Water F20

Added: 2026-05-14

[00:00:00]hello um in today's lesson for greenhouse environment we're going to talk about moisture in the air uh and water and water potential so this is a this is a probably one of the more important lessons i would say as far as managing crop growth keeping it on schedule and managing disease um so there'll be some review of seeing some of your plant science one stuff some of the stuff i've talked about in the greenhouse during our labs but uh let's bring summers together and you will talk about this a little bit about this in the winter again with matt in your greenhouse controls um very important topic so let's let's dig in so i want to cover this is where we get some review water inside the plant specifically in leaves and roots and then we're going to talk about water outside the plant and that's where things like humidity uh are really important so let's review some of the things that water does it carries nutrients through the root zone right all the way up into the plant so it's a way of transporting nutrients sometimes we forget the basics like that carries nutrients through the cell wall so maybe even into the root through the root zone carries nutrients through the vascular bundles and provides pressure to keep your plant from wilting so that's where osmosis comes in and it also a very small percent like one percent or less provides the hydrogen and oxygen required for photosynthesis to happen so some of the water is consumed but the vast majority just goes in the roots and evaporates but it creates a lot of motive energy in the plant in the process and also important and often neglected is it cools the plant through evaporation we'll touch on that a little bit today as well so what is water potential it is energy right we're talking about gravity or osmotic or vapor different forms have different pressure potentials and that is how it drives movement of water through plants is with that energy so just like a waterfall has high energy at the top and lower energy at the bottom which creates motor force to drive the water down that's pretty obvious but it's exactly the same thing in that case it's gravitational potential energy in plants you don't really see a lot of gravitational potential energy i think in plant science too i hope you discussed a little bit about how water gets to the top of uh giant trees and how it has to overcome the weight of the water which is overcoming gravitational potential energy which is a fascinating topic so let's look at physical uh potential energy in the very simplest form if you look at a water pump it sucks water in spins it up and pushes it up against gravity creating uh potential energy in a waterfall again that is that potential energy is relieved when the water is allowed to run back down right so that is physical potential energy and we can also generate physical potential energy which also translates into gravitational potential energy with osmosis so in the sense that a plant has sugars on one side of a membrane and say in the root zone outside the environment water moves towards the sugars creating osmotic energy which then inflates cells or we call we call that turgor pressure where the water now because water has moved in creating pressure it pushes on the cell walls and keeps the plant from from wilting in this case right so water moves in because there's sugars and exerts pressure on the cell walls and that pushes on the plant to keep it pumped up with turgor pressure essentially so that is one of the more basic forms of water potential and that is turgor pressure right so osmotic potential causing uh water to move in now what is interesting to note about turgor pressure is that when you have sugars or solutes they reduce the water potential inside the cell which means that suddenly the water outside the cell has more potential and water moves from low to high so it moves into the cell where there's low water potential right so water moves from high to low just like a waterfall but in this case it's going to move from outside into the lower water potential cells because the sugars have created a reduced water potential that difference is what drives it and as far as actual pressures that are exerted up to 210 psi on the cell walls which is pretty fantastic that's a lot of pressure so again just another picture we go from high water potential to low water potential so when you have sugars dissolved in the cells it creates a low water potential oh that's hard to write and draws water in okay one more time high solutes on this side means water moves in because it has lower water potential relative to low high to no solutes on this side so the water flows through the membrane in uh you may remember from plant science 2 when i talked a little bit about water potential the my example of the cherry tomatoes where your tomatoes are getting ripe and they have lots of sugar creating a really low water potential zone and when that happens water can rush from the plant through the petiole into the fruit causing it to swell remember up to 210 psi pressures a lot this is where some varieties that are grown by farmers end up having thicker skin to reduce the risk of fruit being spoiled from splitting and uh some of you uh myself included have sampled the cherry tomatoes at the college and you may notice that they are much uh meatier with a thicker skin than the million bells or whatever it is sweet cherries that you might get at the garden center and one of the reasons being is that they're bred to minimize fruit splitting to reduce spoilage how can we reduce this from happening hopefully you remember from plant science 2 if you're growing cherry tomatoes in a pot which is your worst case scenario or hydroponically you you do not want to add just straight water to the soil when the fruit is approaching right fully ripe state because if you have clear water it has very high water potential relative to the low water potential in the fruit so what i'm trying to say is if you add a little fertilizer to your watering can you want you won't split the fruit because fertilizer reduces the water potential in the watering can so the difference in water potential between what you're adding to the root and what's in the fruit is less extreme and so your fruit won't split now in hydroponics we're usually running fairly high ecs in the water anyway so it's less of a risk but there is a chance that something happens and you know there's no nutrients the ecs start to come get really low and then you can burst your fruits so water potential from an irrigation perspective is is important now talking about climate let's talk about moisture and water vapor in the climate this is uh this is probably the crux of today's lesson um water can move from liquid to vapor we've all seen it you have a kettle you plug it in it starts to steam um what is happening here is we have a combination of temperature and pressure now pressure is not really that important unless you're boiling water at the top of a mountain or at the bottom of the ocean but for our purposes with crop growing we'll focus on the temperature side of thing because that has the biggest impact on water vapor and it's state so if you're looking at a a pot that's warming up you'll see that water starts to come off the