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Bleached areas on the leaves: III (Responses)

G.Merkouropoulos bsp541 at bangor.ac.uk
Wed Mar 1 12:40:36 EST 2000


About ten days ago I send two messages to this archive asking whether
there are any ideas about the bleached areas I observe on the periphery
of the leaves of the Arabidopsis plants I grow.
The replies I have collected are the following:

Maybe its nutrient stress of some sort, deficiency or toxicity.
Is your room really windy?  Excessive airflow can burn off the edges of
arabidopsis leaves.
I don't know what your husbandry conditions are, but we've found that
Arabidopsis is susceptable to photo-bleaching when grown in light levels
much in excess of about 300 microEinsteins, and particularly when grown
under constant illumination rather than light/dark cycles. You might
check your light levels.
If you are talking about dry, dead, white patches (as opposed to
otherwise healthy-looking tissue like in an albino sectoring mutant), it
may be thrips.  These are very hard to find unless you look very closely
(and they move, so they won't be where the white patches are).
Another possibility--when the Weed Science class used a colleague's
watering cans to water with Liberty herbicide, her plants showed bright,
white sectoring.  Is anyone else using your watering cans?
Temperatures might be too high. We see temperature stress above 23 C.
We found that the edges of some fully expanded leaves became
yellow when they touched the surface of the soil especially if the soil
was too wet.  As you are growing your plants in propagators or
plastic bags I imagine the humidity is very high and there is
condensation on some of the leaves.
We grow our plants in disposable, square plastic pots where each
pot is approx 4 x 4 cm.  The compost was Fisons M3 and are the
pots are watered from below using capillary matting.  The pots are
just the right size so that as the leaves expand they are supported
by the edge of the pot and don't come into contact with the soil.  We
grow our plants under short days so the leaves can get quite large.
The problem with yellowing became very marked when we grew
plants in larger pots as the tips of the mature leaves would curl
down.  Smaller pots also gave problems as the plants became too
large given the fairly extended growth period (6 weeks)
Sounds like your conditions are fine. The only other possibility that
occurs to me is that it may just be a natural part of the lifecycle of
the ecotype you're using. I've been taking a look at nitrogen allocation
patterns in Arabidopsis in a sort of tangent to my Ph.D. research (so
the data aren't as clean as they would be if they were collected for
this purpose) and one interpretation of what I see is that once the
inflorescence starts growing, it becomes the primary nitrogen sink in
the plant--to the point that the rosette appears to decrease its
nitrogen content. I doubt if the plant is actually deallocating
resources away from the rosette, but it's possible that the
inflorescence sink is so strong that even the nitrogen that would be
spent on tissue maintenance in the rosette is being drawn into the
infloresence. Thus, the bleaching may be the result of normal protein
degredation in the rosette not being repaired once the infloresence gets
1.  Arabidopsis is essentially a shade plant, and whilst 170 microE
is low, it may still be high enough to cause problems, particularly
if coupled with another stress.
2.  If the spots you describe are randomly, you may be seeing
nutrient deficiency symptoms brought on by low soil oxygen.  Are you
keeping the soil too moist, and hence anaerobic?  Try adding
vermiculite to your compost mixture.
3.  Are you certain you aren't just seeing signs of senescence of
rosette leaves after the onset of flowering?  Senescence in rosette
leaves follows a clear pattern, and in Arabidopsis, runs from tip to
base, and the region around the main veins remains green longest.

Thanks again to all of you who replied to my query.

G. Merkouropoulos
bsp541 at bangor.ac.uk


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