What REALLY causes root death?

[ScottMcC]

I'm making a foray into the advanced forum briefly.

I got to thinking this morning about what actually causes roots to die. I've heard a lot of conflicting stories, some of which likely old wives' tales, some of which backed by science, and some of uncertain veracity.

Anyway, the most popular story is overwatering leads to rot. Some would clarify and say that keeping the roots too wet, without allowing them to dry is the culprit.

Others contend that roots need access to good air flow, and that overwatering only causes problems if it blocks air flow (and hence cellular respiration).

Some people say that a certain percentage of roots are going to die off regardless, and that this is a natural part of the plant's growth cycle.

Anyway, here's what got me thinking. When transplanting into semihydro, one of the main tenets is to do so during a time of active root growth, because only actively growing roots are able to adapt to that environment instead of standard culture. I've noticed that especially with cattleyas, a good portion of the old roots are going to get soggy and die. This typically isn't a problem though, because the new roots take over pretty quickly.

Based on this observation of the old roots dying, I've come up with a new hypothesis that someone should test. Roots can die from overwatering, not just because of lack of air flow, but because of edema. This edema is a result of roots which were previously adapted to dry conditions, and so are very efficient at absorbing any available water. Unfortunately, this means that they absorb too much when water is more available, and then become fluid overloaded and unable to transport nutrients and waste products appropriately, and so die. The newer roots, however, adapt to the wetter conditions as they are forming, but as a result need constant moisture or they will dessicate and die. This would also seem to explain why aerial roots should remain aerial, and underground roots should remain underground when repotting.

Like I said, the above explanation makes sense to me, but is as of yet untested. I'd like to hear any comments, and if someone has some actual scientific data on this topic, I'd love to hear it.