Episode 233: leaf botany part seven - photosynthesis explained
Transcript
Episode 233
SPEAKERS
Polly Schiffman, Jane Perrone
Jane Perrone
Hey, hello and welcome to the show that shines a light on houseplants! It's On The Ledge, with me, Jane Perrone. In this week's episode, I'm demystifying the process of photosynthesis with my guest, Dr. Polly Schiffman. A reminder, right at the top of the show, that this is the last episode before I take a wee break. 'Wee', as in small, as they say in Scotland! So I'm not doing any episodes in the month of August. I'll be back with a new episode on September 2nd. If you're a Patreon subscriber, your membership will be paused for the month of August, and if you pay annually, you'll get an extra month added on so you don't lose out. And welcome to new patrons, Kay and Hannah, who became LedgeEnds this week, and to all of the Patreon subscribers who filled out the survey. LedgeEnd Nancy was lucky enough to have her name pulled in the random draw for merch. Nancy, I've sent you a message asking for details for delivery of that swag bag, so do get in touch! And if you're not currently a Patreon subscriber and want to find out more, just check out the show notes at janeperrone.com. Cracking straight on this week, we resurrect my leaf botany series for Part Seven, in which terrestrial ecologist, Dr. Polly Schiffman, joins me once again. But this time we're talking about photosynthesis. Now, I think we've all got a basic idea of what it is, but how does it really work? And what does it mean for our plants and for us? That's what we're finding out this week. Dr. Schiffman has been on the show before, talking about guttation and CAM. She works at California State University Northridge, in the US and if you want to go back to that guttation and CAM episode, I'll put a link to that in the show notes, along with all the other leaf botany episodes. So let's harness the power of the sun and get cracking on our photosynthesis chapter.
Jane Perrone
Polly, glad to have you on the show for the latest in my leaf botany episodes! We're talking about photosynthesis today and I think this is one of these things where people sort of think, "Oh yeah, I kind of know what photosynthesis is!" but then if you actually quiz them a bit deeper, a lot of us wouldn't really be able to give a very accurate definition! So let's start with the very basics. What is photosynthesis?
Polly Schiffman
Okay, so, like, really big picture perspective: photosynthesis is a kind of amazing, complicated, but super-important process by which plants - but other things are capable of photosynthesis too meaning, like, algae and some cyanobacteria - basically, what photosynthesis is, is it's a way that plants capture sunlight, which is a form of energy, but it's a not a very useful form of energy - it might, you know, warm things up a little bit, but it's a kind of unuseful form of energy - and make it into a very useful form of energy, which is stored in the bonds of sugars that can then be transformed into carbohydrates and other things. And so it's this process that only a few kinds of organisms can do. Like, we can't do it, or animals can't do it. Fungi can't do it, but plants can and it is really the foundation of most of life on earth. So it is, you know, fantastically important. But yeah, something that, a lot of times, we learn about in school and then sort of go, "Yeah, yeah, that's something that plants do, and has something to do with them being green!". Oftentimes, we forget, you know, what it is and why it matters.
Jane Perrone
Yeah, it's absolutely vital, isn't it? And if it wasn't for photosynthesis, none of us would be here, so we really should take the time to learn a little bit more about it. So let's just go over the basics of the process. What are the inputs? How does it all work and what comes out the other end?
Polly Schiffman
So it's deceptively simple when we think about it in, sort of, the most, kind of, stand-back-and-think-about-it way, where the inputs are three things - sunlight, carbon dioxide, which is gas in our atmosphere, and water - and those are, those three things are transformed through this process, photosynthesis, into some really important outputs, being sugars and oxygen. And sugars can then be complexed into more complicated molecules that are terribly useful because then there are, there's energy stored in the bonds of those sugar molecules that can then be made into carbohydrates that can become starch and other storage molecules, or they can become wood or they can become other kinds of molecules, as well. And then of course, oxygen, and, you know, oxygen is, like, 21%, I think, of our ,atmosphere, the air we breathe. And that, if we, so we rely just to, you know, take in a breath every moment on photosynthesis. So those are the inputs and the outputs. But it's actually a really complicated cascade of chemical processes that probably most people who are into houseplants don't want to think about. But all of it is really driven by the presence of a really important pigment, a green colour molecule that's in plant tissues, called chlorophyll and that is the substance that is absorbing the sunlight into the plants leaves, usually, although certainly stems and even aerial roots and flower parts sometimes, and fruits, can also be green and photosynthetic. But that chlorophyll molecule captures the sunlight and then makes it possible for the plant to then what do what we call 'fix' the carbon and make sugars, and as a sort of a by-product, almost like a waste product, oxygen gas is produced.
