Big Callus--No Roots. Small Callus--Roots
I've been trying to root cuttings of Passiflora membranacea without success for a while. Typically a large callus forms at the cut end, but even after months no roots develop. This species is known to be difficult to root.
I just had my first success with a one month old cutting. In this case little obvious callus tissue has formed. I have seen similar behavior with a distantly related and much easier to root Passionflower, P. x exoniensis x parritae. The cuttings that develop a large callus are delayed in their rooting; those that never form a large callus quickly root. The Passiflora species I've rooted develop their roots above the callus, from what I remember.
So my question is is there anything I can do to inhibit the formation of a big callus and promote root growth? Can rooting hormone promote callus formation at the expense of roots?
It's possible that the rooting hormone I'm using could be part of the problem. It's an ancient bottle of Rootone with 0.2% NAA, perhaps it's largely degraded by now. I've recently gotten IBA at several concentration but have not tried those with these cuttings. I would emphasize, though that formation of the large callus is observed by others who have tried to root these cuttings--I don't think the NAA is the (only) problem. I've given the cuttings to several others who are generally very successful rooting cuttings, including Passifloras, and they have also failed.
I have tried to damage the callus and all of that with no success. With and without bottom heat, different media, etc.