Optimizing plant resource-use efficiency in agricultural systems with Dr. Asaph Cousins

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WSU School of Biological Sciences

Dr. Asaph Cousins’ research and publications

PNNL-WSU Distinguished Graduate Research Program

Contact information:

Dr. Asaph Cousins, acousins@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.

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My guest today is Dr. Asaph Cousins. Asaph is a professor and associate director for research in the School of Biological Sciences at Washington State University. He is also the coordinator of the PNNL-WSU Distinguished Graduate Research Program. His research studies the interaction of photosynthetic carbon dioxide assimilation and water loss via transpiration. His current research focuses on determining leaf biochemical and structural mechanisms influencing photosynthetic water-use efficiency in diverse grasses and crop species, for example, wheat, rice, sorghum, and maize. Hello, Asaph.

Dr. Asaph Cousins: Hi, Drew. Thanks for having me.

Drew Lyon: Thanks for coming in today. So, I often hear people talk about plant resource-use efficiency–nitrogen-use efficiency, water-use efficiency. What is your definition of plant resource-use efficiency?

Dr. Asaph Cousins:  Well, I think of it maybe in two general terms. So, one is how efficient are the plants at extracting resources from their environment? So how effective are the root systems and structures able to access water that’s deep in the soil or nitrogen or other nutrients that are in the soil. But then a lot of what we focus on is how efficient are the plants using those resources in terms of acquisition of carbon or taking up carbon dioxide from the atmosphere.

And then, of course, there’s the downstream process of using that assimilated carbon and converting that into photoassimilates that can be used for yield, whether that’s grain yield or biomass production.

Drew Lyon: Okay. So really efficiency throughout the entire system.

Dr. Asaph Cousins:  Yeah, from start to finish.

Drew Lyon: Okay. What are some different ways to define water-use efficiency, let’s say?

Dr. Asaph Cousins:  There’s a wide range of ways to define water-use efficiency. So, you can think of it in terms of how much water is applied to an agricultural system and how that translates to how much yield is harvested from that plot. But you can also break it down into how efficient is a plant in terms of using water. And so, a lot of what we focus on are various aspects of water uptake by the plant and then how that water is then translocated throughout the plant system, and then how that water is used to allow stomatal conductance or the stomata to open up so that carbon dioxide can come into the leaf to be assimilated for photosynthesis.

So, for us, all those aspects of water-use efficiency are linked. And so, if we can look at traits that are influencing leaf-level components of water-use efficiency, how much water is lost by the stomata via transpiration versus how much carbon dioxide is being taken up, that correlates and translates to whole plant water-use efficiency, which ultimately leads to how efficient is the application of water to an agricultural system in terms of yield?

Drew Lyon: Are there tradeoffs between water-use efficiency and maybe something like yield? Are plants that are water-use efficient always the best yielders?

Dr. Asaph Cousins:  Yeah, that’s a great question. So, water-use efficiency can come at a cost in terms of how much biomass is produced or how much grain can be developed. And so, what we’re looking for are ways to not necessarily minimize how much carbon dioxide is being taken up in terms of water-use efficiency but to minimize how much water is lost and maintain that carbon dioxide uptake–because water-use efficiency in itself can have a drag on yield, for sure. And that’s not exactly what farmers are looking for.

Drew Lyon: Okay. [I] hear a lot about climate change these days. How does climate change influence how much water a plant uses and how much water is available in the soil?

Dr. Asaph Cousins:  Yeah, so it’s a complex question, but, you know, simplistically thinking about climate change in terms of increases in temperature, increases in atmospheric carbon dioxide–both of those can influence the canopy temperature and how much transpiration is needed to cool the canopy. But it also changes the vapor pressure deficit between the plant in the atmosphere around it–so more water is oftentimes extracted from the from the plants, so plants have less water available to them. And then also the soil is oftentimes more arid and either precipitation patterns changing or an increase in soil temperature can lead to increases in evaporation of water from the soil. So, all of those things are interacting to oftentimes negatively impact the availability of water to plants, but also how effective the plants can be in controlling how much water they lose.

Drew Lyon: I know when I was working in Nebraska, in the central Great Plains, that I’d see water-use data for crops grown in North Dakota versus Nebraska versus Texas–and it just took more water to grow the same amount of yield in Texas as it did in North Dakota. So, I assume that has to do with evaporation demand or the environment and just keeping the plant cool rather than using it for yield? Is that a correct interpretation?

Dr. Asaph Cousins:  I would tend to think so, yeah. The higher temperature–growing season temperature–probably influences how much water is just lost by evaporation from the soil, so water is not available for the plants to take up. But also needing to maintain, you know, high rates of transpiration helps the plant canopy essentially cool itself to maintain an optimal temperature. So, in those hot environments, oftentimes there’s a lot of transpiration that’s going on that leads to evaporative cooling.

But also, again, getting back to the vapor pressure deficit and how much humidity is in the air that can influence how much water is being extracted from the plant, essentially.

Drew Lyon: Okay. And you know, one thing about the Pacific Northwest environment’s quite different than a lot of the country in that we basically don’t have much moisture in the summertime. So, when temperatures are high, they really are relying on stored soil water for cooling. So that makes it a real challenge sometimes in this environment versus somewhere where they do get rainfall in the summer.

Dr. Asaph Cousins:  Yeah, absolutely.

Drew Lyon: So, what leaf traits influence rates of photosynthesis and therefore water loss by transpiration?

