Buildup of a acidic thylakoid ΔpH in the presence of excess light resulting the thylakoid lumen activates an enzyme (violaxanthin de‐epoxidase) that converts violaxanthin into zeaxanthin

TLS Online TPP Program

#Id: 4954

#Unit 6. System Physiology – Plant

Type II phytochromes, these phytochromes are also present in etiolated plants. The reason is that the expression of their mRNAs is not significantly changed by light, and the encoded phyB through phyE proteins are more stable in the Pfr form than is PfrA.

TLS Online TPP Program

#Id: 4955

#Unit 6. System Physiology – Plant

Phytochrome is most heavily concentrated in the regions where dramatic developmental changes are occurring: the apical meristems of the epicotyl and root.

TLS Online TPP Program

#Id: 4957

#Unit 6. System Physiology – Plant

very-low-fluence responses (VLFRs), low-fluence responses (LFRs), and high-irradiance responses (HIRs).

TLS Online TPP Program

#Id: 4958

#Unit 6. System Physiology – Plant

Phytochrome responses can be distinguished by the amount of light required to induce them. The amount of light is referred to as the fluence, which is defined as the number of photons impinging on a unit surface area. 

TLS Online TPP Program

#Id: 4959

#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).

TLS Online TPP Program

#Id: 4960

#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).