Plant Responses to Internal and External Signals

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Flowering in day-neutral plants is controlled by plant maturity, not photoperiod.

Slide 54

Critical Night Length

Flowering and other responses to photoperiod are actually controlled by night length, not day length.

Short-day plants are governed by whether the critical night length sets a minimum number of hours of darkness.

Long-day plants are governed by whether the critical night length sets a maximum number of hours of darkness.

Slide 55

Photoperiodic control of flowering

24 hours

Light

Critical

dark period

Flash

of

light

Darkness

(a) Short-day (long-night)

plant

Flash

of

light

(b) Long-day (short-night)

plant

Slide 56

Red light can interrupt the nighttime portion of the photoperiod.

Action spectra and photoreversibility experiments show that phytochrome is the pigment that receives red light.

Slide 57

Reversible effects of red and far-red light on photoperiodic response.

24 hours

R

RFR

RFRR

RFRRFR

Critical dark period

Short-day

(long-night)

plant

Long-day

(short-night)

plant

Slide 58

A Flowering Hormone?

The flowering signal, not yet chemically identified, is called florigen.

Florigen may be a macromolecule governed by the CONSTANS gene.

Slide 59

Experimental evidence for a flowering hormone

24 hours

Graft

Short-day

plant

24 hours

24 hours

Long-day plant

grafted to

short-day plant

Long-day

plant

Slide 60

Plants respond to a wide variety of stimuli other than light

Because of immobility, plants must adjust to a range of environmental circumstances through developmental and physiological mechanisms.

Response to gravity is known as gravitropism.

Roots show positive gravitropism; shoots show negative gravitropism.

Plants may detect gravity by the settling of statoliths, specialized plastids containing dense starch grains.

Slide 61

Positive gravitropism in roots: the statolith hypothesis

Statoliths

20 µm

(b) Statoliths settling

(a) Root gravitropic bending

Slide 62

Mechanical Stimuli