Re: What is the effect of light on plant growth?

Light effects on plants are separated into four main categories: photosynthesis, 
photomorphogenesis, photoperiodism and phototropism.

Photosynthesis is the essential process in which plants convert light energy 
into chemical energy in the form of organic molecules, some of which also serve 
as building blocks for plant structure.

Photomorphogenesis is the effect of light intensity and quality on plant growth 
and development. For example, some seeds require light to germinate. Germination 
of other seeds is prevented by light. Seedlings are etiolated (tall thin  
lacking chlorophyll) when grown without light. Leaf morphology is very different 
in high light and low light resulting in sun leaves and shade leaves. 
Light also has a role in circadian rhythms such as diurnal movements of leaves. 

Photoperiodism is the effect of the relative length of the daily light/dark 
period mainly on flowering. Some plants flower only when the daily light period 
is above a certain length (long day plants), some when the daily light period is 
below a certain length (short day plants), and some flower regardless of the 
length of the daily light period (day neutral plants).

Phototropism is the directional growth of plant parts (leaves, stems, roots) in 
which the direction of bending is determined by the direction of the light 
source.

College intro. botany or plant biology textbooks have basic info on these 
topics. Even more info is found in college plant physiology textbooks.