COMPOSITIONAL AND PHYSIOLOGICAL CHANGES ASSOCIATED WITH THE CHEMICAL DEFOLIATION OF COTTON

Abstract

Five technics were employed in 1950 to alter the carbohydrate content of cotton leaves and study the effect upon defoliation. In 1951 expts. were performed at 4 locations, and the mode of action of a uniform defoliant applied to plants grown under widely different cultural and environmental conditions was compared with plants defoliated with 100 ppm. of ethylene. Chemical changes in the carbohydrate and N fractions were followed by analyses at the time of defoliant appln. and thereafter at 24-hr, intervals up to 72 hrs. during defoliation. Warburg respirometer measurements were made during abscission. Little evidence was found for any correlation between leaf blade carbohydrates and percentage defoliation in the 1950 work. There appeared to be more of an interaction between defoliant treatment and leaf composition during the defoliation process. A similarity in the pattern of degreening (chlorophyll loss) of the leaf during natural abscission, defoliant-induced abscission and ethylene-induced abscission is illustrated and discussed. No definite trends or correlations were found between the carbohydrate and N fractions of the leaf blade and the percentage defoliation obtained in 1951. A rapid hydrolysis of the reserve constituents of the leaf blade and the polar transport of their soluble products occurred under the influence of the defoliant. The results with ethylene, although more pronounced, were comparable with those obtained with the defoliant. The respiratory rate of the leaves sprayed with defoliant, when measured either by oxygen uptake or CO2 production, was approx. double that of unsprayed leaves. The R. Q. indicated that carbohydrates were the primary substrates being oxidized under aerobic conditions. The rapid hydrolysis and the rate of depletion of reserve compounds in the leaf blade and petiole brought about by the polar translocation of the mobile compounds as well as by oxidation in respiration, are believed to play a prominent role in chemically induced defoliation.