In this article we will discuss about the physiology of adventitious root formation.
The process of development of adventitious roots in stem cuttings can be divided in three stages: the initiation of group of meristematic cells i.e. root initials; the differentiation of these initials into root primordia and the development and emergence of new roots including stem rupturing, formation of vascular connections with the conducting tissues of the stem cuttings.
In herbaceous plants, meristematic cells arise outside and between the vascular bundles. These root initials divide repeatedly to form root primordia. Cell division in each of the primordium continues and root tip appears soon after. In the root primordium a vascular system develops and the latter is connected with the adjacent vascular bundle. Adventitious roots arise endogenously and growth through the stem tissues.
In some instances adventitious roots arise from parenchyma cells of the phloem. In woody perennial plants, adventitious roots arise from secondary phloem tissue, vascular rays, cambium or even pith cells. The time at which root initials develop after cuttings are placed in the medium or bed varies. In some plant species root initials develop when intended cutting is still attached to the parent plant.
These are known as preformed root initials and remain dormant till the cuttings are raised and provided suitable environments. In some species callus develops at the base of the cuttings and roots appear from the callus.
In several plant species roots can arise from the leaves which develop primary or secondary meristems. Sometimes roots are also used as cuttings and produce adventitious shoots. However, regeneration of new plants from root cuttings occurs in different ways and is dependent upon the species.
Physiological Basis of Root Initiation of Cuttings:
Plant growth substances:
Auxins, cytokinins, and gibberellins induce adventitious roots at specific levels. Auxins are extensively used for the formation of adventitious roots in wide variety of species. Several chemical compounds also induce root formation.
Auxins are involved in stem growth, root formation, lateral bud inhibition leaf and fruit abscission, fruit development, carnbial activity and several other processes. The precise mechanism of this hormone is not clear. Several of synthetic auxins are being increasingly applied to plant tissues and these are indolebutyric acid, 2,4-dicholorophenoxyacetic acid, naphthaleneacetic acid, etc.
In nature indole-3-acetic acid is synthesized in apical buds and young leaves. Auxin application promotes rooting in cuttings of several species. Auxin application enhances nucleic acid synthesis in the root initials. It is generally suggested that the physiological basis of adventitious root formation is associated with the actual auxin level in the tissues or a balance between auxin and other plant constituents.
Cytokinins:
Evidences are available that cytokinins are synthesized in the root tips and through xylem exudates move up and control protein synthesis and CO2 metabolism in leaves, enzyme formation, leaf ageing and senescence, shoot elongation, stem elongation, branching of inflorescence, fruit set and release of floral and bud dormancy.
Very recently Chailakhyan and Khryanian (1978) have reported that root system plays an important role in sex expression of Cannabis sativa. The effect was based upon the synthesis and translocation of cytokinins by the roots.
Torrey (1976) has suggested that cytokinins play an important rolein the establishment and maintenance of quiescent centre. Cytokinins change in the root which is season and development stage dependent in the plant.
Gibberellins:
They are reported from the roots of several species. It is still not clear whether they are synthesized in the roots or are translocated in them. There is no direct evidence that endogenous GAs control root elongation. In corn roots application of AMO 1618 reduced root elongation suggesting its role in elongation of roots.
Ethylene:
Ethylene has been shown to stimulate rooting in some cuttings. The exact role of ethylene in root developments in not vivid. In tomato mutant diageotropic is deficient in auxin- induced ethylene biosynthnesis and hence roots of intact plant are insensitive to gravity and fail to form lateral roots.
When seedlings are sprayed with IAA or ethrel, there is formation of lateral roots. ABA; Root extracts of many species have been shown to have ABA which chiefly occurs in the root cap. The effect of ABA on root elongation is variable.
Late Professor K.K. Nanda and his students at Chandigarh studied the physiology of adventitious root formation in several plant species. The results of his investigations are briefly reviewed here.
Classification Based on Rooting Responses:
Out of 155 plant species tried, rooting occurred in 70, on 37 without and in 33 with the application of auxins. The remaining 85 species failed to root. The species were easy, difficult and shy-to-root types.
Causes of Shyness of Cuttings to Root:
The causes varied with the species and were mechanical, inadequacy of nutrition, auxin or some cofactors, accumulation of inhibitors, resins, etc.
Seasonal Changes in Rooting Attributed to Morpho-physiological Factors:
Winter dormancy influenced the rooting in Populus, Ficus infectoria and other sp. Renovation of growth activity allowed rooting as well.
The effectiveness of exogenously applied auxin also changed with the season and was related to bud dormancy. Application of auxins also widened the period during the cuttings root. Rooting responses also varied with the nature and concentrations of auxins.
Polarity and Seasonal Changes in Rooting:
The rate and magnitude of polar transport of auxins varied with the season and may cause seasonal changes in the rooting response of stem cuttings.
Effect of Light on Rooting:
Light also played a determinative role in the formation of adventitious roots. In Phaseolus mungo maintained in the dark or exposed to red or white lights rooted while those exposed to far-red light did not root in water or auxin alone. Far-red light possibly increased the level of carbohydrates and decreased the endogenous auxin level.
GA3 Stimulation of Rooting of Stem Cuttings:
GA3 promoted rooting under LD, ND than under SD conditions in Ipomoea fistulosa.
Morphactins exhibited a broad spectrum of responses. In some species they completely inhibited the de novo roots formation.
Seasonal Changes in Metabolic Parameters during Rooting:
Starch content varied with the different phases of rooting during the annual cycle of growth. The hydrolysis of starch was associated with the activity of hydrolases causing mobilization of reserve food materials.
Rooting responses were also associated with changes in the contents of total, soluble and protein nitrogen.
Multifarious Role of Auxin:
Auxin was involved in cell division, differentiation of cambial initials, enhancement of hydrolases, mobilization of metabolites, increase in nucleic acid and protein synthesis.
Auxin-nutrition Balance for Optimal Rooting:
There was a specific need for a proper balance between nutrition and auxin for rooting.
Starch as a Source of Carbon in Root Formation:
Starch is first hydrolysed and then used in rooting. Besides other carbohydrates are also utilized.
Nucleic Acid and Protein Synthesis:
Rooting required synthesis of proteins and the latter was mediated by the synthesis of messenger of soluble RNAs. IAA probably acted as a trigger at the transcriptional level and nutrition provided a carbon skeleton to regulate translation in the synthesis of proteins required for the differentiation of cambial derivatives into root primordia and their subsequent development.
Auxin also indues the formation of new RNAs and even isoenzymes were associated with root initiation and root development. Cycloheximide when added suppressed some of the inhibitory proteins and enhanced rooting.