ABA is synthesized in almost all cells that contain chloroplasts

or amyloplasts and has been detected in every major organ and tissue. ABA is a 15-carbon terpenoid, or sesquiterpenoid, which is synthesized in plants by an indirect pathway via 40-carbon carotenoid intermediates

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#Unit 6. System Physiology – Plant

In addition to the fluence, some phytochrome responses are sensitive to the irradiance, 2 or fluence rate, of light. The units of irradiance in terms of photons are moles of quanta per square meter per second (mol m–2 s–1).

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#Unit 6. System Physiology – Plant

Phytochrome responses fall into three major categories based on the amount of light required; very-low-fluence responses (VLFRs), low-fluence responses (LFRs), and high-irradiance responses (HIRs).

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#Id: 4962

#Unit 6. System Physiology – Plant

Induction-Reversion Responses or, Low Fluence (LF) Responses

Examples include:

Light-sensitive seed germination;

Many seeds require light for germination (like lettuce), whereas others are inhibited by light (wild oats, Phacelia, Royal Paulownia). Phytochrome presumably stimulates GA synthesis/release which in turn, stimulates the mobilization of stored reserves

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#Unit 6. System Physiology – Plant

Light-sensitive seed germination;

This provides energy for the germinating seed and also decreases the solute potential for water uptake necessary for providing the force for the radicle to push its way through the seed.

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#Id: 4965

#Unit 6. System Physiology – Plant

Reversal of etiolation

Etiolated plants are those that have been grown in the dark, light stimulates increased [cytokinin] which stimulates cell division and greening.

increases [GA] which presumably stimulates IAA oxidase production to get rid of excess IAA that causes the long spindly growth.

IAA stimulates ethylene synthesis which maintains the apical hook, treating the apical hook with red light + ethylene maintains the hook.

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#Id: 4966

#Unit 6. System Physiology – Plant

Change surface charge

Red light-treated barley root tips adhered to the sides of a beaker (with a negatively charged surface) but released after far red treatment, caused the surface to become positively charged and far red light negatively charged.