Dr. Asaph Cousins: So, we look at it in two components. One is structural–and that is how does the leaf structure, its anatomy, how do the position of cells within the leaf and the chloroplast that are doing a lot of the biochemistry that drives photosynthesis, how are they oriented inside that leaf and how does that impact the ability of carbon dioxide to actually move from the atmosphere to the chloroplast where that carbon is being assimilated.

And so, the structural component, I mean, you can think of it as, you know, walking through your house, right, if there are many doors that you have to open and close, you know, it becomes difficult to get from point A to point B. So the same is true for carbon dioxide as it moves, you know, into the leaf and then through the leaf, the air space inside the leaf. But then it has to solubilize in the liquid inside the leaf and move across the cell walls to actually get to the site of where photosynthesis is occurring.

So, we’ve been looking at how variation in that leaf structure influences the ability of that carbon dioxide to actually get to the site of photosynthesis.

And you can think of it as there’s a concentration drop. You know, the atmosphere may have a high, relatively high concentration of CO2 available to it, but the concentration of CO2 that’s at the site of that uptake by photosynthesis, there’s a large drop in the concentration. And that gradient in carbon dioxide is influenced by the anatomical traits of the leaf and how hard it is or the diffusivity of that CO2 to move into the leaf.

So, we look at those type of leaf traits, but we also look at the biochemistry. So how efficient are certain enzymes at taking up carbon dioxide? Do they have different kinetic properties or affinities to carbon dioxide that would be advantageous so that they can be more efficient at taking up whatever carbon dioxide is available to them?

Drew Lyon: I remember learning about C3 and C4 plants in earlier study in graduate school and they have different leaf structures in themselves and then bundle sheaths, I think I recall, on C4s. So that affects it. But even within C3 and C4 plants, are there different setups in the leaf that influence how effective they are?

Dr. Asaph Cousins: Absolutely. Yeah. I mean, even within one species there can be lots of variation and so different cultivars, for example, like that influence aspects like leaf thickness, for example. And you know, those are oftentimes traits that will be selected for in agricultural systems–well the leaf thickness can have a real impact on how many cells there are or the size of the cells, the thickness of the cell wall that surround the chloroplasts or the cells that are driving photosynthesis.

And so, all of those traits can impact the availability of carbon dioxide. And the C3/C4 comparison is, you know, maybe one extreme, but you know looking at monocots versus dicots or eudicots, there’s dramatic differences in leaf architecture in leaf structure. And just within the grasses, right–I mean, just between wheat and rice. I mean the position of the cells, the position of the chloroplast, all those things are quite variable and they can be dynamic, meaning that they can change depending on the growth conditions in the environment that they’re growing in.

Drew Lyon: Okay. So, like many things in science, so we tend to simplify it. But the truth is things are a lot more complicated than many of our simplifications might seem.

Dr. Asaph Cousins: That’s what makes it exciting, right? Yeah.

Drew Lyon: So how can studying natural variation of plant traits help promote new ways of selecting these advantageous traits to increase crop yield and resource-use efficiency?

Dr. Asaph Cousins: Well, one way to think about it is, you know, agricultural systems have a limited germplasm that they utilize for food production and there’s a lot of natural variation out there in the world in terms of ways in which plants have evolved and adapted to either large-scale changes in the environment or even microenvironments that they find themselves in.

And so, trying to think about some of those traits that would be advantageous in terms of resource-use efficiency and acquisition of resources and utilizing that for reproductive traits or biomass accumulation allows us to think about those traits in ways that aren’t available in germplasm that is currently being used in agricultural systems. And so, there’s ideas of trying to [transfer] some of these traits from, you know, wild relative, let’s say, of certain species to see whether or not those would be advantageous. Because oftentimes, depending on the historical context, agricultural systems may not necessarily be selecting for resource-use efficiency because oftentimes, as you mentioned previously, that can come at a drag in terms of the yield.

But if we can integrate those resource-use efficient traits from, you know, natural variation, let’s say, that’s out there, that will potentially allow for agricultural systems to not lose the productivity, but to increase how much productivity they get relative to the resources that are required.

Drew Lyon: Okay. Well, very interesting. If our listeners want to learn a little bit more about your research, is there someplace they can go to see what you’re doing? Do you have a website?

Dr. Asaph Cousins: I have a very outdated website, yes, that’s they’re welcome to go look at. I’m in the School of Biological Sciences at Washington State University and so they can find my website.

Drew Lyon: Okay. We can put that in our show notes for our listeners if they want to look at more.

I think it’s very interesting how more of the basic science that you do in the School of Biological Sciences has implications for a lot of what we do in the College of [Agricultural, Human, and Natural Resource Sciences].

Dr. Asaph Cousins: Absolutely.

Drew Lyon: And I appreciate you sharing some of that with us today.

Dr. Asaph Cousins: Yeah, you bet. And I think that that’s a really important point to make that, you know, the basic science that, you know, many of us do in terms of looking at aspects of resource-use efficiency, photosynthetic efficiency, there’s strong application in agricultural systems. And a lot of the work that we do is funded by agencies like the, you know, the Department of Energy that is looking at biofuel production and also the USDA in terms of wheat and maize and rice. And so, there’s a lot of connection between what we do and translating that to larger crop systems.

Drew Lyon: All right. Well, thank you for sharing some of your information with us today. [I] enjoyed having you on.

Dr. Asaph Cousins: Yeah, thanks for having me.

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 podcast 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.


The views, thoughts, and opinions expressed by guests of this podcast are their own and does not imply Washington State University’s endorsement.

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