Pondering photosynthesis

[Paul D]

Humour me.
Lying awake in my bed at 3 a.m. this morning, with the sunlight streaming in, I was thinking about the Crassulacean Acid Cycle (as you do).
It struck me that the north-east of Scotland is maybe not the ideal place in the world to try and grow cacti. But we knew that already.
What I wondered was, do cacti growing here have a different strategy, re photosynthesis, from those growing in more southern climes? My understanding of the CAM is that stomata stay closed during daylight to save water, and open up in darkness, with Malic acid being produced during the dark hours to store CO2 which is then released and used by the plant during the day. Also, that many CAM plants are flexible in being able to move between CAM, C3 and C4 depending on drought conditions. Since we have so few hours of darkness here, in the main growing period, how do our cacti and other succulents fare?
(Yawn).

[sechjoh]

As far as my knowledge is concerned, I think that the temperature during the night is determined, not the light, when it comes to CAM.

[Paul D]

As far as my knowledge is concerned, I think that the temperature during the night is determined, not the light, when it comes to CAM.

Fascinating, thanks Christer. That raises more questions! I will try and find a study.

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[John Dos]

With moderate tº and high relative humidity the plant has no need to protect from evapo-transpiration.

[iann]

CAM is very flexible, but for a particular plant it will be suppressed when night temperatures are too high. The point of suppression varies widely, being quite low in winter growing species but high in tropicals like orchids. CAM, but also photosynthesis in general will be suppressed if day temperatures are too low, but that probably isn't a limiting factor for cacti. CAM plants tend to saturate with CO2 during the night, so shorter nights aren't necessarily a problem, but I don't know how short before it becomes limiting. CAM plants also have evening and morning periods when they directly transpire CO2, before it goes dark or the stomata completely close, again highly adjustable but will be significant in our sort of moderate climate.

Things like changing between C3, C4, and CAM are not particularly relevant in cacti. Certainly most of them don't use C3 or C4. C3 happens in the leaves of a few cacti like Pereskia, C4 not in any that I know of. The mixtures in other succulents are more interesting, with some mesembs showing a sort of "reverse CAM" (CAM cycling) with stomata closed at night and open in the day (this allows rapid growth, but also a rapid switch to CAM idling with stomata permanently closed). Many annual mesembs start out using C3 for rapid growth, then switch to CAM either as a response to drought or to saline conditions. Some Portulaca show C4 in their leaves and CAM in their stems, but this is rare.

[Paul D]

Many thanks Ian. Do you know what causes the stomata to open at night? I guess the plant has some mechanism of making the guard cells turgid or flaccid, independent of water supply?

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[sechjoh]

Some Portulaca show C4 in their leaves and CAM in their stems, but this is rare.

Pereskia have both C4 and CAM

[iann]

Some Portulaca show C4 in their leaves and CAM in their stems, but this is rare.

Pereskia have both C4 and CAM

Weird plants, barely cacti at all

[iann]

Many thanks Ian. Do you know what causes the stomata to open at night? I guess the plant has some mechanism of making the guard cells turgid or flaccid, independent of water supply?

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The water content of the guard cells is altered by their ion content, probably caused by proton-pumping across the cell membranes. Although the exact chemical triggers are unknown, this process is regulated by light, possibly blue light. Additional factors such as temperature, water availability, and temperature, can cause adjustments to the timing and speed of the stomata opening and closing.

[Paul D]

Thanks again. Much appreciated. I found this study which goes into the effects of various environmental factors (light, temperature, etc.): https://www.ncbi.nlm.nih.gov/pubmed/15150072

Many thanks Ian. Do you know what causes the stomata to open at night? I guess the plant has some mechanism of making the guard cells turgid or flaccid, independent of water supply?

Sent from my SM-G965F using Tapatalk

The water content of the guard cells is altered by their ion content, probably caused by proton-pumping across the cell membranes. Although the exact chemical triggers are unknown, this process is regulated by light, possibly blue light. Additional factors such as temperature, water availability, and temperature, can cause adjustments to the timing and speed of the stomata opening and closing.