Dr Andrea Leigh: I’m going to be bringing us back onto land now in the realm of terrestrial plants where I’m really interested in how they are coping with climatic change. I do want to acknowledge the support of the Port Augusta City Council in the ongoing support that they’re providing at the Australian Arid Lands Botanic Gardens where this research is mostly being conducted.
So yes as Bill said I am a Plant Ecologist and I am fascinated by plants. Hopefully by the end of this you will be too. I’m particularly interested in how they cope with stress and namely temperature stress.
Temperature is arguably the most important climactic factor that is responsible for limiting the distribution of organisms across the earth. So it is perhaps of some concern that we are seeing this sort of thing going on. You’ve probably seen this graph or a variation of it I think as Shauna pointed out of the Amazon Basin but on terrestrial systems our semi-arid vegetation is punching above its weight with respect to drawing down carbon but also agriculture. It’s semi-arid systems, the rangelands in Australia, are responsible for supporting our grazing industry. I’m not talking about crops, I’m talking about native vegetation in all of its diversity supporting the animals that we eat if we’re so inclined.
So pretty important systems but those justifications aside I absolutely adore these ecosystems. I love hanging out in them and yes even in the middle of summer and I have a really profound respect for the plants that live here. They’ve living on the edge of what is really biologically able to be tolerated and yet we know surprisingly little about how plants in these systems or indeed any plants but particularly in these systems cope with high temperature stress.
So think about yourselves. If you’re suffering temperature stress you turn on the air con, you go for a swim and animals in these systems can actually avoid it. So an animal in the desert it’s too hot so what do they do? They burrow underground, they will forage only at night, they’ll generally avoid these really, really high temperatures. So they can essentially run away from the heat if they want but plants can’t. They’re rooted to the spot literally and they have to basically cope with whatever climate throws at them and that includes really high temperature extremes.
Who cares? You hate plants, the pot plant in your office died because you forgot to water it. You can’t stand salad. Well let’s just bring you back to basics.
Let’s bring you back to basics. Photosynthesis is what plants do. That is their primary raison-d’être. That’s really why we care about them. So what’s photosynthesis? Again you’ve forgotten. It was a long time since you were in high school. Too bad, I’m going to race you through it really, really quickly.
Here’s a cutaway version of a leaf. You’ll notice two things. One, that it’s green which I think is quite appropriate in the green room. And green stuff is basically chlorophyll. What happens in chlorophyll, photosynthesis, so in anything that’s green, that’s living, photosynthesis is happening or can occur. The other thing that you’ll notice about this leaf is little holes in it which are called stomata and they can open and close. And so into the stomata goes carbon dioxide from the atmosphere. Water is then fed from the roots up to the sites of photosynthesis in the leaf and in the process of transpiration water vapour leaves those stomata and thanks to the energy from the sun photosynthesis occurs and bang we’ve got carbohydrates. See that wasn’t so hard.
Carbohydrates are really good things because that’s what we eat and even if you’re a sworn carnivore you are eating an animal that ate carbohydrates. So guess what, you better like photosynthesis. It’s a really, really important process. So I like photosynthesis even if you don’t so I’ll just keep talking.
So what happens with heat stress? Heat stress mainly harms photosynthetic machinery and that’s what we care about. If we’re looking at how are plants being hurt by heat stress we’re looking at what’s happening to the photosynthetic equipment and if that happens, if photosynthetic equipment is damaged we’ve got reduced growth and reduced growth or reduced good stuff that photosynthesis produces. I’m not going to go into details, good stuff is good jargon; reproduction is also impaired, productivity which is the same thing and survival. Worst case scenario, heat stress is bad enough, the plant dies.
So if we want to predict the future distribution of different species of plants across the Earth what we really need to do is understand how they’re surviving now, how they’re responding now to heat stress. So the best models generate a threshold or they use a threshold of tolerance, say heat stress in our case and they overlay that with the predicted climate for a given region, for a given species, to generate a prediction of distribution of a species across a piece of land. That’s great, these models are getting and better but one thing we’re really struggling with is pinning down this slippery thing called a threshold, the physiological threshold and in our case again we’re looking at a heat stress threshold. What we’re finding in our lab is that that threshold is a bit of a moveable feast and I’m going to just run you through that.
