Compositional and Physiological Changes Associated with the Chemical Defoliation of Cotton.

In a previous publication (13) evidence was presented in support of the auxin-ethylene balance hypothesis as an explanation for natural leaf abscission. It was found that the two commercial defoliants, Shed-A-Leaf (sodium chlorate-pentaborate) and Endothal (disodium-3,6-endoxohexahydrophalate + ammonium sulphate), accelerated ethylene production in cotton leaves. This observation has since been confirmed for other defoliants by another investigator (16). The present study was designed to investigate the role of ethylene in natural and chemically induced leaf abscission. It has also become of interest to re-investigate the role of the carbohydrate and nitrogen fractions of the leaf in relation to defoliation as well as to determine any physiological or compositional changes in the cotton leaf blade and plant during chemically induced defoliation and, if possible, to assess their importance to the basic mechanism of abscission. The first problem has been the subj ect of two recent papers (5, 9). Evidence has been presented by V. L. HALL (9) that the susceptibility of the cotton plant to defoliation varies inversely with reducing sugars, starch, and dextrin in the leaves, whereas his data showed a direct relation between total nitrogen in the leaf and per cent. leaf drop. On the other hand, EATON and ERGLE (5) have provided extensive data showing that the percentage of bolls shed by cotton plants is not directly related to the carbohydrate and nitrogen status of the plant. DENNY (6) has reviewed the literature up to 1933 concerning compositional changes in leaves prior to leaf fall. Although the bulk of the earlier workers reported an autumnal shift in leaf composition, he (6) concluded from his own work with Viburnum dentatutn and Syringa vulgaris that the dry weight of leaves was nearly constant for the period of sampling (41 days prior to frost); no important changes were observed in sugars or polysaccharides, while nitrogen losses were confirmed only for Viburnum. Abscission records were not kept, and therefore it is not known whether any correlation actually existed between leaf drop and leaf composition. BROWN and ADDICOTT (3) have reported upon the anatomy of leaflet abscission in Phaseolus vulgaris and have noted that starch disappeared rapidly from all tissues except those in the abscission zone, and that there was a polar movement of sucrose when applied to the pulvinal region. The sucrose polymerized to starch in the stem. To the authors' knowledge, how-