Transcript:

 

Brad Newbold 0:00
Hello, everyone and welcome to office hours with the METER Environment Team. Today’s session will focus on canopy instrumentation. And we’re shooting for about a half an hour of Q&A with our experts, Chris chambers and Jeff Ritter, whom I will introduce in just a moment. But before we start, we’ve got one housekeeping item. If you’re watching this video and you think of a question you’d like to ask our science experts, we encourage you to submit your questions on our website at METER group.com. Someone from our science and support team will get back to you with an answer via email. All right, with that out of the way, let’s get started. Today, our panelists our application specialists Chris chambers and Jeff Ritter. Chris chambers operates as the Support Manager at METER Group. He specializes in ecology and plant physiology and has over 13 years of experience helping researchers measure the soil plant atmosphere continuum. Jeff Ritter is the product manager for plant canopy and atmospheric monitoring instrumentation here at METER. He earned his master’s degree in plant physiology from Washington State University, where his research focused on leaf level gas exchange and the impact of plant biochemistry on the measurement of the global carbon cycle. Prior to working at METER, he held a research faculty position at Washington State University in the Department of Crop and Soil Sciences. All right. Thanks, you guys, for joining us today.

CHRIS CHAMBERS 1:17
Thank you, Brad. Experts. I hope we sound smart then. All right. Thanks for joining us, everyone. We’re going to talk about just some topics that come up with plant canopy specifically. And we’ve got a few devices that help with that. So let’s jump right in with our first question. What is stomatal conductance?

Jeff Ritter 1:43
Yeah, that’s a good question, a good place to start when we’re talking about plant physiology. And especially if we’re looking at leaf level gas exchange to understand not only what’s stomatal conductance is, but it’d be beneficial for us to take a step back and talk about what what stomata are. But in general stomatal, conductance, the easiest way to think about it is the ease at which gases such as water vapor, or co2, can diffuse into or out of a leaf, there are pores on the leaf called stomata, and kind of like the pores on your skin, except that plant has really dynamic control over those pores. So in certain conditions, conditions, they can open those up, allow for a lot of gas exchange other conditions, they want to close those down,

CHRIS CHAMBERS 2:32
And in a minute to minute timescale, they’re the plants primary way to conserve water, right?

Jeff Ritter 2:38
That’s right toto either conserve water, or in conditions where they might not care about water, you know, in order to maximize how much co2 they’re able to use for photosynthesis.

CHRIS CHAMBERS 2:49
And the trick here is that if you know if the atomata are open, then it’s losing more water. But if they close them up to conserve water, then they’re not letting any carbon dioxide in for photochemistry. So they can’t make any food. And so it’s really a tricky, tricky, tricky trade off that every terrestrial plant has its own strategy for…

Jeff Ritter 3:11
It’s a fundamental push and pull of keep your somata open. You know, you can do a lot of photosynthesis get us to in but you’re losing a lot of water as well.

CHRIS CHAMBERS 3:20
Yeah, sure. Let’s back up one step more, because I, I actually, generally think you find resistance to be a lot easier to think in. And I know it’s kind of old school, that’s fine. I prefer conductance. So we’ve got one of each, and there’s a bit of a preference there. And conductance and resistance are the inverse of each other. Right? So resistance is going to be one over conductance. That’s right. And I maybe it just comes down to the math because I find the math to be a lot easier to work with intuitively. To use resistance.

Jeff Ritter 3:57
Yeah, and you will even with using conductance, you’ve got to convert to resistance to use law, the equations, but I think to make it easier, at least they don’t have one over g plus one over g divided by one over g. Intuitively conductance makes more sense to me, but resistance makes the equations a little bit easier to look at it.

CHRIS CHAMBERS 4:16
Great. So, um, why would you want to measure it?

