XV.PHOTOPERIOD

A. Natural Photoperiod– Yearly variation• 9.5 to 14.5 hrs in

KY• 0 to 24 hrs at

North pole– Cause• Tilt of earth axis

B. Description - plants•Definition–Effect of the length of night on

many morphogenetic changes in the plant; • elongation of internodes• enlargement of leaves• flowering

–Characteristics – Low light

intensity reaction

– red/farred light

– short duration (1-4 hrs) at night

Red

Far red

Pr 660 Pfr 730

Plant in light

Plant in darkness

C. Critical Night Length - CNL1.Definition for long night plant - LNP– The minimum number of continuous hours

of darkness at a given temperature that will permit the most rapid flower bud initiation (a period longer than the CNL will cause delay in initiation).

2.CNL is temperature dependent– Number of hrs of continuous darkness

decreases as temperature increases

3.CNL for short night plant–Maximum number of hours of darkness that

permits bud initiation

D. Night length Control 1. Lengthening the night

• Reduce light intensity to less than 2-3 ft.cds.• Black cloth or

plastic–manual–automatic

Time of covering

Goal: maximum

photosynthesis/ photoperiod response

Normal 5 pm to 7 amIdeal as

much high light as possible but satisfy CNL

ProblemsHeat build-up

2.Shortening the nighta. Incandescent light

–Intensity• greater than 10 ft.cds.• 60 w; 4ft space; 2 ft above plants• 100 w; 6ft space; 3 ft above plants

–Time•middle of night• June & July 1 hr• Aug & May 2 hrs• Sept-Oct & Mar-Apr 3 hrs• Nov-Dec & Jan-Feb 4 hrs

2. Shortening the night– b. Cyclic Lighting• 20% of time

– 10:00 - 10:06pm– 10:30 - 10:36– 11:00 - 11:06– 11:30 - 11:36

Red

Far red

Pr 660 Pfr 730

Plant in light

Plant in darkness

Dark

Red

Farred