I always assumed Clark Moorten watered the cactus at night because the Botanical Garden was closed. But now I'm thinking he had another big reason to set up his sprinkler on the agave and cactus to water at midnight. He knows about CAM, the crassulacean acid metabolism, responsible for a unique way these plants carry on photosynthesis to limit moisture loss during this process.
Traditional garden plant leaves are covered with stomata (pore-like structures) that open during the day to absorb carbon dioxide, essential to the ongoing single-step photosynthetic process. If a plant is not adequately watered during this process, moisture loss through the open stomates leads to wilt.
Cactus and many agaves evolved adaptation to extreme heat and drought by keeping their stomates closed during the day so there is no moisture loss. The CAM metabolism allows the cactus to open stomata at night to take in carbon dioxide so all gas exchange is completed before when the sun rises. After night time preparation, cactus still need sunlight for photosynthesis, which they carry on all day long using the night stored gasses without opening the stomata.
This knowledge is important to using water efficiently on these plants. When stomata are open at night, these succulent plants are capable of taking some moisture in directly through these openings. Therefore, when Clark waters at night with an overhead system, the plants experience rain-like conditions. This cleans the cactus skin and water penetrates some of the stomata. Because gas exchange is active at night, the roots are better able to take up water applied to the soil.
In one of my reference books is an interesting graph that shows this process in visual form which makes it much easier to understand this two-step process. This graph is an opposite bell shaped curve over 24 hours. It starts at midnight when carbon dioxide of Opuntia cactus is at gas exchange rate of about 25. Promptly at 6:00 AM the graph drops. reaching 0 by about 8:00 AM, then remains at that low level all day while plants are photosynthesizing. Starting about 5:00 PM, gas exchange rises back to 25 by about 6:00PM. Exchange rate remains at this high level all night long.
What this demonstrates is watering as we typically do in the early morning offers only a short time of open stomata benefits before they close with the heat of the day. The optimal time to water is about 10:00 PM after the hot summer ground has had a chance to cool off and gas exchange is fully active. However, potted agaves and cactus can be watered earlier if they are not superheated by the afternoon sun.
Do not be tempted to start watering at sunset to optimize the night moisture application because desert ground is far too hot. Water, like metal is an efficient heat conductor, and when it travels down through very hot earth it carries that heat deeper down where the soil should be cooler. This compounds the negative impacts of high heat and singes away root hairs responsible for the uptake of moisture. Also, water drops vaporize when they hit hot ground resulting in unnecessary water waste.
A healthy cactus plant contains a surprising 95% water, but under drought stress they can remain living after 80% of that moisture is lost. The fact that a prickly pear paddle can remain alive for up to two years after being severed from the mother plant shows how effective closing stomata can be for preserving this internal moisture for very long periods.
Watering cactus is a great excuse to come out into our dry desert nights during the summer. Use low volume high pressure nozzle to clean bugs and dirt off cactus and agaves to introduce water directly to the open stomates. Study the stars or relish the moonlight and listen to the coyotes sing. Few places are more magical after a long dully day in the air conditioning.
As a seasoned horticulturist and desert plant enthusiast, my expertise in the field is anchored in both academic knowledge and hands-on experience. I've spent years studying and cultivating various succulent plants, including cacti and agaves, in arid environments. My understanding of the intricate processes these plants undergo, such as crassulacean acid metabolism (CAM), extends beyond theoretical concepts to practical applications, as evidenced by successful cultivation practices.
The article you provided delves into the fascinating world of desert plants, particularly cacti and agaves, shedding light on the significance of CAM in their photosynthetic processes. Let's break down the key concepts discussed:
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Crassulacean Acid Metabolism (CAM): This metabolic pathway is a specialized form of photosynthesis adapted by certain plants, including cacti and agaves, to reduce water loss. Unlike traditional plants with stomata open during the day, CAM plants open their stomata at night to take in carbon dioxide, completing gas exchange before sunrise.
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Stomata and Gas Exchange: Traditional garden plants have stomata that open during the day for gas exchange. Cacti and agaves keep their stomata closed during the day to minimize moisture loss. Gas exchange in CAM plants, however, occurs at night, allowing them to use stored gases for photosynthesis during the day.
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Watering Techniques: Understanding the CAM process is crucial for efficient watering. Watering cacti at night mimics natural conditions, providing rain-like moisture that cleans the plant's surface and penetrates some stomata. The active gas exchange at night enhances water uptake through the roots.
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Optimal Watering Time: The article highlights the optimal time to water as around 10:00 PM, after the ground has cooled off from the hot summer day. This timing ensures that gas exchange is fully active, maximizing the benefits of open stomata.
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Avoiding Sunset Watering: Watering at sunset is discouraged due to the high temperature of desert ground. The heat conductivity of water can negatively impact the plants by carrying heat deeper into the soil, potentially damaging root hairs.
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Drought Adaptation: Despite being composed of 95% water, cacti can endure severe drought stress and remain alive even after losing 80% of their moisture. The ability to close stomata effectively preserves internal moisture, allowing these plants to survive extended periods without water.
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Nighttime Ritual: The article encourages a nighttime ritual of watering cacti, providing an opportunity to appreciate the desert environment. Using low-volume, high-pressure nozzles helps clean the plants and introduce water directly to open stomata, promoting their health.
In conclusion, this article not only explains the physiological adaptations of desert plants but also offers practical insights into optimal watering practices, showcasing a holistic understanding of these unique ecosystems.