Jeff Ritter 4:21
Why would you want to measure stomatal conductance? Well, symbolic inductance is it’s kind of a measurement of how well the plant is able to breathe. So a plant doesn’t breathe like we do when they don’t have lungs and they’re not actually actively inhaling any air, it’s completely a diffusive transport. So if you’re able to measure small conductance, you can get a sense of yield of plants so you can use that in models for for assimilation, carbon fixation. You can also look at water loss of the plant it’s important to know know that for how much water the plant is going through, it’s also important for if you’re trying to monitor stress levels of your plant. So small conductance is a big indicator for stress not only for water stress, but also plants can close their stomata for other stress related events

CHRIS CHAMBERS 5:15
Particularly in that moment in that small timeframe, where if they’re responding to the VPD, or the vapor pressure deficit, you know, the gradient of how much water you’re going to lose, right, the pressure gradient, then the stomata are the primary way to protect against, like rapid changes, in, in that, like a hot afternoon or something like that.

Jeff Ritter 5:40
So being able to understand, you know, how much water your plant is going through, and how much stressing the plant out, stomatal conductance is at least a great starting point for that.

CHRIS CHAMBERS 5:50
So how would you use stomatal conductance calculate water loss, we’re talking about leaf level, when we’re doing something like that, right?

Jeff Ritter 5:57
Right. So stomatal conductance is one parameter that’s important for how much water your plant is using. But there are other things that go into that you mentioned VPD. So the actual concentrations of water, so the concentration of water vapor in the leaf versus what’s outside leave, play a role, but also need to know the total conductance to water loss. So that’s stomatal conductance, and another variable called boundary layer conductance. That’s right now boundary layer conductance, that is, essentially a, there’s a small boundary on the surface of the leaf that impedes the flow of gases through there. And that’s going to be based on wind speed, as well as some morphological aspects of the leaf.

CHRIS CHAMBERS 5:57
And that’s really easy to overlook, and really tricky. I mean, most people model it right and don’t actually measure boundary layer conductance,

Jeff Ritter 6:50
Yeah, it can be a littletricky to measure, we used to do it with, you know, a saturated filter paper technique, where we’ve got to get to know you know, fairly precisely what your, you know, characteristics of your leaf are right and model that with different sizes of filter paper, a different wind speeds, and basically have a lookup table to choose those, so you can directly write, but it’s much easier to use a biophysical model or as long as you know, the wind speed, and some general characteristics of your leaf, the leaf width, and kind of some some leaf shape plays into that, too. So the morphology is going to play a big role in there. That’s right. Yeah. And that’s, you know, plants will use boundary layer a lot to conserve water. So, they will have some plants will have their stomata recessed down into pits. And that helps to conserve water, some will have trichomes around their stomata to to reduce that wind speed there. So it is important to make sure that you’ve got a good boundary layer conductance value or measurement.

CHRIS CHAMBERS 7:45
Or at least know those conditions where it’s not relevant.

Jeff Ritter 7:48
That’s right. Yeah, what sort of conditions are those that won’t be relevant,

CHRIS CHAMBERS 7:53
Um, generally low wind speed days, right? Because your boundary layer isn’t really making making a difference, then it’s all diffusive transport right through there. So it’s just an extra length, right? Of diffusion.

Jeff Ritter 8:06
Yeah. And so understanding that, you know, winspeed plays a big role. So you need a good measurement of wind speed right at the leaf level. So if you’re measuring wind speed, outside of your canopy outside of a stand of trees, it’s very different if then you go into a leaf deep in your canopy, that wind speed is going to be very different from the right hand side of it. So when you’re making leaf level measurements, you need to make sure you have leaf level parameters go on with it.

CHRIS CHAMBERS 8:29
That’s one reason why it’s tricky to scale leaf level measurements up to up the canopy scale. People ask us all the time, we want to do it. Yeah, I’ve got leaf level measurements, why can’t I just treat the canopy like a big leaf?

Jeff Ritter 8:41
Well, you can to varying levels of success. There are what they would call, you know, big leaf models, which is basically you measure a couple of leaves, and then you treat the whole canopy like a big leaf, right. But there are a number of problems that play into that. One of them is just how dynamic leaf level control is to model conductances.

