Plant Propogation and Nursery Management

Introduction

Introduction
Adventitious root formation is an organized developmental process involving discrete biochemical, physiological and histological events in the induction, initiation, development and elongation of root primordia. In general, regeneration of a new plant from a cutting basically depends on two fundamental properties of the plant cell.

a) Totipotency: which states that individual cell contains all the genetic information required for producing a new plant of same kind.
b) Dedifferentiation: which means the capacity of mature cell to return to a meristmatic condition and develop into a new growing point.

In some species, cuttings root very easily as in grapes, while in some species, rooting takes place only after some treatments, where as others do not root at all. Therefore, it becomes mandatory to study the anatomical and physiological basis involved in the process of rooting of cuttings.



Fig. 6.1: Anatomical basis for the type of cuts used in layering

Anatomical basis of rooting of cuttings

• Adventitious roots in stem cuttings of many woody plant species have been reported to originate from various tissues(Fig.6.1). ARF in woody plants generally originate in young secondary phloem, but also arise from vascular rays, cambium or pith (Hartmann and Kester, 1983).
• The origin of adventitious roots has been reported to be callus tissue in difficult-to-root species. Poor rooting in stem cuttings of certain woody species has been correlated with extensive sclerification.
• Thick lignified walls of sclerenchyma tissues were physiological or mechanical barriers to adventitious root formation in poor-to-root species like Fagus, Prunus and Quercus. Generally, four anatomical changes are observed in adventitious root formation.
These anatomical changes are:
• Dedifferentiation of specific mature cells
• Formation of root initials from the dedifferentiated specific mature cells
• Development of root initials into organized root primordia and formation of vascular connection between root primordia and conducting tissues of the cuttings emerging through the cortex and epidermis
• Emergence of roots outside the cuttings
In herbaceous plants, adventitious roots generally appear just outside and between the vascular bundles. In woody plants, one or more layers of xylem and phloem are present and adventitious roots are formed in the stem cutting from the living parenchyma cells. After emergence, the roots develop root cap and other tissues of the root. Adventitious root and shoot usually arise within the stem (endogenously) near vascular tissue, outside the cambium.

Dedifferentiation of specific mature cells: It is also referred as callus formation and rooting process. When the cuttings are placed in a suitable medium, a mass of undifferentiated parenchymatous cells, called as callus, is usually developed at the base of the cutting and only then the root initiation process takes place. There was a belief that callus formation is necessary for rooting of cutting but now it has been established that callusing and root initiation are two independent phenomenons and can occur simultaneously. Though, excess of callus formation may hinder root initiation in some species.
Formation of root initials: Root initials are sometimes developed in the intact stem of certain woody plants, even before cuttings are made from them. These root initials remain dormant in the stem. These dormant root initials are called as preformed or latent root initials. When the cuttings made from such stems are placed in favorable environmental conditions, these root initials become active and roots are developed rapidly and easily from them. Occurrence of root initials is quite common in willow, hydrangea, poplar, jasmine, currant and citron. In some clonal apple and cherry rootstocks and old trees of apple and quince, the preformed root initials show a swelling (outgrowth) on the stem and are more often called as burr knots. Cuttings taken from plants having burr knots usually root better and easily as compared to those having no burr knots.
Stem structure and rooting: It has been found that certain type of stem structures or tissue relationship plays a vital role in adventitious root formation in the cuttings. The development of continuous sclerenchyma ring between the phloem and cortex of the stem is generally considered as an anatomical barrier to the rooting of cutting as in olive and in some leaf cuttings. Similarly, presence of sclerenchyma fibers in the cortex of stem may cause difficulty in the rooting of cuttings in some species as in English Ivy and Hedera helix.