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Drought Tolerance: A Cross-College Collaboration with Tom Sexton

Posted by Blythe Howell | November 23, 2020

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Contact Information:

For questions or comments, contact Tom Sexton via email at tom.sexton@wsu.edu.


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Episode Transcription:

Drew Lyon: Hello. Welcome to the WSU Wheat Beat podcast. I’m your host, Drew Lyon, and I want to thank you for joining me as we explore the world of small grains production and research at Washington State University. In each episode, I speak with researchers from WSU and the USDA-ARS to provide you with insights into the latest research on wheat and barley production. If you enjoy the WSU Wheat Beat podcast do us a favor and subscribe on iTunes or your favorite podcasting app and leave us a review while you’re there so others can find the show too.

[ Music ]

Drew Lyon: My guest today is Thomas Sexton. Thomas in his seventh year of a Ph.D. program at WSU working under the guidance of Doctor Asaph Cousins in the school of biological sciences, Tom is examining the physiological variation in plants, focusing on spring wheat cultivars. Using greenhouse and growth chamber studies, his research aims to quantify which traits are responsible for improved performance in crops under dry conditions while evaluating methods of high throughput screening. Hello, Tom.

Tom Sexton: Hi, Drew.

Drew Lyon: So what do you consider the main aim of your research?

Tom Sexton: Well, I guess I think of myself as a leaf level physiologist. And so really understanding the mechanisms that are regulating, you know, carbon uptake and the loss of water at the leaf level is really what I’m interested in. And so — And again in Asaph’s lab, I think we try and link those processes within the leaf to how plants are responding as a whole. And me specifically, I’m really interested in water relations. This kind of trade-off that happens. As plants are forced to open stomata to get CO2 really to grow, they’re forced to lose water as a consequence. And again under dry conditions that can be a tough trade-off to balance. And how are you regulating water loss throughout the growing season? And so that really has kind of defined a lot of the work that I’ve done is trying to really understand some of the regulatory mechanisms of that process.

Drew Lyon: Okay. So what specific physiological differences are you looking at?

Tom Sexton: So there’s a lot of things that affect how plants respond to dry conditions, what I oftentimes refer to as dry out even though, you know — An agronomist might say the drought is defined as something much more severe, but just in dry conditions in general. You know, plants may flower early to just escape those really dry periods that happen later on in the season, especially in areas like Pullman where we typically don’t see a lot of rainfall during the summer. Other traits can just be things like deep rooting that allows plants to access additional water to really avoid those physiological effects of drought especially again in places like Pullman wherein summers we do have really deep soils where they can hold those water reserves. Osmotic adjustment I think is a real interesting trait where plants accumulate more solutes in the leaves as a way to generate a lower water potential to extract more water from the soil. And also things like thicker leaves that allow them to lose less water, but potentially have more photosynthetic machinery within those leaves. And I guess the main trait — These are all things that are surrounding, you know, how plants respond to field. But I really want to acknowledge how important they are, but at the same time in the greenhouse and growth chamber studies that I do where we’re growing plants in pots, it’s a step removed from the field, and so really the key trait that we look at as we’re — that I look at in my research — Again, Asaph’s laboratory is very diverse, but what I’m really interested in is what we call water use efficiency which we define as the amount of above-ground plant biomass that crops are producing or how much of this plant can you generate for a given amount of water used. And at the agricultural level we might think of this as, you know, how much grain are we getting for a certain amount of irrigation or precipitation? And again in places where it’s not feasible to irrigate, which is large parts of the United States, you know precipitation is really the determining factor. You see a close to linear relationship in terms of rainfall and precipitation that year. And the yield. And so the ability of plants to produce without water is very low. You know, it’s really essential that they have that water. But we’re trying to find individuals that can produce more biomass for a given amount of water that they’ve used, trying to in a sense kind of break that linear relationship that we’re seeing between available water and the amount of, you know, plant biomass variable to generate. And so one of the reasons that we’re really interested in water use efficiency is because this is again what allows plants to grow in dry conditions. And I like to bring up the example of a cactus. A cactus is really good at producing biomass when there’s very little water available. But a lot of the traits that a cactus has aren’t necessarily advantageous traits in an agricultural system. You know, we don’t want to start growing cactuses because they’re not going to grow very fast relative to wheat. They may be highly efficient, but they’re not highly productive. And so that’s what our research is really getting at is how can we increase efficiency to make more, you know — more plants for limited amounts of water. But at the same time maintain high yield and maintain high productivity without limiting how much yield the plant’s really producing. And this is one of the reasons why we have different — And the readers are probably much more knowledgeable about this than I am; I should be careful here — but the reason why they’re cultivars that are generated for lower precipitation regions than higher precipitation regions because if you take those cultivars that do really well relatively under those low water conditions, and you move them to higher precipitation areas, they are outcompeted by the better-adapted cultivars. And so again there’s kind of this trade-off between efficiency and productivity. And so that’s what our research has really looked at.

