Do plants know we're here?

0:00This BBC podcast is supported by ads outside the UK. Fall flavours are waiting for you at Whole Foods Market. Cozy picks all through the store, like maple leaf cookies from 365 brand. Swing by the bakery for pumpkin cheesecake, here for a limited time, and pumpkin turmeric bread freshly baked every single day. Sip the season with 365 brand cinnamon spice coffee and pumpkin spice creamer. while you still can. Spice Up Fall at Whole Foods Market. It felt like everything I believed in was suddenly collapsing. This is the story of an Islamic boarding school in Indonesia with a dark secret of the school's head teacher.

0:44This was someone I respected above anyone else. And the investigation led by students he's trying to silence. If this is leaked to the media, I'll be destroyed. This is World of Secrets, catching my teacher. Listen on BBC.com, the BBC app, or wherever you get your BBC podcasts.

1:08So here we are. This is a particular variety of lavender called Arctic Snow. It's an English lavender. Welcome to CrowdScience from the BBC World Service. I'm Anand Jagatia, and I'm getting my hands dirty in an English garden. And if you just take your fingers and sort of rub it along the foliage here and just smell your fingers, you will instantly remember lavender smell. Yeah, I mean, that's basically as strong as if you have a little bottle of essential oil. But it's much nicer, it's more mellow. Yes, it's quite pungent. I'm trying out some direct physical contact with the plants in this garden because that's what this week's question is all about.

1:48Hi, my name is Gurmit. I'm from India. My question for crowd scientists is, can plants actually sense the presence of humans and respond to the way we interact with them? For example, when we touch, hug or care for a plant, can it detect this in any scientific way? Gamit began wondering about whether plants might be able to detect the presence of humans after hugging a tree at a wellbeing centre. They talk about to hug a tree or talk to a tree that makes us calm and well. then I was very curious to know is there any scientific evidence behind this. So what was it like hugging the tree? Did you enjoy it?

2:28Yeah, when I hug a tree that feels me very calm and happy and it feels earthing. Earthing, I like that word. But I want to know is the trees also enjoy when we hug trees? Yeah, you obviously got a lot out of hugging the tree but did the tree get anything out of it as well? Yeah. So I like that there's sort of two halves to this question. One is, do plants know that we're there? You know, does a tree know if we hug it? But also, what's the impact of that kind of thing on us? Does touching plants or hugging plants have an impact on us as well as humans? We'll try and find some answers and we'll let you know what we find out.

3:06Thank you so much. Thanks, Gamit, for getting in touch with that tree and for getting in touch with CrowdScience. To answer your questions, let's start from the plant's perspective. do they actually have a sense of touch? Yeah, absolutely. They have a pretty decent sense of touch. It helps them in many ways. This is Olivier van Acken. He's a molecular biologist from the University of Lund in Sweden, where he researches how plants respond to their environment. It can help climbing plants to find something to attach to. It can find maybe insects crawling on the leaf so that the plant can prepare before it actually gets bitten.

3:50Also, wind can actually induce these kind of signals because obviously if you're a tree growing on an open plain, you don't want to be blown about too much, so you'll try to become a bit shorter, a bit more stocky, and all of this is response to touch in various ways. So touch is important for plants. It's useful for them to be able to sense the things in contact with them. But Gamit also wanted to know if being touched might be a positive experience for the plant. Well, according to Olivier, most of the time it's probably quite stressful, which makes sense if you think about it from their point of view.

4:29I guess you could imagine if you're yourself standing in a room blindfolded and you don't know what's happening around you and all of a sudden you feel something touching you, you'll probably be a bit stressed about it unless it's like your partner and you know it's going to be OK. But that's more or less, I think, the sensation a plant must have because they can't really know what is touching them. A large part of it is a stress response because it can then help them boost their immune system. Maybe they might get a wound or get cut in some way and then they can stimulate the immune system to stop maybe infections.

