clones are probably physiologically preconditioned for sucker devel-
opment by the factors mentioned. Early growth of suckers may vary with concentration
of carbohydrate reserves (Schier and Zasada 1973).

Any sampling method used to estimate suckering capacity of aspen clones must take
into consideration the large within-clone variation in sucker production. A procedure
that I have found to work well is to collect single cuttings from 30 or more locations
within a clone. Using this procedure I have found significant differences in suckering
capacities among clones (Schier 1974).

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PUBLICATIONS CITED

Batra, Mo We. K. Le Edwards, and T. K. Scott.
1975. Auxin transport in roots: its characteristics and relationship to growth.
In: The development and function of roots (J. G. Torrey and D. T. Clarkson, eds.)
p. 299-325.
Eliasson, I.
1971. Growth regulators in Populus tremula III. Variation of auxin and inhibitor
level in roots in relation to root sucker formation. Physiol. Plant. 25:118-212.
El-Saidi, M. T. 1971. Transport and metabolism of kinetin-8-!*C in Zea mays L. roots.
Ann. Bot. 35:1073-1078.
Farmer, R. E., Jr.
1962. Aspen root sucker formation and apical dominance. For. Sci. 8:403-410.
Heuser, C. W.
1976. Juvenility and rooting cofactors. In: Symposium on juvenility in woody
perennials. Acta Hort. 56:251-261.
Magma Jz Si
1967. Variation in the vegetative propagation of Populus in natural populations.
Bull, Ecol. Soc. Am. 46((2)))75-76-
Robinson, J. C., and W. W. Schwabe.
1977. Studies on the regeneration of apple cultivars from root cuttings. II.
Carbohydrate and auxin relations. J. Hort. Sci. 52:221-233.
Schier, G. A.
1973a. Effects of gibberellic acid and an inhibitor of gibberellin action on sucker-
ing from aspen root cuttings. Can. J. For. Res. 3:39-44.
Sehuier, G. A.
1973b. Origin and development of aspen root suckers. Can. J. For. Res. 3:45-53.
Schier, G. A.
1973c. Seasonal variation in sucker production from excised roots of Populus
tremulotdes and the role of endogenous auxin. Can. J. For. Res. 3:459-461.
Schier, G. A.
1974. Vegetative propagation of aspen: clonal variation in suckering from root
cuttings and in rooting of sucker cuttings. Can. J. For. Res. 4:565-567.
Schier, G. A.
1975. Promotion of sucker development on Populus tremulotdes root cutting by an
aNclaUxan Can. hor, Res. 53558-5400).
penaer, G. A., and J. €. Zasada.
1973. Role of carbohydrate reserves in the development of root suckers in
Populus tremulotdes. Can. J. For. Res. 3:243-250.
Snedecor, G. W.
1956. Statistical methods. Iowa State Coll. Press, Ames, p. 253.
Steneker, G. A.
1972. Size and suckering of trembling aspen clones in Manitoba. Ph.D. thesis,
Univ. Mich., Ann Arbor.
Steneker, G. A., and M. A. Walters.
1971. The effect of root length upon the suckering of trembling aspen. Environment
Canada. For. Serv. North. For. Res. Cent. Inf. Rep. A-X-46, 11 p.
shew. R.. K.
1970. Root carbohydrate reserves in vegetative reproduction of aspen. For. Sci.
16:318-320.
Maberneyer. Fo. and TD, J. Phillips:
1970. The control of growth and differentiation in plants. Pergamon Press, New
York. 303 p-
Aiea. Ls
1971. A rapid method for vegetative propagation of aspens and their hybrids.
For. Chron. 47236-3539.

Headquarters for the Intermountain Forest and
Range Experiment Station are in Ogden, Utah.

Field programs and research work units are
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Young University)

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University of Nevada)

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