So this is a picture of Ellen Curtis who’s been doing a PhD with me for a few years and actually as we speak she’s going into labour, she’s about to have a baby like literally tonight which is why she couldn’t be here. She’d actually registered and then said oh sorry I couldn’t come. So this is Charles Knight who’s a colleague of ours from Cal Polytechnic University in California and together they are here measuring the stress incurred by different temperatures stresses in our temperature torture chambers on different species of desert leaves.
So what they’re doing is using a little piece of equipment here called a chlorophyll fluorimeter, you can forget that in a second, but what it’s doing is measuring the health of the photosynthetic equipment in a leaf and they measure it before stress and after stress, at a range of different temperatures. From those data they can generate what we call a thermal damage threshold which is essentially the point at which damage to photosynthetic equipment in the leaves begins to occur.
So this threshold - actually there’s a whole lot of things around that threshold that are happening. So what it means is that a whole lot of proteins are being upregulated to protect the photosynthetic equipment from getting damage and they’re also repairing damage afterwards and maintaining that threshold is resource intensive. If you think about what we do as humans, we take in food. I eat breakfast every morning so that I can walk to the train and my brain can work. The same thing happens for plants. Having a high thermal threshold is actually sort of costly in terms of resources and resources are finite. There’s not boundless resources to do this.
So if you think about an evolution, personally not so sure that God’s responsible but if God was responsible he or she would say alright there’s got to be a cost benefit analysis. I’m going to make these plants - these species are going to have a higher thermal threshold because they’re more likely to cope with higher stress and these ones, well I’m going to let them have a lower thermal threshold and shunt those resources into something else.
There’s a sort of a cost benefit analysis going on and indeed what we’ve found is that the thermal thresholds vary quite a lot even just in one snapshot in time across desert species, for example by about six degrees. So the temperature at which one plant wigs out and carks it is maybe 46 degrees and another one might wig out and cark it at around about 51 or so degrees. So it’s quite a wide variation in that threshold.
We’re also finding that that thermal threshold varies in space and time. So if we take this spatial example. I’m just going to show you a picture of a dune system in - this is north-west Western Australia. Dunes are as you imagine a big lump of sand but in the desert they’re vegetated so the plants living on that dune have much higher run off. So there’s effectively a bit of water stress, in fact quite a lot of water stress going on in these well drained systems. In between the dunes are these things called swales, a fancy word for a little depression, where the water runs into. So we’ve got dunes and swales so within quite a small space of area there’s a marked difference in water availability and therefore also water stress.
So if you think about water stress this is we are arguing also influence plants’ ability to cope with heat stress. How does that happen I hear you cry? So let’s go back to our cut away leaf. Remember I told you that there’s these little holes in the bottom of leaves out of which water evaporates. As water evaporates leaves cool. Just think about as a human, humans will cool by sweating because water has a high heat vaporisation. As it evaporates off our skin heat leaves our body and we cool down. Well plants do the same thing. They can cool down by transpiring, sweating if you like, losing water vapour out of their stomata but that can only happen if the stomata are open.
We know that in some places some species are more likely to close their stomata to avoid losing water and particularly in desert environments where water stress is going to be likely those species are more inclined to close their stomata.
Now that’s all good because you’re conserving water but just imagine what that feels like. Just think about you or I being covered in Vaseline or glad wrap in the middle of Brisbane. It’s a bit of an ugly thought. It’s not very nice. [laughter] You can’t sweat out that heat. So a leaf that has to have its stomata closed is going to get insufferably hot.
So let’s take this back to another desert scene now. We’ve got a rocky hillslope which is very well drained and we’ve got grasses on the top of that slope. They’re likely to incur quite a lot of heat stress on a 45 degree day if it’s really dry because there’s no water to keep their feet wet, to have their stomata open so they can’t cool. If you compare the same 45 degree day but in an area not very far away, I don’t know, a 100 metres away or something, we’ve got a dry river bed and here we’ve got access to groundwater, seepage, runoff and so on.
So the plants there and it might be a different grass species, for example this native lemongrass here, they’re not experiencing the same heat stress because they’ve got their feet wet, somewhere they’ve got their roots in water and they can have their stomata open, evaporatively cool. So they’re not experiencing the same temperature stress as their buddies up on the hill even though they’re both in 45 degrees. It’s a 45 degree day.