5:01But that's kind of one end of the story. Of course, plants like venous fly traps, you know, the ones that can catch flies, they obviously use it in a more sort of positive way. And they kind of adapted that stress signal into something that alerts them to, hey, now is a good time to close my trap. So, of course, it's not a black and white answer. Of course. And I think that's a really illustrative example, the Venus fly trap. Have you got any other examples that spring to mind of specific plant species that tend to have a reaction to touch? Yeah, you have these sensitive plants. You've probably seen them.

5:38mimosa they're the ones that when you touch the leaf it has all these like leaflets and you touch it and then it sort of collapses on itself very quickly uh yeah you can like stroke them and then you sort of get this domino effect yeah exactly yeah so those ones do it and they they again do it because they think it might scare away some bug that's trying to eat them yeah and i guess in those examples the outcome the response to the touch stimulus is movement and it's visible and with something we can notice but I guess in other plants their reaction might be much more invisible there might be an internal kind of chemical change or something like that so what are some of the ways that plants will respond either internally or externally after they are touched and and why probably a single touch on the plant probably won't give much of a noticeable effect even even a bit later than that so but if you're kind of persistent about it if like in the lab we when we We try to do these experiments.

6:37We touch with a paintbrush or something gentle. We touch them like once or twice a day. And we do this for maybe one, two weeks in a row. And then you start seeing these responses, which are probably cumulative effects of several small contributions of each of these manipulations. And then what we can see is that the plants, they flower later. So they, well, I guess they develop a bit more slowly. They might have smaller leaves. So in a way, yeah, it's like a stress response. So they kind of prioritise survival over being the fastest growing plant. So plants do have a sense of touch, and they can respond to physical contact in a variety of ways.

7:18From climbing plants, which may use it to attach to a surface, to a Venus flytrap, which may snap shut around a juicy fly. But in most cases, plants respond to touch defensively, with a stress response, because there's a good chance that whatever is touching them is going to harm them. So plants might not enjoy being touched quite as much as we may like touching them. Listener Gamit certainly found hugging a tree to be a positive experience. But is there any scientific evidence that close contact with plants is good for our well-being? We've travelled to the perfect place to find out, a well-being garden in the south of England.

8:00Welcome to Wisley, RHS Wisley Garden. We are standing in front of RHS Hilltop, the home of gardening science, so it's a big building dedicated to horticulture scientific research. And in front of it, we are in the Wellbeing Garden. This is Ashby Lavelle-Sax, the Wellbeing in Gardening Fellow at the Royal Horticultural Society. We're surrounded by flowering plants in a very peaceful garden that's been specially developed to make you feel good. This garden was designed specifically to support well-being by including all kinds of beautiful textures. We have moving water, lovely sounds, and we also included a number of aromatic plants so you can experience that scent.

8:40So really all five senses are really elevated in this garden. What is the evidence for the impact of being around plants on our physiology and our psychology? So one of the first things you notice in any garden is colour. It's what people are most drawn to flower colors. And we have done research that shows that actually, if you look over here at these yellow flowers, those warm, saturated yellows, reds and oranges, have found that really energize us and produce positive emotion. And then you said there are also other senses we can think about. Absolutely. So if you think about maybe sound here, we had an interesting study done here at Hilltop where a researcher came in and gave people three different prompts.

9:20first prompt, they just had them walk around for 20 minutes. The second prompt was notice nature around you. What sounds do you hear? And also the third prompt was notice all the man-made sounds, the roadway, the airplanes above you. And then we found that the folks that really paid attention to the natural sounds, the natural features scored the highest in the restoration. Their mood and their relaxation was scored the highest. So my point being is these natural soundscapes can really calm and relax us as well. So listener Gamit when she was hugging this tree she might have been hearing the sounds of the wind in the leaves what other things might have been going on that would have been impacting her her well-being?

9:59So one thing I also want to point out is scent which is very powerful and something we're studying here if you have ever walked through the forest and just wondered you know you felt at peace there's actually a lot going on behind that one of the things specifically is something called a phytoncide. So conifer trees emit these organic compounds. And there's a lot of research that has shown that even just a two-hour walk through a forest with conifer trees activates our killer T cells. And those can really affect our immune system and our sleep and actually help fight cancer even for days afterwards the visit.