Drew Lyon: Okay. Yeah. As I think about it, the — You take something, and you want it to be really efficient, but when you have those good conditions, it just isn’t the racehorse that’s going to produce what you really want. And so there has been that tradeoff, and you’re looking to try to break that trade-off or figure out how you can not have that more efficient plant that can’t take advantage of the really good conditions. Is that — Am I understanding you correctly?

Tom Sexton: Yeah. Exactly. That was a really great summary. And there’s kind of a bit of a debate in the literature as to whether we should be, you know, intentionally breeding for greater water use efficiency or whether we should breed for the exact opposite, breed for plants that are just productive regardless of if they have terrible efficiency because at the end of the day we just want yield. We don’t really necessarily care how it’s done. And so that’s where maybe how I wanted the studies that we did that we’re working on publishing now is we took cultivars that are well adapted to drought, and we measured water use efficiency over their lifetime, and we related this to the known yield that these cultivars have. And what we saw was really interesting. It was that plants that had greater water use efficiency, that were more efficiency, also were producing more biomass. They were more productive. And this was interesting and exciting and what we really found is that of all the photosynthetic and stomata traits that we looked at, the really — The driving force for what appeared to be regulating the water use efficiency was the temperature of the leaves, of the wheat leaves we had grown. And the reason for this, as we developed and looked at other people’s plant environment models — This day if you’re a scientist in the life sciences you spend most of your time looking at models. At least that seems to be where a lot of things are headed. But when we looked at the models, they described what we expected or what we observed really well. And that was that a cooler leaf is going to have a lower absolute humidity at a relative humidity of 100% relative to a hotter leaf that also has a relative humidity of 100% within the leaf. Because that absolute humidity is at lower temperatures, the vapor pressure difference in water between the inside of the leaf and the atmosphere is lower. So the rate of water movement just because of that concentration gradient I believe is lower. And so cooler leaves were more efficient, but they were also more productive because there are two different reasons that we saw leaves being cooler. One of the reasons that some leaves were cooler is they were just transpiring more, and you get the latent heat exchange just like sweating us, we cool down. But the other reason that we saw leaves being cooler was their orientation relative to the sun. Leaves that were more upright, that were more vertically oriented, you know, had less direct, you know, sunlight hitting them. And, as a result, they stayed cooler than more horizontally oriented leaves. And in either one of those cases, regardless of what was causing differences in leaf temperature, the effect being that cool leaves had more water use efficiency, and ultimately resulted in plants producing more biomass.

Drew Lyon: Okay. So the cool leaf, is that kind of what you’re looking for? To find a plant that survives under dry conditions. Or is there — Are there other traits that you’re looking for that help determine or help you decide whether a plant is going to be able to survive drought or not?

Tom Sexton: So yeah. It’s a good question, and there’s — Again that’s the challenge of us. We’re trying to relate these growth chamber studies to field conditions. And that can be really challenging because again we have simplified conditions where we’re better able to control exactly what’s happening whereas in a field condition you have so many things affecting the plant. And again this — This study was exciting because we think thermal imaging is going to be a really valuable tool for identifying efficient cultivars. And not only just because they appear to be more efficient, but also cooler canopy leaves have been shown before as being an indication that plants have better access to water, that they may also have some of these deep-rooting traits that are also advantageous. And so we’re excited to see that, more evidence that this technique could be valuable not only for identifying productivity but also for efficiency, that those two things aren’t necessarily mutually exclusive. You may be able to have both which would kind of be the ideal crop we think. And again we’re a bit removed from the actual growing of crops. And part of the reason why we’re looking at technologies like thermal imaging is because as we’re interested in this leaf-level physiology, in order to scale this up, to actually be able to incorporate some of this valuable physiological variation into cultivars, you have to have a way for the breeders who are, you know, evaluating hundreds and thousands of lines, for them to really screen this on such a massive scale can be a real challenge. And so we’re not unaware of those, you know, real logistical issues in trying to move this from the lab into the field. But the other study that maybe I’d like to highlight that’s related is a different experiment that we did where we looked at again a handful of varieties that had — In this case, we looked at two varieties that were well adapted to these low precipitation regions, and two varieties that were adapted to higher precipitation regions. And we expected to see some differences in water use where the tolerant varieties were — had more conservative water use. They were saving water. They weren’t — You know, they were kind of — We think they might be allocating it out throughout the growing season. That’s kind of what’s been speculated in the literature is that you know, plants that are adapted to low water conditions are really good at conserving water. And saving it for these critical times when they need it late in the growing season. But what we saw is once we withheld water from all these plants, it was the drought-tolerant cultivars that really were using water aggressively. They were putting out more leaves, and they had higher stomata conductance. They were transpiring at greater rates. During this first nine-day period following the — you know, the removal of watering where they were starting to dry down really severely, and this was — It was interesting to us because it was kind of the opposite of what we might have expected. But what we saw was that during the time when those plants are more aggressively using water, they accumulated more biomass, and they had a greater water use efficiency. And again I don’t want to overstate the differences between pot and field experiments, but what this indicated to us is that these drought-tolerant cultivars may not have this conservative water use that people expect, but may instead be spurring additional root growth or again, as I mentioned earlier, they may be accumulating solutes to extract more water from the soil. But it seems that productive cultivars that we’ve developed in the past, not only do they appear to be efficient, but they also seem to be aggressive users of water which is a little bit contrary to what’s been established in — At least from this study, it seems again that productivity and efficiency, you don’t necessarily have to choose. We need to breed for both these traits at the same time.

