Formation of Adventitious Roots in
Decaying Trunks of Old Persea
M.
W. Borys
Prof. Dr., Departamento
Fitotecnia, Universidad Autonoma
Chapingo, 56230 Chapingo,
Mexico; visiting professor, College of Agriculture, Poznan-Poland.
Summary
Three cases of adventitious roots formation were
observed inside the old trunks or of the root base showing advanced process of
wood decomposition. Such roots seem to play a salvation role and may be
indicative of easy rooting of individual seedling trees. Conditions favoring etiolation of healing bark, contact with decomposing wood,
continued presence of leaves, and disequilibrium of root/crown components seem
to favor the formation of adventitious roots.
INTRODUCTION
Old solitary trees are seldom sound. They normally
die due to the stem or root base rotting. This process liberates a mechanism of
adventitious roots formation. In turn, this can be regarded as the salvation
process aimed to replace the dying primary root system and to guarantee the
future functional, physiological, and statical
equilibriums between the above-and underground parts of the tree. Description
of cases observed was considered worthwhile because of theoretical and possible
practical implications.
MATERIAL AND METHODS
The data include observations, documented with
photographs of large avocado trees (Persea
RESULTS AND DISCUSSION
The situations of adventitious root presence in the
three cases (Figs. 1, 2, 3) are different, although their initiation and
further development could have common features. Comparing the situation found
in Figure 2 with those in Figures 1 and 3, one can imagine the following
sequence of events.
Figure 2 shows a situation at a relatively early
stage of development. Figure 3 shows a younger phase; Figure 1 represents the
very advanced stage of restoring the lost root system. I had not until this
discovery located a tree with initial stages of adventitious root formation
inside the rotting wood of the trunk.
In the first case (Fig. 1), at the time the
photograph was made (V 1980) there was no wood decomposing and the roots were
already very thick. All roots were deeply rooted in the soil stratum inside the
cavity formed at the trunk/root junction. Ramifications present above the soil
indicate that roots were developing earlier in the substrate, which was
gradually lost. The situation in this particular case represents an advanced
stage of rescue of the root system.
Fig. 1. [Tecamachalco] Adventitious
roots formed at the trunk base. Old, thick, and some thinner roots inside the
rotted trunk can be seen. They reached to soil after the trunk/root base
decomposed. The adventitious roots are branching inside the soil with vigorous
growth in diameter. The roots, although exposed to the diffuse daylight, do not
show any sign of adventitious shoot formation.
The second and third cases (Figs. 2, 3) depict
earlier stages of formation of adventitious roots. The roots are relatively
thin and long (Fig. 2) to very long: 150-170 cm (Fig. 3). The roots were found
at the early stages of wood rotting, when the decomposing matter possibly was
still in contact with the healing bark. The downward movement of the stratum of
decomposing wood inside the trunk was followed by longitudinal growth of the
roots to remain in contact with the litter. The part of the root in contact
with decomposing material was branching. Due to losses in the stratum level,
those branches appeared above the stratum. In all three cases, roots appeared
at the highest levels of the ceiling of the cavity. In Figures 2 and 3, they
give the impression of hanging from the ceiling of the cavity. They could be
classified as vertical roots similar in function to the primary root.
Fig. 2. [Maltrata] Adventitious
roots inside the cavity formed in the decaying wood of a trunk. The bottom of
the cavity was formed at approximately 1.0 m above the ground level. The
branching of roots occurs at the level of decomposing organic matter of the
trunk (pulverized humid, brownish matter with large fragments still compacted).
Some roots lost their contact with the organic matter and died. The roots seem
to follow the lowering layer of trunk wood in the process of decomposition.
Old, solitary trees are seldom healthy. They are
gradually dying due to rotting at the stem/root junction or any point of the
stem or stem/scaffolds junction. In the latter case, the starting point forms
at a wound from a broken or cut scaffold where the healing process is
unfinished. Such a wound, especially when occurring at the trunk base, weakens
the tree's anchorage, and the tree may fall down.
Fig. 3. [Maltrata] Adventitious
roots formed inside the cavity of an old tree. The upper pan of the cavity,
from which almost all roots originated, is located 17 m above the soil level.
All roots are developing in the organic matter and soil inside the former
trunk. The roots were following the process of wood decomposition and removal
or washing away of organic matter during decomposition, finally reaching the
soil surface, the former basal part of a root system.
The formation of wound bark, especially if it
functions for many years, may result in disequilibrium of growth regulators in
the upper part of the wound (the upper part of the cavity) favoring an
increased level of auxins. It is well known from
horticultural practice that auxins applied to a wound
accelerate the healing process. Roots always initiated inside the trunk, in the
ceiling of the cavity in conditions of low light intensity; thus, higher levels
of auxins may be the prime cause of adventitious root
initiation and growth inside rotting trunks.
The lack, or very low level, of light also should be
taken into account as a possible factor favoring rooting. This has been
repeatedly documented (Barrientos Priego,
etal., 1986; Frolich,
1961; Salazar-Garcia and Borys, 1983). The three
cases observed in this study were of the Mexican race which, according to
general opinion, roots more easily than other races. The adventitious roots
were initiated in the healing bark exposed to conditions favoring the etiolation process.
The rooting of cuttings is conditioned by continued
leaf presence (Reuveni and Raviv,
1981). The leaves of the crown could additionally stimulate adventitious roots
formation. On the other hand, the tree of Figure 2, where the roots were
exposed to high sunlight, lacks the formation of adventitious shoots. Such
shoots are freely formed on roots exposed to the light in Crataeguspubescens
(H. B. K.) Steud. as early as in the first year (Borys, 1990). Avocado roots seems
to be very resistant to such morphogenic action of
sun radiation.
Initially, the process of root formation involves
continued contact of healing bark with wet wood in the process of
decomposition. Microbial activity could add some growth regulators to stimulate
further the root development process. From studies of humic
acids, present in decomposing organic matter, it is known that stimulation of
root development takes place (Niklewski, 1948). The
formation of adventitious roots inside decomposing trunks could result from the
interaction of increased levels of auxins and
formation of humic acids that favor nutritional
composition of decayed matter in contact with healing bark.
As shown in Figure 2, the roots are growing in the
tree's own decomposing body. One could expect a possible alelopathic
influence. The formation of roots and the branching of roots in contact with
decaying wood excludes a negative, alelopathic
effect. To the contrary, it suggests a positive influence of a type
experimentally proved by Niklewski and his students
(1948) with root development in other species.
Adventitious roots can be used to reproduce an individual
tree by grafting onto it the scions of the same crown or by making root
cuttings.
Literature Cited
Barrientos-Priego, A., M., W. Borys,
and F. Barrientos-Perez. 1986. Rooting of avocado cuttings (Persea
Borys, M. W. 1990. Alta
capacidad radical del tejocote (Crataegus pubescens H. B. K.) de generar vastagos
adventicios-comentario. Revista Chapingo (in print).
Frolich,
E. F. 1961. Etiolation and the rooting of cuttings. Proc. Int. Plant Prop. Soc.
11: 227-283.
Leal, F. 1966. Enraizamiento
de estacas de aguacate. Agronomia Tropical 16:
141-145.
Niklewski, B. 1948. Nawozenie Biem Polskich. Ksiegarnia Ludowa, Poznan-Poland.
Reuveni, O., M. Raviv. 1981. Importance
of leaf retention to rooting of avocado cuttings. J. Amer. Hortic.
Sci. 106: 127-130.
Salazar-Garcia, S., M. W. Borys. 1983. Clonal propagation
of the avocado through "franqueamiento."