10:35So when our listener Gamit had her arms wrapped around that tree, it might not have just been the hugging that made her feel good. The colours, sounds and scents that she was experiencing may also have had lasting positive effects on her mind and body. In fact, just smelling the compounds released by certain trees can have a boosting effect on our immune system. We've heard that plants can respond to physical contact, but Gamit also wants to know if they can sense the presence of humans. That's what we'll be looking at. Fall flavors are waiting for you at Whole Foods Market. Cozy picks all through the store, like maple leaf cookies from 365 brand.

11:20Swing by the bakery for pumpkin cheesecake, here for a limited time, and pumpkin turmeric bread freshly baked every single day. Sip the season with 365 brand cinnamon spice coffee and pumpkin spice creamer. While you still can, spice up fall at Whole Foods Market. It felt like everything I believed in was suddenly collapsing. This is the story of an Islamic boarding school in Indonesia with a dark secret of the school's head teacher. This was someone I respected above anyone else. And the investigation led by students he's trying to silence. If this is leaked to the media, I'll be destroyed. This is World of Secrets, catching my teacher.

12:04Listen on BBC.com, the BBC app or wherever you get your BBC podcasts. Next.

12:19You're listening to CrowdScience from the BBC World Service, where we try and wrap our heads around the answers to your science questions. I'm Anand Jagatia and this week we're exploring what happens when we come into contact with plants We've learned about the impact that this can have on us. But what about the plants? Is there any evidence that they can detect us being near them? I absolutely think plants will be aware of other organisms around them. This is Alexander Jones. He works at the Sainsbury Laboratory at the University of Cambridge in the UK, studying plant hormones. In terms of humans, I think they won't necessarily know a human or even an individual human, they certainly wouldn't be aware of that level of specificity.

13:07But they would, in a small room, if a human walked in, they would be breathing out carbon dioxide and the plant would immediately be aware of that change in the air. If the human turns on the light, if they open the shutters, all these things would affect the plant's physiology in a way that the body of the plant would respond to, even on short timescales. According to Alexander, just by being around a plant, we would change their environment in ways that they can detect and respond to in measurable ways. So we can measure genes, gene expression going up or down in response to these stimuli very fast.

13:47We can measure physiological responses like the pores in their leaves opening and closing. These are the things that would happen right away actually in response to, let's say, a large animal entering a closed room. I'm wondering, is there any evidence that plants can distinguish between human touch, but also maybe other kinds of contact, like an insect walking across them, or rain or wind? I absolutely think the plants would be able to discriminate different types of stimuli. In fact, we've seen some of this in our own research, where, you know, A poke may be read differently from a pest inserting a feeding structure into the plant leaf.

14:28And so I think this could take on different forms, but I absolutely think plants are capable of discriminating different types of touch. Alexander has studied the way one species of plant reacts to a particularly invasive form of touch, being eaten. And you might be familiar with spider mites crawling all over your houseplants. They're actually terrible pests. We found that when spider mites feed on a leaf, they will actually trigger a drought hormone. This drought hormone is normally used to save water by closing pores on the leaf. But the plant can do the same thing to defend against spider mites.

15:09And the reason they do this, we think, is because the spider mites prefer to feed through these pores. So if you close the pores, actually the spider mites have a harder time of feeding and causing infestation. The plant has evolved a new way of using an existing hormone, and Alexander says that hormones are really important for plants as a way of responding to their environment. When a plant is responding to an environmental stimulus, we know that it will change the levels of these hormones, these molecules, and that then becomes information that the rest of the plant can then interpret. it. And what's really important is that they can come together and provide information processing to say, well, there's multiple signals present at the same time, and then we're going to actually activate the immune system.

16:00And so we think that's how plants function without a central information processing brain. So despite not having a brain, plants have the ability to respond to touch in very sophisticated ways, using hormones as a way of processing information signals from their environment. And to answer listener Gomit's question about whether plants can sense our presence, plants can respond to changes in the environment that we might cause, like CO2 or light levels. But there's no evidence to suggest that they can detect the presence of humans per se, although it might be possible. And you never know, maybe there is a planet out there that has evolved a way to specifically read some sort of chemical plus physical signature of a human touch.

