Growth and Development of Horticultural Crops (1+1)

The Day-length Requirements for Flowering in Three Categories of Plants

• In short day plants the dark period is critical and must be continuous. If this dark period is interrupted even with a brief exposure of red light (660-665 mμ (μm = 10-6 mm ) wavelength), the short day plant will not flower.
  
Short Day Plant

16 hours dark

8 hours light
A. Short Day Plant flowers short day plant flowers

DARK

LIGHT
B.Critical dark period. Short day plant does not flower interrupted by light

DARK

LIGHT
C. Light period interrupted by dark

• Maximum inhibition of flowering with red light occurs at about the middle of critical dark period.
• However, the inhibitory effect of red light can be overcome by a subsequent exposure with far-red light (730-735 mμ (μm = 10-6 mm ) wavelength)
• Interruption of the light period with red light does not have inhibitory effect on flowering in short day plants.
• Prolongation of the continuous dark period initiates early flowering in short day plants.



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Long Day Plants (LDP)

• These plants require a longer day light period (usually 14-16 hours) in a 24 hours cycle for subsequent flowering. Some examples of these plants which are also called as short night plants are Hyoscyamus niger (Henbane), Spinacea (spinach) Beta vulgaris (Sugar beet).
• In long day plants the light period is critical
• A brief exposure in the dark period or the prolongation of the light period stimulates flowering in long day plants.
Day Neutral Plants

• These plants flower in all photoperiods ranging from 5 hours to 24 hours continuous exposure. Some of the examples of these plants are tomato, cotton, sunflower, cucumber and certain varieties of peas and tobacco. During recent years certain intermediate categories of plants have also been recognized.
  
• They are,
Long Short Day Plants

• These are short day plants but must be exposed to long days during early periods of growth for subsequent flowering. Some of the examples of these plants are certain species of Bryophyllum.
Short-Long Day Plants

• These are long day plants but must be exposed to short days during early periods of growth for subsequent flowering. Some of the examples of these plants are certain varieties of wheat (Triticum) and rye (Secale).