CALIFORNIA AVOCADO ASSOCIATION 



69 



THE EFFECT OF GIRDLING THE AVOCADO 



J. ELIOT COIT 



Certain varieties of otherwise excellent avocados are losing popularity on 

 account of the habit of not coming into bearing as early and as promptly as is 

 desirable for a commercial fruit. Various growers have attempted to overcome 

 this difficulty by ringing or girdling. I have been asked to collect, digest, 2ind 

 present the results so far secured. 



Scientific Consideration 



In the first place, it is well for us to consider the girdled branch from the 

 standpoint of plant physiology. 



Dr. Jacques Loeb of the Rockefeller Institute for Medical Research in 

 New York City, working on the general problem of regeneration of tissues in 

 animals and plants, has brought to Hght new facts in regard to plant growth 

 which have a wide practical application to the propagation, pruning and girdling 

 of trees. To state the matter in a few words. Dr. Loeb has found out that 

 every growing tip of twig or shoot is continually giving off a certain substance 

 which, for lack of a better name, he calls an "inhibitor," which flows down- 

 ward toward the root. This substance represses, slows up, or prevents the growth 

 of buds on the main stem by which it passes. The path followed by this sub- 

 stance is the bark just outside of the cambium. 



The dormant buds at the leaf bases situated some distance down the stem 

 are prevented from all growing out as soon as they are formed by being contin- 

 ually repressed by this downward stream of inhibitors from the growing tips. 

 If it were not for this, all the buds on a branch would immediately grow out, 

 taking all the sap as it passed by them; there would be nothing left for the tips 

 and growth in height could not occur. This discovery explains why tip buds 

 which are farthest from the root grow the fastest. They are able to grow 

 because they are at the apex, and there is nothing beyond them sending down an 

 inhibiting substance. 



As soon as we accustom ourselves to this new idea of a dovmward stream 

 of growth inhibitors, we are in a position, to understand why, when we girdle 

 an avocado limb, the region immediately back of the girdle gives rise to several 

 vigorous suckers. It is because the dormant buds near the cut are freed from 

 the repressing effect of inhibitors, and inasmuch as the sap channels are large 

 and abundant, the new shoots are extra vigorous. We can now better under- 

 stand why, in top-budding avocado trees it is usually better to girdle the limbs 

 above the buds than to cut them off entirely. The girdle relieves the bud from 

 inhibitors and allows it to spring into normal growth; whereas if we cut the 

 limb off entirely, the bud is sometimes drowned out by the excess of sap. 



The effect of girdling a limb therefore is to permit the growth of suckers 

 immediately behind the girdle which appropriate a part of the raw sap, thus 

 checking the vigor of growth beyond the girdle. At the same time, the elab- 

 orated sap coming from the leaves, transported through the bark, is checked by 

 the girdle and soon reaches a concentration which forces a rapid formation of 

 blossoms and fruits. 



