Growth and Development of Horticultural Crops (1+1)

Mechanism of abscission

• The young leaves remain attached to the stem and do not abscise till they become old. However, if the blade or lamina portion of a young leaf is cut, the debladed petiolar stump soon abscises. In case auxin (IAA) in lanolin paste is applied to the cut end of petiole of such a young leaf the abscission of the petiolar stump is greatly suppressed. The intact young leaf does not abscise because its lamina portion contains auxin synthesized by it. These experiments have led to the belief that auxin has controlling influence in the abscission of leaves. This belief is further strengthened by the fact that endogenous auxin concentration in leaves falls considerably at the time of normal abscission.
  
• In yet another experiment, it has been found that if auxin is applied to the distal side (blade side) of the abscission zone of the debladed petiole of young leaf, the abscission of the petiolar stump is prevented. On the other hand, if the auxin is applied to the proximal side (stem side) of the abscission zone of the debladed petiole, abscission is accelerated.
  
• Normally, the auxin level of the stem side of the abscission zone is probably maintained due to basipetal transport of auxin from the stem tip while the source of the auxin on the blade side of the abscission zone is the blade or lamina of leaf itself. The above-mentioned experiments have led to the establishment of auxin gradient hypothesis according to which it is not the presence or absence of auxin but relative concentration of auxin on stem side of the abscission zone or nearly equal concentration of auxin on both its sides will promote abscission while higher concentration of auxin on the blade side of the abscission zone will retard abscission.
  
• The mechanism by which auxin controls abscission is not clearly understood. Besides auxin, other growth hormones especially ethylene may also play important role in abscission.
  
• Recent researches have now shown that the relative concentration of auxin on two sides of the abscission layer has regulatory influence on the production of ethylene that stimulates leaf abscission. At the time of abscission, concentration of auxin in the laminar region decreases with simultaneous increase in ethylene production. This also increases sensitivity of cells of abscission zone to ethylene which now synthesize cell wall degrading enzymes such as cellulases and pectinases. Activity of these enzymes results in cell wall loosening and cells separation ultimately leading to leaf abscission.