16:46We cannot exclude this sort of thing. Now we've looked at some of the downstream consequences of how plants respond to touch but what we haven't explored is how do they actually sense physical contact in the first place? What's actually going on inside a plant when you first touch it. What we assume is that when you touch a plant, if you think of a plant cell like a balloon and you would push it with your finger, you would see sort of around where you're pushing it that the balloon will stretch locally. This is molecular biologist Olivier van Acken, who we heard from earlier. So we think there are channels in the plant's membranes in the cells that are stretch activated.

17:28So when some kind of mechanical force hits the plant, it'll stretch the membrane and those channels will sense it and somehow they will open up. Once these channels are open, it means charged particles called ions can flow through them. And different ions like calcium can enter the cell and that sort of sets in motion a whole signaling cascade. And that cascade itself we're kind of getting to know and we know some important players of how that part inside the cell is working but then really how the first signal is perceived, that is still a bit of a mystery. So we don't know exactly how plants translate that first physical touch into a response, but it's likely that it involves stretch-activated channels for ions like calcium.

18:17And once those ions move into the cell, it can cause a domino effect for all kinds of cellular signals. So we know there are particular proteins that can sort of measure the concentration of these ions inside the cell, and then they will activate it's a bit like a relay race so the first person you know gets a tap on the shoulder and then he starts sprinting maybe has some kind of stick and he passes the stick on to the next one and then this person will continue so you have this sort of protein cascade so one protein activates the next one and the next one and in doing so you can kind of amplify the signal.

18:57And then 10-20 minutes later we can measure like a thousand or two thousand genes have changed just by this initial touch. And then depending on how often this will happen you get these cumulative effects, that's what we think. So just a single touch probably doesn't do so much but if you do it every day, a couple of weeks in a row, all this will build up into something that actually becomes a tangible effect. So that's the impact of touching a plant on the plant. But what about the impact on us? We've heard about the positive impacts of being around plants in general, but listener Gamit wanted to know if there's any evidence that actually touching them might be good for us.

19:41Back at Wisley Garden, I put this to well-being and horticulture expert Ashby. There hasn't been that much research on specifically what happens when we touch a plant. I think we have done research on gardening. You know, what happens if you pot a plant? What happens if you're working with your hands in the soil, touching the soil? So the act of gardening stimulates all five of our senses. In my head, I can definitely think of plants that I don't want to touch, like stinging nettles, or, you know, if you go camping and the grass is really scratchy underneath you. But yeah, I guess the act of hugging a tree, I don't know if you know if there's been any research on that, but it's more than just the feel of the tree or the bark against your skin.

20:21There's also this sense of connecting with a being that's been there for a really long time. There's a sense of feeling how you're connected to the earth beneath you. Also, probably your face is getting a lot of smells and sights as well. There's been research that's shown that spending time looking at nature, being in nature, it makes your problem seem smaller because you get greater perspective and you feel like you're part of something larger than yourself, a bigger ecosystem, and it can be really grounding and soothing feelings. So maybe that's what your listener was describing, not just touching the tree but really feeling connected to something bigger.

20:57And I think the more we can do that in our world today, the better everyone will be. So there is research which shows that being in nature can ground us and connect us to something bigger. Perhaps that's part of what listener Gamit was experiencing when she hugged that tree. There hasn't been much research which has specifically looked at the effect of touch, but we were able to find a scientist who has done one of the few studies in this area. We wanted to find out whether different textures could produce different physiological and psychological responses. In other words, does touching a soft leaf affect us differently from touching a soft, stiff or loved one?

21:43This is Sine Park, who researches the health benefits of gardening and plants at the Department of Bio and Healing Convergence at Kongok University in Seoul, South Korea. In her experiment, 30 adults were asked to touch soft, smooth, stiff and rough leaves on 12 plants for 90 seconds. The plants were placed inside a box so that participants couldn't see them, allowing them to focus purely on the sensation of touch. While they were touching the plants, we measured responses in the prefrontal cortex. After touching each plant, the participants were asked to evaluate how they felt psychologically.

