Plants capture light through their leaves as an energy source for manufacturing food. The conversion of light to energy-rich food occurs through photosynthesis. Light also informs the plant of the factors that regulate its development and indicate the passage of time periods. Light changes in intensity but also in the predominant part of the light spectrum transmitted in the course of a day. More blue light is present during the day (thus the appearance of blue skies) and more red light at the end of the day (hence red/ orange twilight colors).

Only light that is absorbed by the plant can be active in the processes that create its food. The green color of leaves comes from the family of pigments called chlorophylls that absorb light energy for photosynthesis.

The energy transmitted by light is related to the wavelength and frequency of that light.  Green plants absorb light in some regions of the light spectrum better than others. Plants absorb energy from the the purple - blue, high energy, short wavelength end of the visible light spectrum, which they use primarily (but not exclusively) in vegetative growth.  Plants also use light from the lower energy, long wavelength red end of the light spectrum, which is mostly (but not exclusively) used in the formation of flower and fruit. Both of these light frequencies are needed by plants for different growing functions but the precise effects of too much or too little blue or red light varies with the plant species. 

Courtesy of NYBG Plant Information Service