CHRIS CHAMBERS 8:59
Sampling sampling is going to play a role

Jeff Ritter 9:00
Sampling issues is huge. If you are measuring stomatal conductance on leaves at the top and the bottom of the canopy, they’re very different light conditions and stomatal conductance and a lot of species doesn’t respond linearly to like conditions, right? So you can’t just treat it all as one big leaf. So, there are other methods that you can get at things like water loss at a canopy level, sap flux, Eddy Covariance. So that’s not to say it can’t be attempted, but as varying levels of success

It hardly it never matches up with the sap flux desert or the Eddy Covariance right. What are some methods and how do they compare?

some methods of measuring stomatal conductance. So, you know, this is a measurement that people have been attempting for a long period of time. Some of the earlier ones are what we would call it a mass flow approach where they actually pressurize one side of the leaf and then look at that pressure drop. And as long as you have a good seal that pressure drop should be related to stomatal conductance, but there’s a lot of problems with right. And ultimately they found, you know, you can see whether you have open stomata or closed stomata, but you can’t get very good values from that. Now the thing to keep in mind with all of these methods is that, like we were saying, stomatal conductance is a very dynamic variable. So anytime you’re changing parameters around the leaf, or you are putting something up against the leaf, you run the risk of the plant responding to that you try to get your measurement and move on. So if you apply a big slug of pressure to the leaf that can force some of those stomata closed itself. Another one that people have tried is what you call a dynamic porometer, which that’s fairly simple. You’re putting steeled cuvette around the leaf, where you’re measuring relative humidity pump dry air into there and wait for that relative humidity to rise to a certain preset value. That has some drawbacks, and that it can take a while to get to that preset value. And you’re changing lead conditions, right? If that lead is all of a sudden, you know, exposed exposed to bone dry air, the stomata are gonna start to close, right. So if it doesn’t get back up to its present value quickly, the measurement itself is impacted.

CHRIS CHAMBERS 9:55
And the rate of water loss is one of the main feedbacks for leak closers tomato, so it might not necessarily be VPD. The VPD reeled that out didn’t I? So it might not necessarily be VPD that’s driving the water loss. Although generally is but the water loss is going to be the main, the main response variable, rather than just a vapor pressure deficit,

Jeff Ritter 11:43
You then have what you would call a null balance porometer where there’s an active pump of air of dry air over the leaf.

CHRIS CHAMBERS 11:53
Is that just matching the fluxes?

Jeff Ritter 11:54
It’s just trying to keep a preset humidity on the outflow. So from that, by measuring the flow of air that that’s required, you can calculate stomatal conductance.

CHRIS CHAMBERS 12:07
And then there’s steady state porometers.

Jeff Ritter 12:08
Steady state as well, which is like the porometer that METER makes, the SC-1. And that’s actually rather than having one relative humidity sensor and waiting a certain amount of time for it to reach a preset. It has two relative humidity sensors in series. And so by measuring the flow of the diffusion of water vapor over a fixed path length, we can measure flux that way just using the biophysics, it’s right. And there are some other methods as well similar to no balance parameter, if you’ve got a system that is actively pumping air over the leaf and you’re measuring the concentration before and after. So long as you know, the boundary layer conductance you can get stomatal conductance that way that’s, that’s generally in in larger scale gas exchange systems. So and there’s there’s pros and cons to each I mean, we talked about some of them quite a bit. The most important thing with most porometers is the speed at which you’re taking the measurement, the speed at which they actually are clamped onto the leaf to reduce any impacts measurement has something like a steady state measurement that you can do in 30 seconds or less. That’s gonna be just fine. If you’ve got to clip it on there for five minutes, then all of a sudden, you don’t know what’s going on with it or controlling the environment within your leaf. So it says right mimic what you’re seeing outside. That’s right. So anything that you can flow air through, you can control the lights flow air through relative humidity, that’s right temperature. Yeah, so what you’re going to use, it’s going to depend dependent on you know, in some ways, your your budget, and what you have access to.