22:28The study looked at the prefrontal cortex, a part of the brain known to be important for regulating mood. To measure brain activity, they used an indirect marker, levels of a substance called oxyhemoglobin. Oxyhemoglobin is hemoglobin that carries oxygen in the blood. So when a brain reason becomes more active, its oxygen demand generally increased. So changes in oxyhemoglobin can be used as an indirect indicator of cortical activity. Previous research suggests that lower levels of oxyhemoglobin may signify mental relaxation. But Sine's findings didn't completely fit with that interpretation. Oxyhemoglobin concentration in the prefrontal cortex was lowest when participants touched smooth plant and highest when they touched soft plant.

23:29Psychologically, however, both soft and smooth plants were evaluated very positively. Participants had a lower concentration of oxyhemoglobin in the prefrontal cortex when touching smooth plants, and they reported a higher level of relaxation. But it also turns out that oxyhemoglobin was high when participants touched soft plants, even though they still reported high levels of relaxation and comfort. So one important finding is that physiological and psychological responses do not necessarily follow exactly the same pattern. Different textures appear to produce different types of responses. So it seems from this research, which granted is just one study with a small sample size, that touching smooth or soft plants may relax people.

24:26But it's not conclusive from this study that it was definitely the plants that were affecting well-being. Perhaps touching something like velvet or a teddy bear might have had the same effect. However, as we heard at Wisley Garden, there is lots of research to show that being in nature does have an impact on us. We heard earlier that for a plant, touch is usually a trigger for a stress response. Plants simply don't know whether it's a friend or a foe that's brushing their leaves. But there is evidence showing that touching some plants, like crops, might lead to some positive results. For us, at least.

25:05Out on the field, if we think about wheat or barley, and they're still quite young, just after winter or when you've just sown them, then we actually see if we roll the plants gently, so we don't really sort of destroy them, we just gently roll them with a soft roller. This is molecular biologist Olivier van Acken again. And then we do that a couple of times and what we've noticed is that actually the mature plants are more resistant to, it's called lodging, it's when it gets very windy and rainy and then when the sort of plants get blown over and they struggle to get back up again and that's obviously not great for the farmer because he'll lose a bunch of that yield.

25:47So we have some nice evidence that that actually can prevent some of the lodging. So that's something we're trying to actually finish off a study at the moment. Wow. So basically, if you, as you say, have this little roller and you use it to roll up and down the stems of these crops, then when they are mature, they will be more resistant to being blown over by wind and things like that. you have to sort of time it well which is why the research goes quite slowly because you have like one go a year when it's the season to try this and then okay this happened and the next year you can say oh maybe let's do it slightly differently and then oh yeah that worked that didn't work and then there's the next year so it's sort of slow going research yeah and i suppose it really speaks to the kinds of evolutionary strategies that plants have to employ because i guess they're just slower than animals, right?

26:39They operate on a different timescale, like you were saying. They can't change very quickly. And so actually, they're picking up on maybe fairly subtle cues when they're very young to push them in a different direction and kind of maximise their resources to predict the future, really, of what kind of environment they might be in. Yeah, exactly. I guess what we do by doing these treatments, I guess you could think of it like if it doesn't kill you, it only makes you stronger. Olivier's research could mean bigger yields for crops. Even though touching plants in this way might be stressful for them in the short term, it could make them more resilient in the long term.

27:17Gamit, you wanted to know if plants can sense our presence. And while they might not be able to distinguish us from other animals, or even from generic changes in light or CO2, plants are surprisingly sensitive to changes in their environment. and they can certainly respond to being touched, even if they aren't aware of who's doing the touching. As for tree hugging, there isn't much data on how the physical sensation of connecting with a plant impacts us, but there is lots of data to show that being near to plants in general can improve our health and well-being through all of our senses. Thanks for reaching out with your question.