Drew Lyon: Very interesting. That does go contrary to many things I would just think about what a drought-tolerant plant is doing versus a plant, not under stress. So you’re looking also — looking at high throughput screening techniques. What are you hoping to do with that?

Tom Sexton: Yeah. As I kind of mentioned earlier, you know, we’d really like to see some of these physiologies be able to be moved, you know, from the greenhouse or the growth chambers into field studies. And again the way that’s really going to happen is through some sort of technology that’s going to allow us to identify a variation of physiology at a big scale. And maybe I’ll back up a little bit here and during the green revolution from the ’60s to about the turn of the century we saw these massive increases in yield in a lot of cereal grains as a result of these dwarfing varieties that were created. And some of them right here at WSU. I think that’s a cool bit of history. And this really steep increase at the — a steep increase in the rate of yield, you know, per acre was driven by these new traits that were introduced by these dwarfing varieties, you know some resistance to diseases, lodging resistance, and a higher amount of grain — or a higher amount of plant biomass that was allocated to the grains. But it seems like that these traits may be reaching their physiological optimums. The traits that we’ve really been selecting for have been bred to be the absolute maximum that they may be able to be reached. And again around the turn of the century it seemed like yields in wheat appeared to be plateauing, that again we’re maintaining high yields, but we — It seems like the rate of increase and those yields is not getting any higher. And so there’s — Again the research seems to indicate that we need to be again as a scientific field — We need to be looking at new traits to select for. And that’s where a lot of this physiological work is rooted in is we need to kind of understand what’s happening at the — I think at the leaf level. A lot of people would argue at the root level. And I think both is true. Both are true. But — And so again so the high throughput technology we’re looking at, as we mentioned, was thermal imaging which I think is really valuable for looking at some of those water use traits. But the other things that my research has looked at is carbon isotope discrimination which is — It gives you an indication of the relative concentration of CO2 within the leaf. And so it — What you’re measuring when you measure carbon isotope discrimination is a ratio of 12 carbon which is the everyday carbon that we see as 99% of all the carbon that’s in the atmosphere if CO2 has 12 electrons, 12 neutrons, and 12 protons. But then there’s this stable isotope of carbon 13C and this has an extra neutron. So it gives it just a little bit higher mass, and it’s stable. It doesn’t break down. And plants again, as the name implies, they discriminate against the 13C and preferentially fix 12C. And so if you look at plant tissue, you’re going to less 13 carbon than you would in the atmosphere, in the air we’re breathing right now. And this has been a tool that’s been used for I think about 40 years now. And so it’s not new by any means. And part of the — It’s valuable because it gives you an indication of how physiologically kind of active the plants are, how efficient they are in terms of how open are their stomata. How much CO2 are they getting into the leaf? But the challenge is that there’s a lot of different processes that affect this discrimination. So we’ve tried to define a little bit more in some of our research, and for example, as plants open up stomata, they get more CO2 into their leaves. They increase their photosynthetic rates. But, as a result, they’re losing a lot of water. And so a higher discrimination that occurs as a result of this greater CO2 typically indicates a lower efficiency. Well, one of the things that this carbon isotope discrimination doesn’t include is the rate of water diffusion because again you’re only measuring the carbon side of the reactions. You’re really not seeing these transpirational losses. And so trying to combine these discrimination measurements with thermal imaging may allow as a way to get a better estimate of how efficient plants are being. And so really just trying to get a better sense of really what those measurements of discrimination are indicating is part of the goal of our research. And, you know, we’ve had mixed success where we’ve seen certain varieties that are really well reflected in their carbon isotope values. And other ones that appeared to have other processes that were influencing it. And we’re trying to get a little further in describing what some of those differences are, and if there’s ways that we can use those or avoid some of those confounding influences. And again we’re a long ways away from being able to really interpret some of that data that comes back from the breeders and from just a conversation I had with Arron Carter I know he measures — He’s been measuring carbon isotope discrimination from the grain of a lot of his winter wheat for a lot of years. And again he’s told me that he isn’t able to make any valuable predictions from it. It hasn’t told him anything useful. And again to me, that indicates that there’s something else going on, that there’s other physiological effects of this — affecting discrimination that we’re not accounting for. We need to combine that with some of the measurements to really get at that physiology. And to segue a little bit, the last technique that we’ve examined in our lab is what’s referred to as hyperspectral reflectance which is the reflected light coming off a leaf that wavelengths all throughout the visible as well as the near-infrared, and then the shortwave infrared. And, as you might expect, in the visible region you get most of that reflectance coming back in the green which is why leaves appear green to us. And these measurements of reflectance are really rapid and high throughput and nondestructive of leaves. And they can tell you a lot of things about the leaf. For example, there’s something called a SPAD meter that I’m not sure if you’re familiar with, but it is

