The Absorption, Translocation and Metabolism of 1-Naphthaleneacetic Acid Applied to Apple Leaves

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Abstract
Summary Four days after placing known amounts of a 20 p.p.m. aqueous solution of 1-naphthaleneacetic acid (NAA) (14C carboxyl-labelled) on the upper surface of detached apple leaves in a greenhouse, approximately 10% of the applied radioactivity was found within the leaves, 10% was still on the surface, and 80% was lost. Apple leaves were found not to decarboxylate NAA, and the disappearance of carbon 14 from the leaf surface is attributed to the breakdown of NAA by ultraviolet radiation. The carboxyl group is lost, leaving a neutral compound, inactive in auxin tests, whose ultraviolet absorption spectrum is similar in general form to that of NAA. When a 50 p.p.m. aqueous solution of NAA containing a non-ionic wetting agent is placed on the surface of an apple leaf held in darkness at 25° C., absorption proceeds at approximately 1% per minute until the surface deposit is dry. Thereafter, absorption continues much more slowly and at a rate depending on the relative humidity of the air and the temperature. In experiments on translocation a 50 p.p.m. solution of radioactive NAA was applied to the basal leaves of fruiting spurs of Miller's Seedling. Over a period of 4 to 5 days, 17 to 28% of the carbon 14 which entered the leaves was translocated to the bourse and fruits, but between one half and three-quarters of this translocated radioactivity was in a form other than free NAA. When radioactive NAA was fed into detached apple leaves through the transpiration stream, 80 to 90% was rapidly converted into a watersoluble addition compound (I), neutral in reaction and lacking auxin activity. Over the course of several days this compound was gradually replaced by a second addition compound (II), which is acidic in nature. Compound II built up more rapidly in Cox’s Orange Pippin than in Bramley’s Seedling. The possible significance of these results is discussed, particularly in relation to the erratic performance of NAA in the field when it is used for the control of fruit drop or as a fruit-thinning agent. 1-NAPHTHALENEACETIC ACID (NAA) is a growth regulator employed in fruit-growing practice for reducing the pre-harvest drop of apples and pears, and as a fruit-thinning agent for apples. One of the chief disadvantages associated with the use of this compound is the variability in response shown by different varieties of apple, and by the same variety when treated under different conditions or in different seasons. These erratic responses make it difficult to predict the results of any particular application of NAA and offer a serious limitation to its use, particularly as a fruitthinning agent. The aim of the experiments described in this paper was to obtain quantitative data relating to the behaviour of NAA when it is applied in aqueous solution to the surface of an apple leaf. By using NAA labelled with carbon 14 in the carboxyl group, the penetration of this compound into the leaf, its translocation from the spur leaves to the bourse, and its metabolism within the tree were studied. Attention was also devoted to the destruction of NAA by ultraviolet radiation, a discovery that was made during the course of the investigation and which, like the other factors studied, was thought to have a bearing on the variable performance of NAA in the field.

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