Plant growth regulating chemicals are nowadays extensively applied on grapevines in India to produce marketable quality berries and enhance the yield of the crop.
Among other regulators, gibberellic acid, chloro-phenoxy acetic acids such as 4-CPA, 2,3,4-T, growth-retardants, viz., CCC, Phosfon-D, ALAR, and Ethephon have been on use to a large extent. It may be stated that grape berries treated with these chemicals produce harmful effects in the body whether they are consumed as fresh, or made into products like raisin or wine. Toxicity is higher with higher concentrations of the chemicals used and particularly when they are applied shortly before harvest. Of late, there is, however, a preference in the distilleries to use organically grown grape berries which are grown without the use of chemicals.
In this connection, it may be brought to notice that where the vegetative growth of the vines is satisfactory, some specialized horticultural techniques prove to be highly effective in augmenting the quality and productivity of the crop. The four techniques that have greater effectiveness have been described in the following.
(1). Thinning:
In many grapevines, over-production of bunches is observed to be an occurrence. This seems to be desirable in making the increase in yield but that does not happen. On the contrary, such a bearing badly hampers yield and also marketable quality of the berries. The adverse effects of such over-loaded vines are delay in maturity of the berries, their improper color development, uneven ripening, lowered soluble solids content, higher production of shot berries, and premature drop.
Hence, in such a situation, the cropping intensity is needed to be regulated by disallowing super-abundant berries to develop in a vine. This could be brought about by maintaining a reasonable number of flowers to develop in a vine with the removal of the rest. Such a specialized form of pruning is termed thinning. Removal of the parts may be done manually by clipping with pliers, secateurs, or a pruner. The advantage of thinning lies in that, the flowers and fruits that are left unpruned in a vine and allowed to grow do not face competition in respect of air, light, space, moisture, and nourishment as would happen if all these were left as such without clipping, i.e., without thinning them. Thus, intelligently done thinning operation is a contrivance to step up the quality of the berries and also enhances yield behavior.
There are in fact, three types of thinning operations that are practiced in grapevines as stated below and each type has some specific purpose of its own. These have been described below.
(1) Flower-cluster thinning:
In this type of thinning, the desired number of flower clusters are clipped off soon after they are borne in the vine. Clipping should be done scrupulously taking care that under-developed or misshapen clusters are not left unpruned. The direct benefit of this form of thinning is a high set of fruits. The method is, however, not very suitable for varieties that produce compact clusters. Drastic pruning should also be avoided in grape varieties that set poorly, such as in Thompson Seedless, Gold, etc.
Cluster thinning:
In this thinning, entire flower clusters are removed and this should be done just before the berries have set. The method particularly centers attention on the grading and sorting of flower clusters and can be performed easily. This type of thinning is considered by many viticulturists as the most useful of the three types. The method is largely practiced in many countries for the production of raisins and wine grapes.
Berry thinning:
This type of thinning is done by removing only parts of clusters and not the whole of them. Clipping of the berries should be done when they have not sufficiently grown. The end of the main stem and several branches of the clusters are pruned in this method. Among other benefits, this type of thinning greatly increases the size of the berries, advances their maturity, and markedly improves their quality. The practice is much suited to grape varieties that produce highly compact forms or very large-sized clusters.
(2). Girdling (Phloem Block):
In the body of the higher plants, water, minerals etc. are taken up by roots from the soil and these are translocated to the leaves through the xylem vessels. From these materials and the gases that are taken up from the atmosphere, complex organic food, and other compounds are synthesized in the leaves. These elaborated materials in the leaves are then sent downwards through the phloem vessels. Now, if their downward movement is somehow arrested on some part of the stem, i.e., by making a block in the phloem vessel, the materials would not be able to cross the blockade and would go on accumulating above the arrested part, i.e., the blocked part. Accordingly, the part would be sufficiently enriched day by day with the accumulation of these elaborated materials.
Such enrichment of the part makes it invigorated and for this reason, flower induction, fruit set, and fruit development will take place above the girdled part, and for this, fruit drop will also be checked. This principle is adopted in the technique, known as girdling, or, phloem-block. This is described below.
It should be noted that by this technique, although the continuity of the phloem is arrested continuity of the xylem is not arrested. Hence, the upward conduction of the materials is not hampered by this technique.
Mode of operation:
Two circular cuts are at first given completely around the part of the stem where girdling is to be performed. This can be done with an ordinary knife and two parallel cuts should be given maintaining a space of 1 – 1.5 cm.
However, a specially made double-bladed knife could be used. With such a knife, the two circular cuts could be given in one stroke. Such a knife can be easily prepared by parallelly fixing two replaceable sharp razor blades with screws 1–1.5 cm apart in the grooves which should be made on a small handy wooden block. On giving the cuts with any of the knives, several longitudinal incisions are to be given side by side between these two ends in such a way that each of the longitudinal cuts starts from the upper circular cut and extends downward up to the cut at the lower end. After this, the piece of bark between the transverse cuts is removed by pulling out one by one with the blunt end of the knife. In this way, when all the bark pieces are removed, a complete removal of bark will occur from the part of the stem.
Girdling is normally done on the stem or the matured cane but many viticulturists in the U.S.A. perform this on the trunk itself with the idea that it would affect the whole vine uniformly and would also keep it small.
Precautions:
The following considerations should be taken into account in carrying out the girdling operation. (i) The practice is not warrantable unless the vine has made copious vegetative growth because, in insufficient vigor, this may devitalize it. (ii) Too many girdles done on the stem parts on the same vine and in the same season may cause them to dry up. (iii) Making girdles on the same vine every year may hamper its growth. (iv) Wounding that is caused by girdling may lead to disease infection and hence, fungicidal paste should be applied on the cut surface.
(3). Ringing:
The ringing technique consists of entwining the selected part of the stem with a thin galvanized iron or metallic ring of 1–2 mm diameter twice or thrice at the same position. After that, it is twisted with a wrench to make the ring very tight, and then both ends of the wire are cut off with pliers. Thus, it is looked that a metallic ring is tightly fixed on the same part. As the stem increases in diameter with growth, it exerts pressure on the ring and thus, a circular cut is formed. The ringing technique is sometimes wrongly termed girdling.
The ringing technique is not as efficient as girdling. By ringing, phloem continuity is disrupted for a short period only because the healing process takes place readily and makes it joined. But some injury caused by this technique helps in flowering, fruit set, and development.
(4). Notching
The notching technique consists of cutting out only a small piece from a mature stem part, such that a V–shaped notch cut is formed on it and a wound is formed.
By making a wound in this way on a vine stem, a mechanical shock is produced on it. Such a shock acts in releasing some stimulus in the vine and causes flowering. The explicit usefulness of the technique is, however, not much clear and it is not much used in the vines. Too many notches may also cause breakage of the stem. There is also the chance of disease infection through the notch cut. The method is not recommended unless the vegetative growth of the vine is prolific.
Dr. Bibhas Chandra Manumdar
M.SC Agri, PH.D
Prtofessor in Horticulture institute of Agriculture Sciences
Calcutta University
Kolkata, West Bengal – 700029
Mob- 09830272846
Email.- profbcmazumdar@yahoo.com
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