plants turning purple ??? growing in pots in green house

[bulspider]

last year was my first year growing in a green house ..I started every thing in pots they all did great till they started to bloom , I had tomatoes,melons,cucumbers and a few other plants , the bottom leaves started to turn purple and slowly worked its way up the plant and there growth stunted can some one please help ???

[serial_killer]

First guess is cold, what are the air temps and what is the temp of the water you give them?