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Relative humidity levels affect a plant’s transpiration levels. Many succulent plants use CAM photosynthesis, which inhibits daylight transpiration, leaving them unaffected by low or high humidity levels. Other succulents use their structure to harvest dew point water.
How Succulent Plants Manage and Use High Humidity
All plants use photosynthesis to convert carbon dioxide into sugar and oxygen, utilizing the sun’s power. Sugar is the starting point for the creation of other molecules and stores energy.
Plants can use three photosynthetic pathways to convert sunlight, water, and CO2 into sugar, with oxygen as a by-product. These are C3, C4, and CAM.
C3 Photosynthesis Pathway.
This is the most common form of photosynthesis in about 95% of plants. The first product is a 3-carbon molecule or 3-phosphoglyceric acid.
C4 Photosynthesis Pathway.
Found mostly in tropical plants in high-humidity regions. The C4 pathway converts sunlight energy into a C4 carbon molecule or oxaloacetate acid, a more efficient pathway than C3.
CAM Photosynthesis
CAM, or Crassulacean acid metabolism, combines the C3 and C4 pathways, storing solar energy during the day and converting it into sugar energy at night.
How CAM Photosynthesis Helps Succulents Survive Extreme Conditions
Plants that use C4 or CAM are more efficient at conserving water in arid environments or environments with high relative humidity. CAM is familiar in succulent and other plants like Aechmea, Agavaceae, Cactaceae, Crassulaceae, Euphorbiaceae, Guzmania, Liliaceae, Neoregelia, Orchidaceae, and Vitaceae.
The stomata can remain closed throughout the day, using the stored carbon dioxide to insulate the plant from its surroundings. CAM is up to ten times more effective at preventing water loss than regular C3 plant respiration.
Most plants use the C3 pathway and are adapted to moderately cooler outdoor air, an average amount of water vapor, and normal light levels. During the day, their stomata are typically open to presenting air conditions.
CAM plants can keep their stomas closed in low humidity (dry air). The organic cycle is fuelled by the internal recycling of stored carbon dioxide fixed at night. The whole process makes me think of bagpipes.
How Relative Humidity Affects Plants
Water is essential in plant metabolism, turgidity, and temperature management. In managing temperatures, a plant transpires, giving off water through its pores (stomata).
Transpiration mainly serves to cool the plant and access CO2 needed for photosynthesis. The process uses osmosis by which water molecules pass through the plant’s stomata from high humidity to an environment with a lower humidity level.
The current relative humidity level is integral to how fast a plant will dehydrate (or not). Let’s first get a better understanding of humidity and relative humility. The best illustration is what happens in a sealed terrarium.
Transpiration in a Sealed Terrarium
A sealed Terrarium offers an excellent opportunity to observe how the interplay between plant transpiration, air temperature, vapor pressure, and dew point recycles water.
Plants in a terrarium transpire as they heat up. If the amount of water vapor in the terrarium is low, the transpiration through its leaves and stem will be rapid, drawing water from the roots via the xylem.
Because the terrarium is wind-free, the transpiration process increases the relative humidity in the leaf’s immediate vicinity. This high relative humidity level slows the transpiration rate, conserving water.
The relative humidity around the leaves is compromised as the temperatures in the terrarium rise because warm air can hold more water vapor than cold air. To compensate, transpiration increases, as does water usage.
If the water reserves dry up, the loss of water vapor from the leaves will cause a drop in turgidity, and the leaves will become flaccid and droop. If there is sufficient water, transpiration will continue until the relative humidity in the terrarium matches the leaf’s absolute humidity.
The evaporating water absorbs energy from the leaves, causing them to cool and slowing transpiration. When the air temperature drops, the water vapor held as humidity will form condensation – initially at the coldest spot.
The colder air causes the vapor pressure to rise, decreasing the air’s absolute humidity capacity and causing the water vapor to condensate. This is known as the dew point of water vapor.
In a sealed terrarium, the dew point transforms gas water vapor into liquid water and rehydrates the roots.
The sealed system allows us to visualize nature’s events in a closed loop, showing the effect that relative humidity has on transpiration and vice versa.
Humidity, Absolute Humidity, and Relative Humidity Measures
Humidity
Humidity is how much water vapor there is in the air. If it is measured, it is either expressed as relative humidity or absolute humidity.
Absolute Humidity
Absolute humidity measures in grams per cubic meter or ounces per cubic yard. It is the maximum water vapor mass a standardized air volume can hold.
Below is a sample of absolute humidity expressed as a 100% relative humidity measure at different temperatures:
| Temperature | oz./cubic yard (g/m3) |
|---|---|
| 122°F (50°C) | 2.24 (83.0) |
| 104°F (40°C) | 1.38 (51.1) |
| 86°F (30°C) | 0.82 (30.4) |
| 68°F (20°C) | 0.47 (17.3) |
| 50°F (10°C) | 0.25 (9.4) |
| 32°F (0°C) | 0.13 (4.8) |
| 14°F (-10°C) | 0.062 (2.3) |
| -4°F (-20°C) | 0.024 (0.9) |
Relative Humidity (RH)
RH is the comparative percentage of actual water vapor in the air compared to its total capacity at a given temperature and air pressure.
Let’s say the temperature is 68°F (20°C), and the absolute humidity is 6.7 g/m3; then the RH would be 50% as the air only holds half of what it could hold at the same temperature.
How Air Temperature Affects Humidity
The 100% above is the maximum amount of water vapor a specific volume of air, say a cubic meter, can hold at different temperatures—it does not mean it’s raining.
You will notice that the warmer air holds much more water vapor than the cooler air.
However, if the temperature drops above 100% saturated air, say from 86 degrees Fahrenheit to fifty, each cubic yard will have 57 ounces of water vapor it cannot accommodate, causing precipitation.
For all the absolute humidity readings above, the relative humidity is 100%.
The water vapor holding capacity triples between freezing and 68°F (0 and 20°C).
Any drop in temperature would trigger the dew point for air that has reached 100% absolute humidity.
The Effect of Dew Point On Succulents
For millennia, cacti have been harvesting water vapor to water themselves. So inspiring is their efficiency at harvesting water; the concept has been proposed to solve water shortages in arid regions – see cactus kirigami.
They do this by harvesting fog, a low water vapor column below the dew point. Nighttime high humidity isn’t surprising in desert regions, as most plants only transpire at night when the air temperature is more relaxed.
Their ribs and spikes protect them from herbivores and harvest water vapor when the dew point air temperature is reached. How much water vapor can they harvest this way?
According to the cactus kirigami study, about 4ml per cm2 per hour translates to about half a gallon per night per cactus.
In Closing
Succulents are unique plants that survive against incredible odds – drought and high relative humidity. I know I harped on about the interaction of plants with the hydrosphere, but mark the page for future reference.
