Re: why is yellow food colouring sucked up by flowers more quickly?

One possible explanation would be if the transpiration was greater for the 
flower in yellow food coloring than for the other colors. For example, if the 
flower in the yellow had larger leaf area or was wilted more to begin with or 
was in brighter light or in a draftier spot, it would have a greater 
transpiration rate and absorb the food coloring faster.

Another possible reason might be if there were air blockages in the xylem (water 
conducting tubes) in the stems of the other colors and not in the yellow flower. 
It is best to recut the ends of cut flower stems underwater to remove any air 
blockages. When recutting underwater remove about 2 cm of the stem. 

What you might want to do is repeat the experiment only combine equal volumes of 
three different food colorings (blue, red and yellow) and have a race to 
determine which one reachs the flower first. This experiment would eliminate any 
of the possibilities in the first two paragraphs assuming the food colorings do 
not interact with each other and precipitate. If the flower turns yellow first, 
then what would be happening is that the blue and red colorings were slowed down 
because they were interacting with the cell walls of the xylem. This is the 
principle of chromatography which is used to separate different molecules. Leaf 
pigments (green chlorophylls, orange carotenoids and yellow xanthophylls) are 
often separated by paper chromatography. Paper is made of xylem cell walls.


Reference

Witham, F.H., Blaydes, D.F. and Devlin, R.M. 1971. Experiments in Plant 
Physiology. New York: Van Nostrand Reinhold. (See Chapter 14 on paper 
chromatography of leaf pigments)