So it was our hypothesis that plants in these high water environments would have a lower thermal damage threshold than plants in well drained water stressed environments because the stress that they experience is going to be lower because if you remember that having a high thermal threshold is costly so why do it if you don’t need it. If you can cool evaporatively, have the threshold lower and shunt your resources into something else, growth, reproduction, whatever.
So that’s exactly what we found when we measured thermal thresholds for a whole range of species, over 40 arid species. We noticed that those who are native to areas of higher water access have lower thermal damage thresholds than those that are used to water stress if you will.
So plants experience heat stress in different ways depending on where they live. What about time? Does time affect how plants experience stress?
Think about as a human - I’d like you to imagine yourself at the end of February after you’ve been experiencing two months of days in the thirties and you get a 35 degree and you’re like nah, whatever, I’m just going to drink a beer and go for a bike ride at two in the afternoon because I’m good with 35 degrees. If you give yourself that 35 degree day in Spring you sort of get a bit panicky, you think you’re going to die because you’ve just been experiencing 24 degrees for the last month and before that it was 15. So suddenly you haven’t acclimatised so that 35 day feels really bad. So does the same thing happen for plants?
Just think of plants in the desert. It’s been raining. It’s been 20 something degrees for the last month. Are they soft compared to that same area in summer where you’ve been having drought stress, it’s been 43 degrees for two weeks in a row? I think that there’s a good likelihood and in fact that’s exactly what we find, that plants will experience stress in a different way and accordingly will ramp up the thermal damage threshold across the year and they do this by a process of acclimatisation.
What this means is that plants need to experience some sort of stress in incrementally, ever increasing amounts throughout the year to kick start those processes of thermal protection. All the proteins start turning on as soon as they experience a bit of stress. So it just keeps happening. Their thresholds get pushed up by up to five degrees between winter and summer irrespective of the difference in threshold among species at each time. On average thresholds increase by five degrees and that’s just amongst the desert plants that we’ve measured.
So a little bit of stress is a good thing but how much stress is enough stress versus too much stress? This is another Bureau of Meteorology graph from 2012 in October, a Spring day and the bars in orange are the high temperature extremes experienced in the Northern Territory relative to the grey bars which is Australia. You can see that there was a heatwave, actually this was 42 degrees in the Northern Territory in this particular year. If you look carefully you can see that there hasn’t been a whole lot of slow incremental increases in that temperature. So look I wasn’t there, I didn’t get a chance to chat with the plants in the Northern Territory but if I was I reckon they’d be saying - freaking out, I’m dying here.
So let’s just think about this. We’ve got the basic tips for a plant in stress management. We’ve got long term adaption, evolutionary processes going on which are selecting plants to have good genes in areas that are more prone to heat stress. So that would be a good tip is make sure you have good genes. You don’t have much choice in that though.
So if you happen to be evolved to places where you’re going to get less water stress and therefore heat stress and we get more droughts which we’re certainly predicted to have in some parts of the world then you’re kind of in trouble in areas like in a desert where there’s flood plains and so forth where you normally have a lower thermal threshold then you’re going to be in trouble.
The other idea would be to experience a little bit of a hardship along the way. Acclimatise by experiencing some stress which will give a short term shot in the arm if you will to increase your thermal threshold and cope with the high stress when it comes but there’s a caveat there as well and that is if we have a sudden heat wave without an incrementally increasing ambient temperature things could get a little bit messy for you.
So plants distribution in the future what’s it going to look like? Well I still don’t know but I can tell you that we’re starting to get a little bit closer to understanding the thresholds that plants have. I’ve told you that thermal thresholds vary among different species and I’ve told you that they vary spatially and also temporally and with this kind of information we’re getting a much better handle on being able to predict the distribution of different species under a future climate scenario.
That’s all I have to say just now so thank you for listening.
12 May 2015
plant ecology, leaf function, climate change, temperature extremes
Andrea Leigh looks at leaf function and survival at extreme high temperatures. High temperature extremes are predicted to increase in both intensity and frequency in the future. Learning how and which plants will survive these increased extremes will be crucial.
About the speaker
Dr Andrea Leigh
Andrea Leigh is a plant ecologist with a particular interest in arid environments, primarily in Australia. Her research looks at leaf function and survival at extreme high temperatures. High temperature extremes are predicted to increase in both intensity and frequency in the future. Learning how and which plants will survive these increased extremes will be crucial.
UTS Science in Focus is a free public lecture series showcasing the latest research from prominent UTS scientists and researchers.
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