Abscission Remarkable ability to discard parts organs that

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 Proteins, DNA, starch, etc + aa, glucose aa signal + enzymes

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 Met + IAA - C 2 H 4 + Abscission

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