Propagation-Past, Present and Future
Charles E. Hess
Department of Horticulture and Forestry, Rutgers University, New Brunswick, N.J.
When man began to select plants for particular horticultural characteristics, it was necessary to develop techniques of reproduction which would bypass the variability introduced by sexual reproduction. Tremendous variability can be seen in seedling populations of many ornamental plants with only a small percentage resembling the parents. Of course we must not overlook the positive benefits of genetic variation since it is the reservoir into which plant breeders may dip for new hybrids. However, once the desired combination of plant form, flower characteristics, hardiness, disease and insect resistance is obtained, then it is the responsibility of the propagator to reproduce the plant in identical units for all to enjoy.
There are several options available. Scions of the desired variety may be grafted onto seedling root stocks, branches may be layered, or cuttings may be taken. Grafting is one of the most precise forms of propagation in that exacting care must be taken in matching stock and scion, both physically and botanically. Also, timing and the maintenance of the environmental conditions during union formation are extremely critical. In addition to the technical problems of grafting there is still the possibility that variability is introduced by the seedling understock. Also, shoots may develop from the understock and overtake the desired variety.
Layering is a very safe method of asexual propagation - it could be called the natural way. The portion of the plant to be propagated is bent in such a way that the branch is under the soil or rooting medium. Little equipment is needed since the plant part being propagated is not severed from the mother plant until a new root system is formed. Water and nutrients are supplied to the layer by the mother plant during the rooting period. Layering is a very satisfactory technique for the home gardener. However, it is usually possible to produce only a few new plants from each mother plant; the procedure requires quite a bit of room, and it may require a year or more for the development of a root system substantial enough to support the new plant.
Propagation by cuttings is a compromise in that some equipment is required for the procedure but the requirements are not as precise as those needed for grafting. Large numbers of plants may be propagated in a relatively small area and the plants will be on their own root system.
In the past 20 years propagation by cuttings has been intensively studied. Substantial gains have been made in the techniques used and in our understanding of the processes taking place within the cutting during root initiation.
When a leafy cutting is severed from the parent plant, one of the first requirements which must be met by the propagator is the control of water loss. The leaves of the cuttings contain stomatas which allow the exchange of oxygen and carbon dioxide, but also facilitate the loss of water vapor. If water loss is unchecked, the cuttings first wilt and then die.
One of the early techniques to control water loss was the use of a bell jar which traps the crater vapor as it evaporates from the cuttings and the medium. The humidity builds up within the bell jar to the point where an equilibrium is reached between water escaping from a leaf and water returning to the leaf. In addition to being a moisture trap, the bell jar is also a heat trap and the units must be protected from exposure to direct sunlight. Contemporary examples of the bell jar principle are an inverted mason jar or a polyethylene bag placed over a clay pot.
Larger scale propagation units utilizing humidity control were constructed to increase the efficiency of operation. The Wardian or grafting case and plastic tents are examples. Some unique approaches have been developed over the years, one of which was particularly designed for Rhododendron cutting propagation. This is the unit known as the Nearing Frame. It is essentially a cold frame with one very important addition. The frame is protected from direct light by building a reflecting surface on the south side of the frame. The only light which reaches the cuttings is indirect light or light which is reflected from an adjacent surface. The Nearing Frame is a very successful, low-maintenance propagating unit which has been used to propagate a wide number of Rhododendron varieties including the more difficult, red flowering types. It has been used with equal success by the ambitious amateur and commercial propagators.
Although grafting cases and particularly plastic tents were and still are very successfully used to propagate Rhododendrons and many other woody ornamentals, the search continued for greater automation and the ability to propagate the particularly difficult-to-root cuttings. In the early 1950's, the major developments in mist propagation were begun. The use of a spray of water over the cuttings provided a new dimension in moisture control. Not only was moisture introduced in the environment by intermittent misting, rather than depending upon the cutting as the principle source of water, but the cooling effect of the water evaporating from the surfaces of the leaves during the "off" mist cycles reduced internal leaf temperatures up to 10°F. The lower leaf temperature reduced the tendency for water vapor to escape from within the leaf and it also reduced the rate of respiration or food utilization by the cutting. Since mist eliminated the need to trap the water vapor around the cuttings, the heat trap was also eliminated and it was possible to place the cuttings in full or nearly full sunlight. The latter factor is particularly advantageous since cuttings exposed to higher light intensity have a greater potential for photosynthesis or food manufacturing. The combination of a reduced respiration and increased photosynthesis greatly enhances the ability of a cutting to initiate a root system.
Other environmental improvements have been introduced over the years. The use of supplemental heating in the rooting medium-bottom heat has been very beneficial for many species of plants. Electric heating cables or hot water pipes incorporated into the surface of the bench are equally effective. Some newer materials that provide a uniform source of heat rather than the point source associated with a heating cable are being introduced. Low voltage heating (16-24 volts) has been used to reduce the potential shock hazard of 110 volt power supplies, particularly when heating is used in combination with mist propagation. The medium itself has also improved. Although sand or sand and peat moss meet the requirements of a rooting medium, there are two major disadvantages. They are weight and lack of uniformity. The particle size and the pH of sand can be particularly variable. Peat moss-perlite combinations are very useful for Rhododendrons. Perlite provides an essentially inert, sterile and lightweight medium which can be purchased in a uniform particle size.
More recent refinements in regulating the environment of the cutting during root initiation has been the introduction of nutrients to the mist system and the use of lights to extend the length of day. The response of cuttings to the latter two treatments varies considerably according to the species and variety used. In general root initiation is not substantially increased by the use of nutrient mist or lighting, but the subsequent growth of the cuttings after root initiation can be greatly enhanced.
The interest which has been devoted to the regulation of the environment surrounding the cuttings has not distracted from investigations to find out what is going on within cuttings. The Dutch propagators used to insert a grain of wheat into the base of difficult-to-root cuttings. In 1935 the probable explanation of the stimulatory effect of the wheat grain was found in the discovery that plant hormones are synthesized in the leaves and buds, and translocated to the base of the cutting where they accumulate and stimulate root initiation. It is believed that the germinating grain in the base of the cutting provided a source of auxin which was absorbed by the cutting. The discovery of indoleacetic acid and its ability to stimulate root initiation led to the introduction of a number of synthetic auxins which are currently used to enhance root formation. The synthetic auxins such as indolebutyric acid and naphthaleneacetic acid are more effective than the natural auxins because they are not readily metabolized or destroyed by the cutting.
Other substances have been supplied to cuttings in combination with the auxins. The most frequently used materials include vitamin B1, boron, and fungicides such as Captan. Although vitamin B1 is essential for root growth, most plants synthesize enough that little or no response is realized from external application. Boron is a minor or trace element which has the ability to partially correct the detrimental effects of an oxygen deficient medium. Both vitamin B1 and boron primarily influence the growth of roots after they have been initiated rather than stimulating the initiation process itself. The fungicides protect the cut surfaces of the cutting which are quite vulnerable to attack by plant pathogens. In addition, some of the fungicides stimulate root initiation directly, particularly when they are used in combination with an auxin.
Even though we have made significant strides in regulating the environment used for cutting propagation and we know some of the internal factors which regulate root initiation, there are still many plants which are considered impossible to root from cuttings. One explanation is that difficult-to-root cuttings contain a ring of fibers which block the emergence of the root primordia. It is true that a correlation can be established with the continuity of a fiber ring surrounding the phloem of a cutting and the degree of difficulty of rooting. Wounding the stem of a cutting is a technique used to overcome this problem. However, there are other plants which do not have fiber rings, and do not respond to auxin treatments, or the best environmental conditions available. We believe that there are two areas in the biochemistry of root initiation that are still not completely understood. One area is the mobile components involved in root initiation. The mobile component is made up of substances essential for root initiation synthesized in the leaves and buds and translocated to the base of the cutting by the phloem and cortical tissues. Included are the auxins, sugars, amino acids, vitamins and a group of substances we have called the rooting cofactors. The rooting cofactors are made up of phenolic, terpenoid and two unknown compounds which react synergistically with auxin in stimulating root initiation. Plants which are easy-to-root contain more of these substances than do difficult-to-root-cuttings. For example the easily-rooted clone, Rhododendron 'Cunningham's White' contained all four of the rooting cofactors in good supply whereas the difficult-to-root clone, 'Dr. Dresselhuys', contained only two of the cofactors. We must complete the identification of the rooting cofactors and determine their mode of action.
The second area which is not completely resolved is a non-mobile component involved in root initiation. Evidence for a non-mobile component in root initiation may be found in reciprocal bark grafts between easy-and difficult-to-root cuttings. The easy-to-root bark roots well when grafted on a difficult-to-root cutting. However, the rooting capacity of barks from a difficult-to-root cutting is not enhanced when grafted on an easy-to-root cutting. If only a mobile component was regulating root initiation, it would have been expected that the bark from the difficult-to-root cutting would root when placed on the easy-to-root cutting. We now believe that there are certain proteins or enzyme systems lacking in the difficult-to-root tissues and their absence limits the ability of the cutting to root. Although all the raw materials may be present at the base of the cutting, the enzymes are essential to catalyze the synthesis of the substances needed for root initiation. Research is currently underway at Rutgers University to isolate and identify the enzyme systems which are involved in root initiation.
One of the most recent developments in asexual propagation and one which will have an influence or future techniques is that of meristem or apex culture under aseptic conditions. Precise control of the physical environment is possible including the supply of nutrients, vitamins, and growth regulators in the medium. The tissues used are highly meristematic and their differentiation can sometimes be modified by altering the content of the medium. In the case of the orchid, the one plant most successfully propagated by meristem culture, one bud may be multiplied into many thousands of plants in a six-month period. In the case of tobacco, a complete plant has been grown from a pollen grain and a single cell has been used to grow an entire carrot plant.
These experiments have stimulated research in many laboratories to utilize the meristem techniques to propagate ornamental plants. A number of problems have been encountered. The major difficulty is the development of a media which will support the growth and controlled differentiation of the tissues. Each species and variety of plant appears to have its own particular set of requirements. Secondly, there are opportunities for the introduction of genetic variability. Most frequently the variation is in the form of polyploid tissues, or tissues containing more than the normal, diploid, number of chromosomes. However, in spite of the problems, meristem culture is already being used commercially for orchid production and in the development of disease resistant clones of asparagus. The potential is tremendous for the production of disease-free plants under precisely controlled conditions and also for learning some of the biochemical and physiological processes involved in the differentiation of unorganized cells into complete organisms.