Anthocyanins Many tropical species including some Passiflora can develop purple undersides to their leaves. This picture of P. 'Flying V' clearly shows where exposure of the underside of a twisted leaf to direct sun has caused the exposed area to turn purple. In contrast the top surface of the leaf (not pictured) is a uniform shade of dark green. The purple colour may act as a sunscreen (in a similar way our skin produces melanin to protect itself after exposure to light) and protects the leaf chloroblasts (small granules containing chlorophyll which produce energy for the plants) from being damaged by the intensity of the light. The purple undersides (rather than tops) to the leaves may be an adaptation to try to get the best of both worlds, the deep green top surface maximising energy absorption and the purple underside helping the leaf chloroblasts avoid overheating and /or possibly acting like a mirror reflecting light back up to them. Purple growing tips These are observed on many Passiflora e.g. P. actinia, where the leaves emerge purple & folded up before slowly opening & becoming green. Sally Pert advices however that in very hot sunny weather (38°C/100°F) in Spain she does not see the purple tips. So rather than to protect the delicate tips from the sun, perhaps the adaptation is more to make growing tips harder for herbivores to see against a green background and in the very hot weather the plants are simply unable to make the pigments required. Dr. John MacDougal ''I would note that the purple coloration under leaves of shade-loving plants has been shown in many cases to be an accessory pigment to aid photosynthesis in addition to being protective in some ways. Probably Passiflora are similar, at least in the deep shade. The light on the forest floor is primarily green (having been filtered by the green canopy) and the purple can catch this and relay the energy to the green chlorophyll (same reason why deep-sea algae are red). This purple underside can be seen in many tropical shade plants. The red/purple growing tips are a different story, and are more likely radiation protection, as you suggest, and /or anti=herbivore in some way. Incidentally, the latest view is that tricolor vision (adding the ability to see red) evolved in primates not to help find fruit (the old hypothesis), but to find reddish new growth of tropical plants to eat.'' | 
Passiflora pigments - anthocyanins
