Orchid Stomata, or how and why a plant can hold its breath all day

Q. What do orchids have in common with cacti and other arid climate plants? 
A. Their stomata and a special pattern of photosynthesis. (Well yes, they also are both grown by eccentric people, but that is the wrong answer and I am grading this exam.)

Most plants open their breathing pores (stomata, singular stoma) in the day and close them at night, but cacti and most orchids and a few other groups of plants open them at night and close them during the day. This night-open pattern is coupled with a biochemical pathway that allows these plants to use carbon dioxide in a special way. Most plants open their stomata during the day to enhance delivery of carbon dioxide to the photosynthetic chloroplasts. The "fixation" of carbon dioxide from the air using light is one of the defining adaptations of plants - the carbon in their stems, roots, leaves, and flowers all comes from carbon dioxide in the air. Our carbon does too, indirectly via plants. Okay, that makes sense - plants can only perform photosynthesis when there is light, so they open stomata to let carbon dioxide into the leaf when there is light and they close them up to conserve water at night, when they can't use the carbon dioxide anyway.

So how can orchids and cacti fix carbon dioxide? They hold their breath all day! Or more precisely, they only open their stomata when it is dark! First clearly demonstrated by Aubert in 1892 with many details worked out later by others, the key is that these plants open their stomata at night and store the carbon dioxide as the small molecule malate, then convert it back to carbon dioxide during the day. (Malate, by the way, is tart tasting and is abundant in many fruits including apples - genus Malus, from Latin for apple. The compound is named for apples.) Photosynthesis then uses the released carbon dioxide in the same way as nearly all plants do (via a cycle called C3 photosynthesis). The nighttime storage consumes energy, but the daytime photosynthesis captures more energy (from light) so the energetic balance is positive. For reasons that are more confusing than enlightening, this is called crassulacean acid metabolism or CAM. Short version: orchids take a great big breath at night and hold their breath all day.

Cacti and many other dry adapted plants use CAM because it allows them to keep their stomata closed during the hot daytime. This conserves water, which is inadvertently lost through the open stomata, much as we lose large amounts of water breathing. Cool night air lets them acquire carbon dioxide but lose little water.

Water vapor condensing from exhaled breath.

And why do orchids do the same, including ones that grow in wet cloud forests? Well, it is probably for the same reason - to conserve water. You should be crying foul about now: but my Masdevallia plant grows naturally in 90% humidity with daily rainfall! Despite this, epiphytes are often water challenged (though not as much as cacti) because they lack the large underground root systems that provide abundant water to most plants. Indeed, not only orchids but most other epiphytes, such as Bromeliads, use CAM. This water challenge also contributes to the fact that so many orchids have waxy thickened leaves or pseudobulbs. You may have noticed that a lot of orchids look like succulents, despite growing in wet areas.

And there you have it - cacti and orchids, sisters under the skin. And a blog post about orchids with human breath as the only picture. I am very proud of that.