CHRIS CHAMBERS 13:48
And also the complexity and of the instrument you hope to work with, right? What are some best practices when measuring stomatal conductance? So to get good data, one thing that jumps out at me looking at any stomatal conductance ver- dataset, is the variability, like within a plant. Within a plant within species, you can you can really get a high spatial and temporal variability with stomatal conductance.

Jeff Ritter 13:49
Yeah. So, but there’s a lot that goes into that. I mean, it’s, there’s some variability and you know what instrument you’re using. But every plant every species itself is going to have varying levels of control species to speak or individual, to individual leaf to leaf, right. So I have found that a lot of a lot of plants that are used in agriculture, especially any ones that are clones of each other, they’re going to have very low variability leaf to leaf. A lot of plants that you find out, you know, what we would call wild type plants, plants that aren’t agriculturally important. They can have very high variability, the amount that you sample is going to have to be dependent on how much variability you’re seeing. So keep that in mind when you go out. But also keep in mind that every species kind of has its own baseline for stomatal conductor’s you can’t go out and measure a well watered nut tree. And then compare that to some prairie grass and say, Oh, this progress must be stressed, because it’s got very low conductance must be closing its stomata.

CHRIS CHAMBERS 15:20
That might very well be a C4 plant as well. And so completely different water carbon concentrating mechanism that uses less water.

Jeff Ritter 15:27
That’s right. So you need to know something about your plants and get a sense of what it means for your plants to be happy or stressed.

CHRIS CHAMBERS 15:34
And compare like the like, shade leaves versus sun leaves, you don’t want to compare, even if they’re getting the same irradiance, they were produced in under shady conditions or some conditions, your leaf anatomy is going to be completely different, or even time of day.

Jeff Ritter 15:49
Or even time of day. So your plants, you know, even if you were to control the light condition around your leaf, plants do follow a certain set circadian rhythm where they will start closing down at the end of the day, keep in mind leaf age can play a role as well, if you are looking at a leaf that is starting to senesce versus a fully, you know, a new newly emerged leaf that can also play a role. So just make sure that you kind of understand what your question is, and what you’re trying to measure before you go out. So you’re not, you know, comparing things that shouldn’t be compared directly.

CHRIS CHAMBERS 16:23
And having that baseline is is really helpful. You know, once you’ve got a baseline of how your plants are behaving, then you have a high VPD day, you think everything’s shut down, and you go out and put the measurement on it and they’re wide open, then you’re plant, even though you’ve got a high vapor pressure deficit, the plant, it’s still likely has plenty of water, and it’s just doing its thing.

Jeff Ritter 16:44
So I would recommend going out, you know, measuring around the same time each day having some record of the light level, and time of day that you’re measuring. And you know, as much metadata as you can position of the leaf, where on the plant it is, etc. What about in a specific instance of like a vineyard?

CHRIS CHAMBERS 17:09
Yeah, so just to just to get kind of on the same page. Vineyards generally operate at a irrigation deficit, right, where you don’t want them to be fully watered, because it affects the the fruit quality. So it’s a case where you don’t want just maximum production, you don’t want to keep everything well watered and cranking all the time. And at some point, you want to reduce the water, have them, have them work a little bit at, you know, resource allocation so that you know, you’re just not flooded with water. And so the SC-1, something like the SC-1 can be really helpful for determining whether or not you’re over watering, or underwatering. If everything is completely shut down, you might not want it to be there either.

Jeff Ritter 18:04
Right. And I think what’s important to keep in mind with something like a vineyard is when you’re not just trying to measure the stress, but you’re trying to manage that stress too. So like you’re saying with irrigation in deficit to try and force those plants some amount of stress. And I’m not a viticulturalist. But you know, different varieties of grapes even are going to need different amounts of stress to get optimal fruit quality. So you just need to know what what your plants should be at when you go out there. And so a white grape I don’t think requires as much stress is something like a red wine grape.

