Agriculture Reference
In-Depth Information
name 'adventitious', meaning 'coming from without, accidental, casual, in an
unusual position' ( Oxford English Dictionary ), is misleading. Such roots are in fact
internal in origin and, far from being exceptional, are the norm for most of
the plant kingdom (Groff and Kaplan,
). Most of the
so-called adventitious roots of apple and pear are more properly described as
shoot-borne roots, although 'adventitious' roots can also be regenerated from
root-pruned seedlings. The capacity to produce such roots is dependent on
two basic characteristics of plant cells: totipotency, i.e. the fact that each living
cell contains the genetic information needed to reconstitute all plant parts and
their functions; and the ability of previously developed, differentiated, cells to
re-differentiate and develop a new growing point.
Shoot-borne roots of apple and pear arise in two ways:
; Harper et al. ,
As pre-formed roots which develop naturally on stems. These generally
lie dormant until they are placed in environmental conditions suitable
for development from primordia into actual roots. They may, however,
develop into swellings called burr-knots and show evidence of root growth
even when still above ground.
As wound roots which develop de novo when a shoot is severed from the
parent plant and used as a cutting. In this case there is a clear sequence
in which previously differentiated cells de-differentiate, root initials form
from de-differentiated, newly meristematic cells near vascular bundles,
these develop into root primordia and these primordia grow outwards
through the stem tissue to emerge while also forming vascular connections
with the vascular tissues of the stem (Hartmann et al. ,
). The term
de-differentiation is used here to describe a specific stage of differentiation
into a different anatomical structure.
There are many features in common between the development of pre-formed
roots and wound roots, but it is simplest to consider them separately.
Pre-formed shoot-borne roots are developed by the nursery practices
of stooling and layering. In these the stems of rootstock cultivars are covered
in soil while still attached to the parent plant and harvested as rooted shoots
suitable for budding or grafting with a scion cultivar. Most of the commercial
apple rootstocks are rooted in this way. They also develop from that part of the
rootstock stem which is buried in the soil when the composite tree is planted in
the orchard and also from any part of the scion trunk which becomes buried.
The new roots which develop from the below-ground rootstock stem in the
orchard are of particular benefit with respect to tree anchorage if the trees are
deep-planted (Rogers and Parry,
): the new roots which may
emerge from scions if these are partially buried are usually undesirable because
such an 'own-rooted' tree loses the specific benefits conveyed by the rootstock.
; Parry,
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