Jane Perrone
It sounds, as you say, like the the further you get in, and the more you focus down on this, I'm sure that - I mean, there are, I'm sure whole PhDs and scientific studies and many, many more things have been studied on photosynthesis, and, indeed, you have previously appeared on the show, talking about Crassulation Acid Metabolism, which is obviously connected to photosynthesis in terms of how plants operate and how cacti and succulents work - but what I want to know is connected to timings. Is photosynthesis a process that the plant can turn on and off at will, or is it something that's happening all the time? What happens if the plant doesn't photosynthesise for a while?
Polly Schiffman
Well, it's pretty much happening, if there's enough light, even a little bit of light, plants will often be doing at least a little bit of photosynthesis. Typically, the more light there is, at least up to a maximum level, I mean, they can't, you know, photosynthetic, photosynthesise an infinite amount if they have an infinite amount of sunlight, but they will increase the amount of photosynthesis with more and more light. And so that light-capture process that the chlorophyll is doing, is something that, at least in the wild, is happening during daytime. The carbon fixation part, where the carbon dioxide is transformed into sugars, that doesn't actually require light and that can happen in the dark. It doesn't have to happen in the dark, but it can. And so, depending on the plant, and where, you know, it lives, it might be photosynthesising a lot if there's a lot of light available, or, you know, somewhat less if there's less light. And, of course, the capacity that plants have for photosynthesis is typically related to their evolutionary adaptations to their, you know, ancestral habitat. So, if we have a houseplant, that's a tropical Aroid that originated in a Malaysian forest in the tropics there, it might be in the understorey, and then it might not be exposed under those conditions to a lot of direct sunlight, it might be at least somewhat shaded a lot of the time, or if it's an epiphyte, maybe it's getting a little bit more light. And so those plants may not have the capacity to photosynthesise at a super-high level. If exposed to a lot of light, some plants can, they're flexible, and they can ramp up if they have more light exposure, but some plants can't - they're kind of stuck photosynthesising at a lower rate because that's just kind of where their evolutionary trajectory took them when they were, you know, wild plants.
Jane Perrone
I guess this is something that houseplant growers, possibly, has crossed their mind at some point, knowing a little bit about photosynthesis, which is possibly a dangerous thing, and thinking, "Gosh, if I put growlights on my houseplant 24 hours a day, surely they're gonna grow loads!?". I'd imagine that's not a wise idea?
Polly Schiffman
Yeah, I mean, that might be the case, but it might not. And sometimes, you know, I think it's best to think about if you know something, at least about, you know, kind of roughly where that plant lives in its wild habitat, you can kind of imagine, you know, how much it might need. But of course, you know, even if you know this is a tropical forest plant, this is a desert plant, you know, trying to simulate that in an indoor, home environment is not the easiest thing in the world to do, especially if you live in the temperate zone. But, so, yeah, you can, you know, supplement light with growlights, but then maybe the plant will grow more, and maybe it won't. And some, a lot of times, I think it just involves trial and error experimentation and hopefully you don't kill the plant in the process, although I think we all have, you know, not noted precisely the what the growing conditions are considered needed for a plant and we subjected them to something that's kind of horrible as we discover, maybe too late!
Jane Perrone
Yeah, we've been there, done that! I mean, I think one of the things about a lot of the houseplants that are, I would say, you know, ones that have been around for a long time and are ubiquitous, is that they are plants that have been able to adapt so amazingly to really varying conditions. You know, I'm thinking of things like the devil's ivy, Epipremnum aureum, and, of course, Monstera deliciosa. You know, they can just adapt to all kinds of conditions and that's why they make such good houseplants. I imagine, you know, those, if you had them in, you know, 24/7 light, they'd probably be fine, but, as you say, experimentation is the key and when it comes to photosynthesis, are there other factors that can impact it? I'm thinking of things like temperature and water supply. Do those also need to be in place in order for photosynthesis to work well?
