Abscission Remarkable ability to discard parts/ organs that may be re-made later –Leaves –Fruits –Anthers –Floral whorls after fertilization –Bark Eucalyptus Birch

Abscission program Highly programmed series of events: 1.Mobilization 2.Formation of cork 3.Detachment

1. Mobilization Transport the most valuable materials, esp. N, back to the main body: –Proteins –Chlorophyll –Starch –DNA aa enzymes Proteins, DNA, starch, etc aa, glucose signal + +

2. Cork formation –Under abscission zone: Layers of parenchyma on both sides of abscission zone make and inject suberin and lignin into walls (protective cork).

3. Detachment –At abscission zone: Layers of parenchyma cells secrete cell wall enzymes (glucanases and pectinases); self digest Take up water, swell and weaken, part falls off

C 2 H 4 is the primary internal signal C 2 H 4 causes cells to read genes for: –Amylases, glucanases, pectinases that weaken walls –Glucose and oligos increase – change in osmotic potential causes water to enter –PAL synthesis Directly stimulated by C 2 H 4 or by oligos Makes? –Lignin (hardening & defensive agent) & suberin (waterproofing) of cork cells –Anthocyanin purple  red color in leaves and ripe fruit –Defensive compounds (phytoalexins)

Hormonal controls on abscission IAA can stimulate C 2 H 4 production IAA reduces organ’s sensitivity to abscission signal (C 2 H 4 ) –Cocks abscission ‘gun’ but puts on the safety –If [IAA] declines, abscission zone sensitive to C 2 H 4 ; abscission Abscission Met C2H4C2H4 IAA + - +

Environmental signals for abscission Injury Water stress Aging Shading Competition Flooding Fertilizer ABA increases [C 2 H 4 ]; thus?? CK + affects [IAA]; thus?? Abscission Environ. signals C2H4C2H4 Alter [IAA, ABA, CK] Three tier signal system: