This is great info, TZ! Thank you!

Sarah

"TZ" wrote in message
om...
That CAM plants, and Phals in particular, only open their stomata at
night is probably incorrect.

Tests measuring physiological levels associated with CO2 uptake show
that when the plants are sufficiantly watered CO2 uptake buy the leaf
throughout the afternoon is comperable to that at night. This begins
after the plant has used up all of its stored CO2 stored the night
before by the CAM pathways. The plant basically shifts from CAM to
normal C3. The only way this high rate of CO2 uptake could happen is
if the stomata open again in the afternoon in response to CO2 need
within the leaf. The same tests show a brief shut down when the
lights are turned off followed by the usual CAM nightly high levels of
CO2 uptake.

The Phals only stopped the afternoon CO2 uptake, exhibiting the CAM
nighttime only CO2 uptake, after a week of total chemically induced
water stress (not just a dry pot but equal to a bare root plant that
has not been watered in a week).

This makes sense. CAM has evolved several times as a water
conservation device. Why should a CAM plant be inhibited from maximum
growth when it does not need to conserve water during the wet season?

As for nutrients passing through the stomata....leaves are covered
with micropores that the nutrient ions can pass through, and stomata
are fairly rare on the upper surface of orchid leaves anyway so the
fact that stomata are closed may not have significant bearing on the
amount of foliar uptake.

What is important in any case is that nutrient liquid remain on the
leaf surface for as long as possible before drying, so foliar
fertilization is most efficient on rainy-high humidity days when the
plant does not need misting.


If anyone has more specific/contradictory information from
experimental results your input would be welcomed.