Anatomical aspects of hormonal regulation of abscission in citrus – The shoot‐peduncle abscission zone in the non‐abscising stage

Although mature citrus fruits [Citrus sinensis (L.) Osbeck cv. Shamouti] did not abscise at the peduncle‐shoot abscission zone (AZ–A) when incubated in ethylene environment, abscission processes did occur in a limited number of cell layers situated in the inner bark, the starch sheath region, and in the pith of AZ–A. These processes were regulated by 2,4‐D and ethylene treatments. Cells responding to the “separation processes”, particularly in the ethylene treatment, underwent either (a) cell wall swelling, dissolving and breakdown, or (b) growth and expansion in a radial plane. Further away from the dissolving area, the response of some cells of the mid and outer bark took the form of divisions or growth in a circumferential plane, while other cells remained unchanged. Non‐responding tissues of the outer bark formed a “sleeve” of undissolved cells, and the vascular cylinder produced no abscission in AZ–A. It is concluded that the partial cell wall dissolution in AZ–A explains the increased activity of cellulase and polygalacturonase in the non‐abscising AZ–A of the mature fruit (Greenberg et al. 1975. Physiol. Plant. 37: 1–7).