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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. |
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