Drew Lyon: Yes. I use one.

Tom Sexton: It uses two of those reflectance wavelengths to give an indication of greenness or how much chlorophyll is there. And so again a simpler version of this exact same technology, but instead of using two wavelengths, the reflectance gives you about 1,000. And so you’re getting information about the plant’s reflectance which again can give you health and water status and nitrogen status and a whole lot of other information that we’re not exactly sure. And that’s kind of what the challenge is. There’s so much information coming back that we’re not entirely sure what it means or what it’s relating to or what’s really driving some of those signals. But again our goal from our research is that if we can develop models from all that reflectance data to make predictions of things like water use efficiency and photosynthetic capacity, you know, what is the plant’s real ability to fix carbon, we may be able to incorporate some of those measurements into these high — into these really large scale breeding programs in order to again screen for some of these efficiencies that appear to have a lot of room for improvement on them since they haven’t been selected on before just because of how challenging they are to measure.

Drew Lyon: Okay. So it sounds like you work with the wheat breeders here, and that your research may one day inform the wheat breeding process. How do you see your research, what you’re doing today, impacting wheat growers in the future?

Tom Sexton: Yeah. I think yeah. You described it well, that I — That yeah. I’d like to be able to understand the physiology well enough to, one, identify what traits need to be selected for. And there’s a lot of work from a lot of labs around the world that are trying to really hone in on what traits are going to give us the most improvement in the yield that we’re seeing under again the environmentally stressful conditions we’re seeing of water limitation as well as likely increases in temperature that will again place more evaporative demand on plants and an increase in water requirements. And so can we identify those traits? And can we develop the tools that would allow breeders to actually select for them? And so that’s where I’m optimistic that this research is able to go to improve our ability to use some of these tools to make some more informed breeding decisions. And to increase the array of traits that breeders maybe have at this disposal to select for as they’re looking at different cultivars because again they’ve got an incredibly challenging job, and you know, as you’re looking at so much information — And again this is a little bit outside of my area since I spend all of my time in the lab, but I’d like to be able to translate those findings that we find in a greenhouse to some way that we can look at them in a field setting where we can really identify what’s a great cultivar and how can we — How can we make those selections more rapidly and easier to see things that, you know, we can’t see when we’re just looking at plants? We can’t see how efficient we are. We can’t see how quickly they’re using water. But if we could, I think we could generate a better plant because a lot of the drought-tolerant traits that I talked about earlier, these are traits that maybe help survival, but do they help yield also? And again kind of going back to this idea of we want plants that are efficient and productive and yield well and have high survival. And try to combine all these in a way that maximizes plant productivity. Again it’s what I’m really interested in, and it’s what I’d really like to provide to those breeders. And again it may be a long ways off before we’re really able to conclusively say how we’re going to do that, but I’m hopeful that will happen.

Drew Lyon: I’m very pleased to see and to understand the interactions within WSU that’s not just the college of agricultural, human, and natural resources working on plants. We also have very capable scientists in the school of biological sciences, and they’re all working together to understand plants better, and that should help farmers in the long run. Thank you very much, Tom.

Tom Sexton: Yeah. Yeah. Thanks for having me. Yeah. I think it’s a great community here, and it’s been a lot of fun to work with scientists from different fields. Yeah. There’s always more to learn, and it’s fun to talk to people who have, you know, expertise in so many different areas.

Drew Lyon: I agree.

[ Music ]

Drew Lyon: Thanks for joining us and listening to the WSU Wheat Beat podcast. If you like what you hear don’t forget to subscribe and leave a review on iTunes or your favorite podcasting app. If you have questions or topics, you’d like to hear on future episodes please email me at drew.lyon — that’s lyon@wsu.edu –(drew.lyon@wsu.edu). You can find us online at smallgrains.wsu.edu and on Facebook and Twitter @WSUSmallGrains. The WSU Wheat Beat podcast is a production of CAHNRS Communications and the College of Agricultural, Human and Natural Resource Sciences at Washington State University. I’m Drew Lyon, we’ll see you next time.

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