Though we understand relatively well what controls the maximum height of trees, what controls maximum rooting depth is not well understood. If roots are moving water from their tips to the shoots, then similar constraints should apply to roots as stems. A root cannot go deeper than its xylem can resist the negative pressures of moving water that height. Even if it is belowground, the same physics apply.
But is there another constraint besides this? Is maximum rooting depth determined largely by hydraulics? If moving water was not a limitation, e.g. in wet places, what would constrain rooting depth? Or lateral extent for that matter.
I suspect that phloem and sucrose transport could be just as big a constraint on rooting depth/extent as hydraulics are aboveground. I'm not sure I understand the details on this, though. If a typical plant tried to produce a root 10m long, could it move enough sugar through its phloem to sustain the growth of the root tip as well as intermediate tissue? What about 100m? Some plants can apparently go this deep, but could any plant?
I have to admit, I'm not even sure what to begin measuring here. This summer, we're suppose to start doing root cross-sections on long grass roots, e.g. 2 m long, and look at the anatomical characteristics of xylem and phloem. Maybe this'll start to shed some light on what's hidden belowground.