Re: Capillary action in flowers with hot water?

You certainly have an interesting observation. I assume that the 150 degree F 
temperature has no effect on the dye itself. You could confirm that by allowing 
the dye solution to cool and then see of it colors the flowers as well as the 
80 degree F dye solution that was never heated.

One possibility is that cells at the base of the stem are being damaged by the 
heat and water is not moving up the stem to the flower. It seems likely the hot 
water would cool before it gets to the flowers. Do the flowers in hot water 
wilt after the treatment? If so, that would indicate the stem base was being 
damaged by heat. If you later place previously heat-treated flowers in room 
temperature dye, does the dye then color the flowers? If not, then that would 
also confirm heat damage to the stem.

Another effect that may be involved is that air solubility in water decreases 
as the temperature increases. This could possibly result in air bubbles forming 
in the flower stem xylem and cause an air blockage to water flow. This is 
termed an air embolism. Cut flower stems are recut under water in order to 
remove any air embolisms.

Another possibility is that the hot dye is being bound or inactivated as it 
passes through the flower stalk, possibly by heat-damaged cells or substances 
that leak out of them. Do you get the same results with all dye colors? Have 
you tried splitting the base of a flower stem and placing half the split stem 
in one temperature dye and the other half in the other temperature dye? 

Technically, you are not dealing with capillary action. Capillary action is not 
really responsible for upward movement of water in intact plants or food 
coloring in cut stems of living plants. Capillary action occurs in initially 
empty, small diameter tubes with both ends open. The empty tube is stood 
upright with the bottom end just under the surface of water, and water moves up 
the empty tube. The smaller the tube diameter, the higher the water rises. 
Unless you cut both the top and bottom of the stem and the plant xylem is empty 
of water to begin with, in which case the plant would most likely be dead, then 
it does not meet the conditions for capillary action. 

What occurs in water and dye uptake by cut flowers is mainly water flow in 
response to a water potential gradient from vase to stem to flower. Dye is 
mainly carried along with the water, although there may also be some diffusion 
of dye. Water moves from higher to lower (more negative) water potential. It is 
recommended to slightly wilt the cut flowers prior to dye application in order 
to speed water uptake by the flower.
 
Reference


Postharvest of Cut Flowers