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Understanding Photoperiodism in Plants: Effects, Control, and Critical Night Length

This guide explores the effects of natural photoperiod variations on plant growth and development. It details how the length of night influences morphogenetic changes, such as internode elongation and leaf enlargement, and the critical night length (CNL) for long and short night plants. Methods for manipulating night length, including the use of light intensity, black cloth, and incandescent lighting, are also discussed. Understanding these factors helps optimize flowering and maximize photosynthesis for better plant health and yield.

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Understanding Photoperiodism in Plants: Effects, Control, and Critical Night Length

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  1. 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

  2. B. Description - plants • Definition • Effect of the length of night on many morphogenetic changes in the plant; • elongation of internodes • enlargement of leaves • flowering

  3. Plant in light Red Pr 660 Pfr 730 Far red Plant in darkness • Characteristics • Low light intensity reaction • red/farred light • short duration (1-4 hrs) at night

  4. C. Critical Night Length - CNL 1. 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

  5. D. Night length Control 1. Lengthening the night • Reduce light intensity to less than 2-3 ft.cds. • Black cloth or plastic • manual • automatic

  6. Time of covering • Goal: • maximum photosynthesis/ photoperiod response • Normal 5 pm to 7 am • Ideal as much high light as possible but satisfy CNL • Problems • Heat build-up

  7. 2. Shortening the night a. 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

  8. 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

  9. Plant in light Red Pr 660 Pfr 730 Far red Plant in darkness

  10. Red Farred Dark

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