Polly Schiffman
One of the ingredients for photosynthesis, is water. So you have carbon dioxide, sunlight and water, and all three of those things are necessary. And so moisture obviously is needed for it and plants have these little tiny pores, little tiny holes in their leaves, usually mostly concentrated on the under surfaces of of leaves, and they can be on other tissues as well, but mostly on leaves, and those little pores on the under surfaces of leaves, called stomata, can open and close, the plant actually can regulate the aperture sizes of them, and they vary from plant to plant, and even from leaf to leaf on a single plant and certainly among species. But anyway, the plant is opening and closing those stomata, depending on conditions and so, if there isn't very much moisture in the soil, it might close its stomata in order to avoid losing moisture by evaporation out of its leaves through those little holes. But when that happens, it's closed its stomata, and that means that carbon dioxide gas, which is one of the other ingredients for photosynthesis, it can't get into the leaves and so photosynthetic rates will go down when plants close their stomata and they don't have to close them all the way necessarily, they might just close them a little bit and that will reduce the amount of water loss but also the amount of carbon dioxide coming in. And so there's a complicated sort of set of factors that are going on here, not only involving light, but also moisture and temperature. So, if it's warm, the plant might close it stomata to reduce water loss again and in doing so, it will, you know, reduce the amount of carbon dioxide coming in and that reduces photosynthesis.
Jane Perrone
I remember seeing a picture of stomata and they have these little, I think they're called guard cells, that are the things that kind of open and make them open and close and they do look like little mouths, I think! But it's interesting to think of them, as you say, like they're not necessarily shutting completely, but they might be getting smaller in response to conditions and adapting as things go. I mean, can they react really quickly?
Polly Schiffman
Almost certainly within minutes. I don't know if they probably can even react faster than that, but the studies that I've seen typically will be looking at stomatal sizes, say throughout a day, and in the middle of the day when it's hot and bright, and, you know, perhaps more stressful for a plant, they will actually close, you know, and they call that the midday depression, which is when water loss will go down because, or evaporative water loss out of the leaves, that transpiration, will be down because the stomata have closed, at least somewhat, dealing with that hot part of the day. And then, as the day progresses and it gets less stressful in the late afternoon, the stomata open again and they can regulate, certainly, on that level. Now, in nature, you know that that is something that happens all the time, you have these diurnal fluctuations. Water amounts typically don't fluctuate that rapidly, I think, in a natural environment, although, certainly, drying up can happen across a day, and, certainly, you know, over a short period of a few days, but yeah, so plants are responding instantaneously, or close to instantaneously, to their kind of atmospheric and soil environment in that way.
Jane Perrone
Can you confirm for me what I think I know, which is that the old, sort of, rumour that having plants in your bedroom is a bad idea because they give out carbon dioxide at night, is not something we need to concern ourselves with. I'm guessing that's based on a misconception about the amount of carbon dioxide the plants produce?
Polly Schiffman
It absolutely is. You don't need to worry about that at all. First of all, carbon dioxide is a tiny fraction. I mean, it's a shock ingly small fraction of all the gases in the atmosphere. Right now, because of elevated carbon dioxide due to industrial processes, you know, burning fossil fuels and deforestation and the decomposition that is associated with that and burning, our atmos global carbon dioxide level is about 0.04%, so a tiny fraction of 1% of all the gas in our atmosphere is carbon dioxide, but it's a hugely impactful fraction. So if you have a few houseplants in your bedroom, they're hardly impacting the amount of atmospheric gas one way or the other. And plus, plants are photosynthesising, they're taking carbon dioxide out of the atmosphere and producing oxygen. And, of course, oxygen can also be toxic at super-high levels, but it's about 21% of the atmosphere. And whatever plants, even if you had every square, you know, centimetre of your bedroom covered with plants, you probably would have a very negligible impact on the ratios of oxygen and carbon dioxide that you would be experiencing.
Jane Perrone
It makes sense, when you think about it, otherwise how would humans have evolved to live in forests? If this was the case, we would never have been able to exist in those environments, were there some huge carbon dioxide load created by plants that would impact on us. There are lots of other misconceptions about photosynthesis. I'm sure when you've been teaching students, you've come across some humdingers!
