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Cloudy days

  • Thread starter jimscott
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jimscott

Tropical Fish Enthusiast
We all know that temperature waffles up and it waffles down, while photoperiod goes in one direction or the other. What is happening to the plants when it's cloudy? Are they deriving any benefit from it at all?
 
I would assume as long as you can see out your window and observe the squirrels, plants are still photosynthesizing.

-Ben
 
Yup, they're still photosynthesizing.

Remember, chloroplasts are not bound by membranes within the cell, and as such they're free to move around. When there's lots of light, chloroplasts will line up behind one another (like stacking, with the sunlight coming from above) so that only the the few blocking the extreme sunlight will get damaged (this's why plants in lots of sunshine are often a lighter green.) In dark conditions (eg shade,) chloroplasts will move and spread out, so that as many are getting light as possible (thus, plants which grow in lower light conditions are often a darker green; think forest floor in a forest with a dense canopy.)

While a a few days of reduced light due to clouds wont generally cause any major visible differences in leaf shade (IME,) they're compensating for the weather and making the best out of it.

An excellent example is Germany. Parents will sometimes need to get vitamin D supplements for their children because they aren't getting exposed to enough sunlight, so their bodies don't produce enough Vit D on their own (there are years when you get just a few days of sunshine.) Yet areas like these often have vast amounts of vegetation. Hamburg, for instance, looked at from above looks more like a forest than a city due to the large gardens and extensive greenery, when it fact it is a pretty large city.

There are definitly cases when things are better when they're cloudy. Certain plants will close their stomata when there's too much sun (otherwise they risk drying out,) this hinders gas exchange and if it occurs for an extended period of time, a plant will use O2 instead of CO2 in the photosynthetic process, this reaction ends up actually just wasting energy for the plant. A bit of cloud darkness isn't a problem, it's generally too much sun that you should be worried about! Furthermore, it's better to have a sun-adapted/acclimated plant in shade, than a shade-adapted/acclimated plant in sun.

To sum it all up: Photosynthesis is still going on, you gotta try REEEAL hard to get it to stop.
 
[b said:
Quote[/b] ]An excellent example is Germany. Parents will sometimes need to get vitamin D supplements for their children because they aren't getting exposed to enough sunlight, so their bodies don't produce enough Vit D on their own (there are years when you get just a few days of sunshine.) Yet areas like these often have vast amounts of vegetation. Hamburg, for instance, looked at from above looks more like a forest than a city due to the large gardens and extensive greenery, when it fact it is a pretty large city.

Not sure about that! Germany has long hot sunny summers and even if you lived in a forested city you'd still get plenty of vitamin D.
 
[b said:
Quote[/b] ]Not sure about that! Germany has long hot sunny summers and even if you lived in a forested city you'd still get plenty of vitamin D.

Trust me on this one, there's areas in Germany where is important. I'm German, my parents are, my grandparents are (etc etc,) so we know about raisin' kids there.
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Granted, there are areas where you get some dern HOT days in summer. Germany is pretty far north (and I'm sure you appreciate, Alexis, being in the UK,) I believe Hamburg and Anchorage share the same longitude. I know the Vitamin D thing sounds far fetched, but unless I'm horribly, horribly mistaken and making up that my mom ever said this; it's true.
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Germany has a continental climate so doesn't get affected so much by the rain from the Atlantic that we often have to put up with. I've no doubt people buy vitamin D supplements - it's probably in the multivit I take every day!
 
Jonathan, thank you for the scientific information! Glad to know that they adapt to cloudy days. Man, it has been very UK in Western NY!
 
[b said:
Quote[/b] ]Jonathan, thank you for the scientific information! Glad to know that they adapt to cloudy days. Man, it has been very UK in Western NY!

My pleasure, Jim! Haha, I was looking at the weather map today in the paper, I looked at NY and thought "Poor Jim." lol It may be a bit rainy and windy here, but at least it aint freezing!
 
Maybe those Germans needing Vitamin D supplements are vegans who avoid processed foods (many are fortified with vitamin D).  Getting enough Vitamin D can be a challenging part of a vegetarian life.  Maybe not as big of a challenge as not eating bacon, but a challenge nontheless.

As for what happens to plants growing in less than optimum light, their growth is less than optimum.  Meaning lower growth rates, fewer flowers, and so on.  That CO2-O2 thing is photorespiration and I'm pretty sure it's more a function of heat than light intensity.  Plants using C4 photosynthesis aren't affected, which is why corn and some weeds (pigweed & C4 grasses) take over the garden in the hottest part of summer.  Moisture has a role too, but I think temperature is the major player.
 
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[b said:
Quote[/b] ]Maybe those Germans needing Vitamin D supplements are vegans who avoid processed foods (many are fortified with vitamin D). Getting enough Vitamin D can be a challenging part of a vegetarian life. Maybe not as big of a challenge as not eating bacon, but a challenge nontheless.

