1. Alkaline conditions produce a vibrant blue and acidic conditions can produce purple, red or pink variations possibly through shifts in sodium and potassium in the plant.(Andrea)
2.The proximate cause of a change from blue to pink flowers is a change in pH, from more alkaline to more acidic.
As for the ultimate cause, I can think of three hypotheses:
1) A change in soil pH as the temperature becomes cooler.
2) A withdrawal of valuable ions from the blue petals (evolved to attract bee pollinators) after the flowers no longer have a function (similar to the withdrawal of valuable nutrients from tree leaves, causing color change, in autumn).
3) An evolved response to attract cool-weather pollinators. When the weather is warm enough for bees to fly, the plant makes its petals more alkaline and therefore blue, the color to which bees are most attracted, and bees are the morning glory's most efficient pollinator. As the weather becomes too cool for insects, the plant makes its petals more acidic and therefore pink, a color that attracts the warm-blooded hummingbirds.(Angelina)
Certainly hypothesis #1) was true for us this year. The phenomenon started with cooler days and evenings.
I like the idea of hypothesis #2) as I always wondered what triggered leaf color change in the fall.
Finally being romantics, Fred (Angelinas brother-in-law) and I prefer hypothesis #3). We both love a good story.
What do you think?