Dear ECOLOGGERS: One of my intro biology students posed a question for me last week that I cannot answer. Can you help? I am doing a unit on plant physiology, covering how trees take up water from the soil. I have told students that a turgid living plant cell is one where the pressure potential balances the osmotic potential. Thus, a turgid cell will have a water potential of zero. Right so far?
Now, however, I come to how root epidermal and cortical cells create water potentials lower than the soil water potential. Now, I have assumed that they do it by modifying their cytoplasmic solute potential to give a water potential that is lower than that of the soil. Is this right? Or am I deluded, here? If so does this mean that cells in the root endodermis and cortex are flaccid, and not turgid, all the time they are taking up water from a drying soil? If they are turgid, they wouldnt be able to create the required gradient .. And then to move water out of the leaf vascular tissue and into the leaf parenchyma, at least through the symplast, the cells again would have to have a negative water potential, and thus also not be turgid. But arent cells in the plant normally turgid to give the plant structural integrity? Where am I going wrong, here? Is most transport in the apoplast, where transpiration pull can operate? My students question was, how can the root cells have a negative psi to take up water, if they are turgid? How DOES that water get into the root? Lordie, I took Plant Water Relations from Paul Kramer only 35 years ago but this is not what I spend my time thinking about. Any help would be most welcome. Please reply to me, and I will summarize to the list. Thanks. Lyn Loveless
