Subject: The onset of plant water stress......

From: Joey Norikane <[email-masked]>
Date: Mon, 29 Apr 1996 18:49:38 -0700

I am curious about the onset of plant water stress. i.e., when 
the plant's transpiration rate exceeds the rate of water uptake
>from  the roots.  Under such conditions, I understand that the
plant will continue to transpire, by using some of the water
stored in its cells (cytoplasm) and by reducing stomata openings.

My question is, the initial expenditures of cytoplasm come from
which plant cells?  For a C3 plant, are the cells closest to the
stomates the first to relinquish cytoplasm?  this would also mean
that the cells on the bottomm side of the leaf, right?

I would appreciate any insight into this question.  Thanks in advance.

Sincerely,
Joey Norikane

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From: [email-masked] (Bob Vickery)
Date: Wed, 01 May 1996 22:51:04 +1000

In article <[email-masked]>,

>I would appreciate any insight into this question.  Thanks in advance.

I vaguely remember some work done, perhaps 15 years ago, by G. Farquar and
others, that seemed to show that the average water molecule travels about
half the leaf's thickness before escaping out a stomate.  That would mean
that most water comes from somewhere near the middle of the leaf.  The
cells nearest to the stomates are covered by a layer of waterproof cuticle
and don't transpire all that much.  
In any case, the cells are in good contact with each other and water moves
freely between them.  There would not be much difference between them in
the amount they shrink when transpiration starts.
Some plants have all the stomates on the bottom sides of leaves, lots of
them have stomates on both surfaces, some water plants have them only on
the top surface, some don't have clearly defined top and bottom surfaces.

Bob Vickery
[email-masked]
[email-masked]