Polly Schiffman
Oh, yeah! And, in fact, I, it's not really a misconception, but when I was a student, I took a biochemistry course, and I remember this vividly on the first day of class, and it was general biochemistry, it wasn't about plant biochemistry, and the professor said, right off the bat, that he didn't like plants, he wasn't interested in them and he didn't even like eating them and so, therefore, he was not going to talk about plant biochemistry, which I thought was kind of hilarious! But, you know, I think, basically, he didn't want to talk about photosynthesis because, I mean, that is kind of the primary, you know, biochemical phenomenon you should be teaching in a course like that! But I also get it, of course, because I am a university professor and I teach this topic. I'll have students in my class who might be interested in biology, but they're not really interested in plants. They think they're interested in animals, or they want to study to become a physician, and so they're really interested in, you know, cells and molecular processes, and they don't really care about plants. But plants are the source of our nutrition, everything from, you know, vitamins to, you know, the energy that we need to keep ourselves alive. Every organism that can't photosynthesise relies on a photosynthesiser, or a whole bunch of them, for food, and, you know, nutrients, and so not just to breathe, but to just keep ourselves running energetically, we have to consume plants, or animals that eat plants. So they're hugely important and I think people naively think, "Oh, you know, I'm not into plants, so I don't need to know about them!" but we kind of do! And especially, you know, nowadays. I mean, this is sort of off the topic of houseplants, but, you know, the fact that plants are photosynthetic means that they are the best sequesters of carbon dioxide that we can really control in our environments and so planting more plants means that we're taking more carbon dioxide out of the atmosphere and because carbon dioxide has been increasing in the atmosphere, we really should try and do that as much as possible. Now, you know, a houseful of houseplants is going to have a really negligible impact on that, but if we tried to do more reforestation of areas that used to have forests but don't any more, that certainly would be important and affecting all of us, you know, who live on this little planet floating around in space, and it's all because of photosynthesis.
Polly Schiffman
It's not just shade, you know, because the plants are transpiring moisture out of their stomata, it's actually doing evaporative cooling at a, you know, city-scale. So, if we have more plants, we have less of what's called the urban heat island effect, where cities heat up because of all those surfaces like buildings and asphalt and cement and concrete and rocks, that dissipate heat into the atmosphere and warm up our urban environments. But if we have a lot of trees, yes, they shade, but they also do evaporative cooling, and with the moisture that they're taking from the soil, that helps cool as well and that, you know, is good for all of us, and especially in places where they don't have very many trees, planting them, I mean, it may feel like "Oh, it's not going to make a difference now!" but trees grow pretty fast and yeah, we can make a difference and we should protect the trees we have by making sure they get enough water. Like, where I live here in Los Angeles in California, we are in a, you know, another drought and the thinking is, if you need to water anything, water your trees, forget the lawn, but water the trees.
Jane Perrone
Exactly! And I've just seen so many interesting posts on social media over the last few days, during the heatwave we've been having here, showing the amazing impact that plants can have on dealing, when in the midst of a heatwave, actually, the more plants you've got around you, the better. I saw a picture of two greenhouses, one was surrounded by plants on the outside of it and the other one wasn't and temperature difference was about 10 degrees because the plants were just shading the greenhouse and helping it stay cool. And the same with shots of paving with weeds growing in between qnd you can see that the weeds were way cooler than the stone and obviously having an impact there.
Polly Schiffman
I totally concur and I've got a crabapple tree that I planted last year, that I'm taking out bowls of grey water to, from my washing up bowl, and it's all ending up under that tree to try to make sure it keeps establishing well because you're right, that's what we need to be doing. Any other photosynthesis misconceptions you can think of, that you've heard?
Polly Schiffman
It's not a misconception, but I would call it a fun fact, and that is in photosynthesis, there is this really important enzyme and enzymes are proteins that drive chemical reactions and so obviously, photosynthesis is a biochemical process with a lot of chemical reactions and one key enzyme in that process is this chemical called - it's a mouthful! - ribulose one five bisphosphate carboxylase oxygenase, which we, as biologists, botanists, have given a nickname because we don't want to say that long name all the time! We call it Rubisco. And Rubisco is super-important in the carbon fixation process, but the reason I wanted to mention it, is because just the presence of that one kind of enzyme in plants is the primary reason we need to fertilise plants, because Rubisco is the most important protein on earth and in a given plant leaf, it's about 30% of the nitrogen in that plant leaf, is just that it's nitrogen in Rubisco. So if you need to fertilise your houseplant, it's probably because of the presence of that one super-important photosynthetic enzyme Rubisco and the activity that it's doing constantly, you know, when there's photosynthesis going on, it's fixing carbon fixing carbon fixing carbon and when the plant's growing, it has to make more Rubisco, it needs to grab more nitrogen out of the soil, so we have to fertilise and so I think that's it's not really a misconception, but it's definitely kind of a techno-geek fun fact!