Speaker 1 18:37
And there’s a lot of research out there too. So it’s you can find a good starting spot in the literature. What is leaf area index? And why would I need to measure it?

Jeff Ritter 18:49
Sure. So leaf area index or what we abbreviate to LAI is it’s a measurement of

CHRIS CHAMBERS 18:55
basically the number of layer of leaves. You have over a certain…

Jeff Ritter 18:58
Yeah, so so leaf area index is it’s it’s a measurement, basically, of leaf area in your canopy.

CHRIS CHAMBERS 19:04
Projected leaf area.

Jeff Ritter 19:06
That’s right. And so you know, we take it on a square meter basis in the canopy. So you can essentially look at how much leaf area is there at least modeled from whatever parameter you’re using. So there are different ways to get at it. Yep. You know, the more old school technique and still a decently accurate technique, you could do some direct sampling, or you actually do sampling. That’s right, you can actually take leaves off and then run through a device that measures how big they are. And then from that you can get out how much how much leaf area is in that canopy. There’s a lot of indirect methods as well that are much less labor intensive, some of which are based on reflective light, where you can put something above the canopy and look at how much light is being reflected back. Or you can use satellite based techniques. See where it’s actually looking at light reflected back up to the satellite. And so those can be great for long term continuous monitoring. But they also have some issues with with resolution and what happens when the canopy is canopy saturation, that’s right because some that reflected light kind of saturates out your canopy continues to densify. So another great technique is- technique is using intercepted lights. So basically, if you know the light conditions at the top of the canopy, and underneath the canopy, you can get a sense of how much light this absorbs. And then just knowing a little bit about your canopy architecture, the leaf, leaf distribution you can get at the leaf area index.

CHRIS CHAMBERS 20:44
And how much light is intercepted is directly correlated to biomass production. Right? Right. So that is one of the most useful things that we bury index, and one of the quickest ways to get that on a difficult canopy without destroying it, is to estimate its intercepted radiation by the canopy.

Jeff Ritter 21:02
That’s right.

CHRIS CHAMBERS 21:03
So best practices for sampling with the LP-80. And one of the main things to keep in mind is that canopies are extremely variable, we’ve already talked about that a little bit with stomatal conductance and it’s no different for light interception. And that’s why you see generally instruments measuring over an area or a transect, or something similar like that, to kind of capture the variation density, in the canopy. So sample sample sample.

Jeff Ritter 21:36
And also be aware of, you know, the sort of canopy you’re measuring is going to play a big role, the makeup plan going in, you’re gonna have very different plan, if you’re measuring in row crops versus going to standard trees, if you’ve got to, you know, you don’t want to be hiking for an hour and your standard trees, and then you know, the only measurement of the light conditions at the top of the canopy they were an hour ago, well, those are probably changed. So you need a good way to make sure you’re making almost simultaneous measurements of both and so you know, just make sure you have a plan going in based on the canopy that you’re using.

CHRIS CHAMBERS 21:38
And the light, the light conditions do make a big difference. If you’ve got a really cloudy day, you have a lot of diffused light, the light coming in from the sides is going to be this kind of give you a different give you a little bit different picture of the canopy. As opposed to just a direct beam, that’s where it’s just the canopy in between you and the septometer that’s, that’s influencing your reading.

Jeff Ritter 22:39
Yeah, and there’s, you know, there’s a couple of plant specific things that you need to keep in mind when you’re using something like a septometer, you need to know something about the canopy architecture, you need to know whether it has very horizontal leaves or vertical leaves. A lot of plant canopies are going to have kind of a default value for that. And so you don’t need to worry about it too much. But right if you’re if you are working in a, especially in some sort of a mono crop where strawberries or something along those lines, make sure you do some research into what sort of leaf area angles you’re seeing.

CHRIS CHAMBERS 23:15
And your leaf area, your leaf angle distribution can change with life stage of the plant. That’s right. So if you’ve got a very young plant, it might be have a very different canopy shape.

