How do betalain pigments differ from anthocyanin pigments in Caryophyllales?

In the plant order Caryophyllales, the primary distinction between betalain and anthocyanin pigments is that they are mutually exclusive; species within this order possess either one or the other, but never both.1

While most flowering plants rely on anthocyanins to produce colors ranging from red to blue, certain families within the Caryophyllales utilize betalains—a distinct group of pigments comprising betacyanins and betaxanthins—to achieve similar coloration.1

The two pigment types differ fundamentally in their chemical nature and evolutionary distribution:

• Anthocyanins: These are water-soluble flavonoid pigments found in the vacuoles of plant cells. Their color expression is highly sensitive to the acidity or alkalinity of the cell sap.2
• Betalains: These pigments are restricted primarily to the Caryophyllales order and serve as a parallel evolutionary development to anthocyanins. Their presence is considered a significant taxonomic trait that helps define the relationships between various families within the order.1