Jane Perrone
I'm gonna be using that in a conversation very, very soon! I love that. That is good to know!
Polly Schiffman
Actually, another fun fact, you know, a lot of houseplants are, you know, a lot of tropical forest kind of understory species and epiphytic plants that don't get full sun all day long and in nature, sometimes those plants get very little sunlight on a, you know, a continuous basis. They rely on these, this phenomenon called sun flecks, which are little blasts of sunlight and if you've ever walked under a forest canopy, you've seen them, they're little patches of sunlight on the ground, and they might be lighting up that that leaf, or that plant, for a short time and then the light is gone and maybe it's gone for the rest of the day. Or maybe it's windy, and the lights are kind of bopping around because the little patch of light is is moving as the leaves move in the tree canopy and cause the light not to be consistently in that spot for any meaningful length of time. And there are tropical plants especially, but other forest plants as well, that are really adapted to using that very ephemeral availability of light to live their lives. And those plants, you know, we then take, sometimes take them into our homes and supply them with, you know, a kind of continuous light and maybe that's why sometimes they don't do well. I think plants are pretty flexible and they can tolerate, you know, even living, you know, maybe adapted to sun flecks, maybe in a home environment they can tolerate longer periods of light and even thrive in that, but maybe that's one reason sometimes they don't, is they're really not adapted to that exposure to long, long periods of light. But there's really no way to know. And, you know, given different species have different levels of ability to deal with that kind of situation.
Jane Perrone
I guess, you know, there's been a process over the last, gosh, well, 200 years or so, of plants being, you know, experimented with, and some of them have fallen away because they just didn't work as indoor plants and other ones have absolutely thrived. And I guess, also, looking through the history of indoor plants, we can see that, you know, conditions in our homes have changed so dramatically that, you know, what worked in Victorian times didn't necessarily, doesn't necessarily work today and I'm thinking of, you know, researching my book, I've come across lots of images of English ivy Hedera helix being grown, which I guess quite liked the sort of very cool, damp nature of a lot of Victorian homes in Britain, but are not quite so happy in our centrally-heated homes. So yeah, conditions have changed, but the old faithfuls, I guess, are the ones that, you know, could really seem to be able to adapt to all these different factors that we have going on.
Polly Schiffman
First, houseplant species that have a long history of living in home environments, there's been selection that's gone on in the nursery trade, I'm sure, that is selecting for houseplants that will tolerate, you know, those kinds of conditions. And either individuals of that species, or different types from maybe some areas, you know, are collected from some areas in the wild, rather than others may be, you know, just not able to live in a home environment, where other individuals will be great sources of tissue for growing up more plants to, you know, live in a home environment, which is really different than out in nature. So, yeah, I think you're right.
Jane Perrone
Yeah, it's fascinating stuff. And as you say, the plants are probably being selected out for the ones that really work, as opposed to the ones that just look a bit, that don't last the course over the years. So we shall see. But it's been fascinating to talk to you, Polly, and hopefully that's made photosynthesis a little bit clearer for our listeners, so thanks very much for joining me today!
Polly Schiffman
Well, you're welcome. Glad to do it!
Jane Perrone
Thanks so much to Dr. Polly Schiffman for joining me this week and to you for listening and I hope the month of August slips away pleasantly enough until I'm back with you again. And if you've got suggestions for episode topics, plants you'd like to cover for the autumn, then please do let me know, I always love to hear from you. Oftentimes, when I get emails from people, they say, "Oh, I didn't think I'd get a reply!" I do try to reply to most people, so do email if you've got an idea suggestion, constructive criticism or a question for the Q&A. Right, that's me signing off until September, so enjoy your rest and I'm going to enjoy mine! Bye! The music you heard in this podcast was "Roll, Jordan, Roll" by The Joy Drops'; "The Road We Used To Travel When We Were Young", by Komiku, and "Overthrown", by Josh Woodward. All Tracks are Licenced under Creative Commons. Visit the show notes for details.
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We all know photosynthesis is vital for plants, but how does it work? Dr Polly Schiffman joins me to explain.
This week’s guest
Dr Polly Schiffman is a terrestrial ecologist at California State University Northridge in the US. You can also hear her in On The Ledge episode 148 explaining CAM and guttation.
Listen to the other episodes in the houseplant botany series here:
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CREDITS
This week's show featured the tracks Roll Jordan Roll by the Joy Drops, The Road We Use To Travel When We Were Kids by Komiku and Overthrown by Josh Woodward.