Jeff Ritter 23:26
Corn is a great example of that where it’s highly vertical when it’s young. And then as it ages, it becomes very horizontal in places.

CHRIS CHAMBERS 23:33
So why would you want to use a leaf wetness sensor to measure leaf wetness directly rather than just using relative humidity near the dew point as a proxy?

Jeff Ritter 23:46
Well, I mean, one of the things that’s going to drive, plant disease, pathogens, those sorts of things, it’s going to be the actual water on the leaf itself. So you want to have a more precise measurement of leaf wetness in order to do a lot of those disease prediction.

CHRIS CHAMBERS 24:06
Especially for models.

Jeff Ritter 24:08
Especially for a lot of those models, they require that input. So it’s difficult to predict when water is going to be deposited on the surface of the leaf when it’s going to condense on the leaf. It is far easier if you have a good instrument to do so to measure it.

CHRIS CHAMBERS 24:28
It’s far more reliable.

Jeff Ritter 24:29
That’s right, it’s going to cause less errors in your model. And when those models are, you know, leading decisions to how- when you should spray when you should make certain management decisions, then it’s important to get it right.

CHRIS CHAMBERS 24:45
And some relative humidity sensors the accuracy decreases the closer you get to the dew point to as you call it 95% relative humidity and to be condensation risk or 100.

Jeff Ritter 25:01
Right. And so it’s with something like a leaf wetness sensor, you don’t have to care. Right? Right. It’s just whenever that whenever the water actually condenses.

CHRIS CHAMBERS 25:09
And gives you a better approximation of how long the leaf is wet too, because it includes the drying cycle as well, right?

Jeff Ritter 25:16
To do IRT, I just really didn’t really touch on the importance of leaf temperature.

CHRIS CHAMBERS 25:22
Yes. And leaf temperature is extremely important for like, it’s a really, if you’re going to boil if you are going to boil plant canopy, like light status, or photo protection status down to one variable without measuring something crazily complex, like solar induced fluorescence or something like that. But canopy temperature is one is one of the single most useful variables, I think

Jeff Ritter 25:54
It’s definitely up there. And it is easier to measure that on a canopy level than right something like measuring stomatal conductance where you’re going from leaf to leaf.

CHRIS CHAMBERS 26:03
So an infrared thermometer can give you that temperature and especially when you compare it to air temperature.

Jeff Ritter 26:09
That’s right. And it’s very important to understand the control that a plant has on leaf temperature, you can have a plant with, you know, it’s leaves are out in the bright sun on a hot day. But the leaves are actually quite a bit cooler than air temperature. If that plant is well watered, all of a sudden that plant is facing a drought, it’s got to close its stomataso it can conserve water. And now what heats up above air temperature. That’s right. So if you are taking a VPD measurement that is just based on air temperature, that could be off significantly to a leaf that can be several degrees different from the ambient air. So whenever you take stomatal conductance measurements on a leaf level, or if you’re doing any sort of canopy level, you need to either know the leaf temperature, not the air temperature, or you need to know the canopy temperature itself. And so an IRT is a great tool for that.

CHRIS CHAMBERS 26:59
And aside from that, it can be a way to not measure stomatal conductance but get kind of an index of okay, this leaf is so many degrees hotter than the air temperature, the stomata are most likely completely closed and in the the plant is down regulating its production and protecting itself from the possibility for cavitation or, or, or drought stress.

Jeff Ritter 27:28
It can be an indicator of, you know, loss of yield if a plant especially if it’s hot enough it can do to be heat damaged. And I can give you an early warning for that. Yep.

Brad Newbold 27:39
All right, that’s gonna wrap it up for us. Thank you for joining us today and we hope that you enjoyed this discussion. Again, if you have any questions we didn’t answer please contact us via our website metergroup.com. And finally, you can subscribe to the METER Group YouTube channel and accept notifications to see previous episodes of office hours and to get notified when future videos are available. Thanks again. Stay safe. Have a great day.