The problem was just with regular ol' people. Besides, somehow "vegan" and "vegetarian" just don't register in the same sentence as "German."
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[b said:
Quote[/b] ]As for what happens to plants growing in less than optimum light, their growth is less than optimum.

True, but optimum light isn't necessarily maximum light. So there could be a plant that Jim's growing that'll be doing a lot better on a cloudy day than on one with a clear sky.

[b said:
Quote[/b] ]That CO2-O2 thing is photorespiration and I'm pretty sure it's more a function of heat than light intensity.

True, it's a function of heat, but I figured I'd include it as heat and light intensity are generally related.

[b said:
Quote[/b] ]Plants using C4 photosynthesis aren't affected, which is why corn and some weeds (pigweed & C4 grasses) take over the garden in the hottest part of summer. Moisture has a role too, but I think temperature is the major player.

C4 and CAM plants are adapted for dry conditions (and therefor we can generally assume hotter, and more intense light.) Moisture and heat are both the major players because moisture is often relative to heat. It's a conflict between maintaining good gas exchange while limiting water loss.

An excellent example is a succulent (CAM.) During day it's too hot (and thus the humidity is too low), if they have their stomata open, they'll dry up and die. So what CAM plants do is they divide the functions of photosynthesis with time. The plant will open up it's stomata during night when it isn't so hot and take up as much CO2 as possible and store in within its tissue. When the sun comes back up, the stomata close, but the plant can still perform photosynthesis because of the stored CO2. Very interesting, love this stuff!
 
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I think heat is more significant than light for photorespiration because the light intensity is declining during the time (July-August) the C4 plants begin to dominate.  But do you know if there are any C4 alpine plants?  That's a high light/low temperature environment and it wouldn't be heat that favors a C4 plant up there.

Not all CAM plants are succulents (and vice versa).  Many orchids are facultative CAM plants - switching between C3 & CAM depending on growing conditions.  At least some (most, maybe all?) Bromeliads are CAM.  I think CAM Bromeliads do it full time and pineapples are one of them.

Plants have a lot to do so, even when photosynthesis is reduced on a cloudy day, they keep busy with other things. In fact, many don't take full advantage of a sunny day and stop photosynthesizing with the sun still fairly high in the sky.  It wouldn't make sense to an economist, who'd expect a plant to maximize its earnings by photosynthesizing from dawn to dusk.  But it makes sense to the plants who actually make a living from it.

It can be a strategy to avoid photorespiration during the heat of mid & late afternoon or it might have something to do with water stress.  Maybe plants turn their attention to some other physiological process or maybe they just like to kick back and rest after a good day's work.  Maybe all of the above.
 
  • #13
[b said:
Quote[/b] ]I think heat is more significant than light for photorespiration because the light intensity is declining during the time (July-August) the C4 plants begin to dominate. But do you know if there are any C4 alpine plants? That's a high light/low temperature environment and it wouldn't be heat that favors a C4 plant up there.

Indeedy, I believe heat is more significant, I just made the connection (as I said before) because heat and light intensity are often directly related. As for the alpine C4 bit, I'm not currently in school so I can't ask anyone, but I'll try to do a bit of research! I would think that there woudln't be many (if any) because to my knowledge C4 is more helpful in dry/Nitrogen defficient cases.

[b said:
Quote[/b] ]Not all CAM plants are succulents (and vice versa). Many orchids are facultative CAM plants - switching between C3 & CAM depending on growing conditions. At least some (most, maybe all?) Bromeliads are CAM. I think CAM Bromeliads do it full time and pineapples are one of them.

I'm not sure if all bromeliads utilize C4 (including CAM) photosynthesis since Spanish moss is in the Bromeliaceae family (they might all do it, I'd just think that Spanish moss, for example, doesn't. I'll look in to it!). I just used a succelunt as an example, but thanks for mentioning this, I wasn't aware that some plants actually utilize multiple pathways! I suppose when you think about it, that it sort of makes sense, sill, new to me!

[b said:
Quote[/b] ]Plants have a lot to do so, even when photosynthesis is reduced on a cloudy day, they keep busy with other things. In fact, many don't take full advantage of a sunny day and stop photosynthesizing with the sun still fairly high in the sky. It wouldn't make sense to an economist, who'd expect a plant to maximize its earnings by photosynthesizing from dawn to dusk. But it makes sense to the plants who actually make a living from it.

It can be a strategy to avoid photorespiration during the heat of mid & late afternoon or it might have something to do with water stress. Maybe plants turn their attention to some other physiological process or maybe they just like to kick back and rest after a good day's work. Maybe all of the above.

Out of curiousity, do you happen to know if they would continue photosynthesis at times where it's normally decreased if there's less sun? IE, if there's less sun, do they not "worry" about getting burned? Man, just when so many good questions arise, I'm out of school! I'll have to remember these for my next semester of horticulture. And hey, if they just like to kick back, then more power to 'em
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Thanks for the additional info, herenorthere.
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