Foliar Abscission in Cotton. I. Effect of Age and Defoliants on the Respiratory Rate of Blade, Petiole, and Tissues of the Abscission Zone

1. Respiratory measurements were made with cotton leaf tissues from untreated and defoliant-treated plants grown under greenhouse and field conditions. 2. The results indicated that leaf blade, petiole, and pulvinoid tissues have different respiratory rates. Different defoliants caused respiratory stimulation or inhibition, depending upon the chemical applied, the time after treatment, and the tissue involved. 3. Both the physiological age of the leaf and the time of sampling during the day significantly affected the rate of respiration, but the differences between age groups were the greater. The respiratory rate decreased with age in leaf blades but increased with age in abscission zones. Abscission-zone tissue was the more reliable for detecting respiratory changes associated with the events of the abscission process. The maximum rate of respiration in the abscission zone coincided with abscission. 4. A detailed comparison of the effects of two defoliants, 3-amino-1,2,4-triazole and disodium-3,6-endoxyhexahydrophthalate, demonstrated that leaf age was an important factor in regulating the rate of defoliation. The rate of leaf fall by age group, as induced by the two defoliants, was closely correlated with the respiratory rate in the abscission zone.