I sometimes water my mounts by putting them into a rainwater bath for a number of hours, up to overnight. If I don't forget, this has never been a problem. But I have forgotten occasionally, and killed or damaged plants.
Vandas and Schomburgkias don't much mind immersion just over 24 hours. Dendrobiums, Encyclias and Epidendrums have been the most damaged when I forget. The thinner the leaves on the Den, the more damage there has been.
Many orchids us a method called crassulacean acid metabolism (CAM) to collect carbon dioxide (CO2) from the atmosphere. CAM plants open their pores at night, and take in CO2 at night. They store this CO2 as a 4-carbon organic acid. This method of storing carbon dioxide is often abbreviated as C4 metabolism. The CO2 will be converted to sugar the next day, when the sun shines, using standard photosynthesis pathways.
CO2 diffuses through water about as fast as it diffuses through air. CAM plants immersed in water can take in their normal amount of carbon dioxide at night, plus a lot more water than they would get merely by a root watering. A lot of orchids in dewy or rainy climates will have a film of water over the whole plant for the whole night.
The CAM method for fixing CO2 may have evolved before the method used by most terrestrial plants, abbreviated C3. This abbreviation is because most land plants use a 3-carbon organic acid to trap CO2. Getting adequate CO2 is a major problem for water plants, most of which are thought to have evolved before land plants. The CO2 level in the Earth's atmosphere is thought to have been lower when plants were evolving. CAM metabolism seems to be more efficient at low CO2 levels than is C3 metabolism. DNA coding for CAM metabolism enzymes has been found in almost all plants studied to date, even in plants that don't use it.
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