Re: What effect will continuous 24 hour light have on the growth of the radish?

Complete question:

I am doing a project based on photoperiodism for an agroecology class.  I 
can't seem to find much information that explains possible effects on 
plants that are grown under continuous light.  I selected the radish to 
grow, simply because of the fast growing period.  I will be allowing my 
control group to see only regular daylight in regular amounts.  My 
experimental group will be in regular daylight and then under artificial 
grow lights with a max. of 130 Watts.  The light will be continuous for 24 
hours.  I am using icicle radishes and plan to measure the growth based on 
the length and diameter of the radish, as well as taste testing both 
radish group at the end.  I don't know what sources to use to research 
this area or even what to hypothesize regarding my project.  Could you 
please let me know what possible effects the continual light will have on 
the radishes as well as text/information to research from?   I greatly 
appreciate and anticipate your response.  Thanks!!!

Reply:

Radish is considered a qualitative long day plant meaning it absolutely needs a 
photoperiod longer than a critical level in order to flower. 

A college plant physiology text will have a chapter on photoperiodism, and 
there has been a lot of research in the area. Ask your librarian to assist in 
your search for literature. Good sources are Horticultural Abstracts, 
Biological Abstracts, Science Citation Index, and computer databases such as 
BIOSIS and Agricola, the latter by the National Agricultural Library. Tomato is 
harmed by continuous light and gets leaf symptoms. Many indoor plants do not 
seem adversely affected by continuous light.

For strict photoperiodism experiments, a low level of electric light is used to 
extend the photoperiod so there is no significant photosynthesis that may 
affect development and confuse the results. Typically a 75-watt incandescent 
bulb suspended 4 feet above the plants is sufficient.

Having enough electric light to increase photosynthesis is also a valid 
experiment, just not a strict photoperiodism study. For edible crops, such as 
radish, growers want to speed the crop cycle to get a harvestable crop as soon 
as possible.

If possible, you may also want to measure leaf area and plant fresh and dry 
weight. Weight is a good measure of plant growth. 


Reference

Salisbury, F.B. and Ross, C.W. 1985. Plant Physiology. Belmont, CA: Wadsworth.