Biology of adventitious root formation on cuttings The

Biology of adventitious root formation on cuttings

• The biology of what actually triggers adventitious root formation is largely unknown… • But here’s what we do know…

Adventitious root formation • Roots • Arise from tissue other than existing roots (e. g. stems, leaves) • Absolutely necessary for cutting propagation to work

Two types of adventitious roots Preformed (latent) Wound induced • Root primordia are preformed but lie dormant • Emerge in response to environmental conditions • Easy to root species • Develop only after the cutting is taken • In response to wounding • De novo = “anew” • Direct – cells in close proximity to the vascular system (easy-to-root taxa) • Indirect – from callus (difficult-to-root taxa) – – Salix (Willow) Hydrangea Populus (Poplar) Ribes (Currant)

Emerging preformed root initials of Hedera helix

Preformed adventitious roots on corn

Preformed adventitious roots on Kalanchoe panamensis

Aerial (preformed) roots on Ficus

Two types of adventitious roots Preformed (latent) Wound induced • Root primordia are preformed but lie dormant • Emerge in response to environmental conditions • Easy to root species • Develop only after the cutting is taken • In response to wounding • De novo = “anew” • Direct – cells in close proximity to the vascular system (easy-to-root taxa) • Indirect – from callus (difficult-to-root taxa) – – Salix (Willow) Hydrangea Populus (Poplar) Ribes (Currant)

Response to wounding 1. Outer cells die and form a necrotic plate, wound is sealed with suberin, xylem plugs 2. Living cells behind the plate begin to divide and form callus (parenchyma cells) 3. Some cells near the vascular cambium and phloem begin to divide and initiate new adventitious roots (where exactly is still unclear!)

Callus

Response to wounding 1. Outer cells die and form a necrotic plate, wound is sealed with suberin, xylem plugs 2. Living cells behind the plate begin to divide and form callus (parenchyma cells) 3. Some cells near the vascular cambium and phloem begin to divide and initiate new adventitious roots (where exactly is still unclear!)

Direct formation of wound induced roots 1. Dedifferentiation of parenchyma cells 2. Formation of root initials from cells near vascular tissue (meristematic) 3. Formation of root primordia (more organized) 4. Growth and emergence of the new root, connect to existing vascular tissue

Root initials (slightly organized) Root initial

Direct formation of wound induced roots 1. Dedifferentiation of parenchyma cells 2. Formation of root initials from cells near vascular tissue (meristematic) 3. Formation of root primordia (more organized) 4. Growth and emergence of the new root, connect to existing vascular tissue

Root primordia (more organized) Root cap Meristematic cells

Direct formation of wound induced roots 1. Dedifferentiation of parenchyma cells 2. Formation of root initials from cells near vascular tissue (meristematic) 3. Formation of root primordia (more organized) 4. Growth and emergence of the new root, connect to existing vascular tissue

Growth and emergence of new root Root initial

Indirect formation of wound induced roots • Callus formation (non-directed cell division; parenchyma) • Cell differentiation • Root initials • Root primordia • New roots and vascular connections

An important difference Leaf and root cuttings • Have to generate adventitious roots and adventitious shoots Stem and leaf bud cuttings • Have to generate adventitious roots only

Adventitious shoot formation • Arise from any plant part other than terminal, lateral or latent buds on stems • Kalanchoe panamensis

Rubber plant • Adventitious shoot formation is the limiting factor!

References • Principles of Propagation by Cuttings, Dr. Fred Davies, Department of Horticultural Sciences, Texas A&M University, College Station, Texas • Hartman, H. T. and D. E. Kester. 1997. Plant propagation: principles and practices. Sixth ed. Prentice Hall, N. Y.

A word about auxin • Auxins will only speed up and enhance rooting of cuttings. • Difficult to root species or cuttings taken from physiologically mature stock plants may not respond to auxin.

Proximal and distal Distal Proximal

Cone of juvenility • Adult = reproductive Chronologically “newer” but physiologically “older” Juvenile = vegetative Chronologically “older” but Physiologically “young”

Hardwood Cuttings