Well, water normally passes from root hair cells to cortex to endodermis, and from there to the xylem vessels. All of this occurs down the water potential gradient by apoplastic or symplastic pathway through these cells. I was wondering how water potential differs in each of these cells for water to be transported. BTW I'm confused why my teacher said that the water potential in xylem vessels(the tracheids) is highest among all the cells I mentioned. Then how come water can move down the water potential gradient?

  • $\begingroup$ It's the "higher" when you take only the value, or mathematically speaking the absolute value. Suppose the water potential outside the root is 0 KPa, and inside the tracheid is -10 KPa (I'm making up those numbers). We tend to say that the potential inside the tracheid is higher, but mathematically speaking it's lower. Water will move from exterior (0 KPa) to the tracheid (-10 KPa), going from the higher pressure to the lower pressure, not the other way around. $\endgroup$ – user24284 Jun 5 '16 at 14:19
  • $\begingroup$ So the water is moving from less to more negative water potential, but at the end of the day it will be the tracheids which will have most of the water, right? $\endgroup$ – James Jun 5 '16 at 14:23
  • $\begingroup$ Don't think about "from less to more negative", it's mathematically confusing. Water moves from higher pressure to lower pressure. If the pressure in a pond is -2 KPa and the pressure in the tracheid is -10 MPa, water will move from the pond to the tracheid (-2 is higher than -10). If the pressure in the leaf spongy mesophyll is -50 MPa, water will move from tracheid to the mesophyll. And if the pressure of the air is -100 MPa, water will move from the mesophyll to the air. In the end, water will move from the pond to the air. $\endgroup$ – user24284 Jun 5 '16 at 14:28
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    $\begingroup$ Thank you so much for explaining. It's all clear to me now. $\endgroup$ – James Jun 5 '16 at 14:54
  • $\begingroup$ No worries. And just correcting what I wrote: -2 is higher than -10000 is the correct sentence between the parenthesis, because I compared -2KPa with -10000Kpa (-10MPa). $\endgroup$ – user24284 Jun 5 '16 at 15:08

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