surface as vapor steam and if you close the top as in have a sealed lid the pressure builds or at least the water vapor starts to accumulate it's not escaping anymore and eventually it reaches what we call equilibrium that's essentially what what we mean by saturated so i'm just highlighting the words on the slides rather than reading everything you can go over this in your powerpoint as well but what we're trying to talk about here is water reaching a saturated state so if this air is continually allowed to escape right then saturation may not be achieved whereas if it's closed it might and again we're obviously you were talking about greenhouses so think of it as greenhouse vents closed and greenhouse vents open so air can escape so saturation can happen when your vents are closed all right versus venting it out not a lot of room here to draw sorry about that but i think you get the idea so condensation is something that happens when you've reached saturation and the air can no longer hold the moisture that is being presented to it so in that case condensation starts happening on any kind of cold surface because it has a lower temperature so it can't hold as much water vapor and it condenses um this is where i'll introduce relative humidity you've all heard it before in the weather forecast you know when it's a really humid day in the summer we feel a lot hotter so what is relative humidity it gives us a measure right that's what it says here a picture of how much moisture is currently in the air so it's expressed as a percentage you see that here um and it's always relative to 100 saturation that's why we have the term relative humidity so relative to absolute saturation we are at what percentage on our way right so relative to temperature um this is an interesting graph the red line represents 100 humidity at any particular point in time based on temperature changing so obviously as the temperature increases we can hold more water so our relative 100 percent relative humidity goes higher right so this on the left here is grams of water per kilogram of air so as we get warmer we can physically hold more grams of water in a unit of air right so and it's exponential so as the temperature drops let's say we reverse the flow and the temperature drops the air can hold less moisture and it drops fairly quickly so this is important when it comes to managing a suitable climate in a greenhouse because we need to be careful that we monitor our humidity for reasons of plant growth and disease prevention so how do we measure it uh i think matt talked to some of you and crops your crops class about uh the wet bulb thermometers that are in the greenhouse again you'll cover this in your greenhouse controls class in the winter but essentially there's a box hanging in the different zones in the greenhouse and it has two thermometers in it a normal one which we call a dry bulb thermometer and one that has the bottom of the bulb attached to a wick that is wet with a little reservoir of water and that wet wick actually evaporates and as it evaporates it cools the thermometer so we end up with uh a difference or a delta between the two thermometers and that goes into an algorithm to give us our percent relative humidity obviously the drier the air the more rapidly this evaporates the cooler this becomes so as we approach 100 percent humidity what happens what would you expect if we're approaching 100 humidity what would happen with this wick it will evaporate less which means the delter becomes smaller effectively i'll just draw a red line in here if we had a really humid environment this thermometer would be almost identical and if it is a hundred percent it will be absolutely identical to the dry thermometer because there's no evaporation happening off the wick so there's no cooling so it's this differential that allows the computer to calculate relative humidity and we call that a wet bulb thermometer uh there's actually handheld spinning ones this is a hand grip here and you dip this wick in some water and then you spin it around like a a party clapper thing and as it spins through the air this evaporates and then you stop it and you look at your two thermometers this one's your dry and this is your wet and you look at the difference and you plot it on a graph and it tells you your relative humidity uh this is the a greenhouse box similar to the ones we have in the college and there's a water reservoir and there's a wick that goes up to a thermometer and that's where the evaporated cooling happens and just out of curiosity just our interest i should say on the top here that's right here is a infrared picture of a plant i think it's a rabidopsis and it's showing that the leaves are quite a bit cooler than the rest of the soil and the surrounding area and that is specifically because of evaporation uh that's an important thing to remember that um when plants are actually evaporating moisture when they're transpiring they cool their leaves and that's really really important in when you have heat stress on crops so it's important to make sure you don't have too high high humidity so that the plants can actually transpire for many reasons but one of them is to reduce stress and if you remember your plant science too by keeping the leaves cool through evapotranspiration they reduce or limit photorespiration which is going to hurt our plant's ability to make sugars so all right so we have 100 relativity in this graph that's just a reminder this line and as you move up and down the temperature range it changes how much moisture the air can hold right so at 10 degrees celsius it can hold 10 grams but at 50 degrees celsius it can hold 95 grams of water so it's a pretty huge difference in in water content at different temperatures so what's the take-home let's say 70 relative humidity is an acceptable humidity for your crop right suddenly 100 is achieved because your air's water capacity water holding capacity has changed and what would that scenario be let's say at the end of the day right the sun is going down and your greenhouse is cooling right the temperature is dropping in your greenhouse because the sun's going down the exact same moisture content which had a nice happy 70 relative humidity remember that graph as the temperature drops it can't hold the moisture anymore and it quickly reaches 100 percent and if you're really unfortunate it actually starts to surpass that which means you get condensation because the air can no longer hold the moisture that's how rain happens you've got a cloud and as the temperature drops the cloud moves to a higher altitude the air condenses out because the air can't hold the moisture the moisture condenses out i mean and it forms water droplets which fall in the form of rain and in the greenhouse as the temperature drops and the air can't hold the moisture it forms water droplets as in the form of condensation on your structure or worse yet on your leaves which then creates the moisture for spores to germinate for things like powdery mildew so what do we do for powdery mildew we need to remove the moisture at the end of the day so for growing crops that are prone to cup powdery mildew this is standard growing practice where towards the end of the day you open the vents but that may not be just enough oftentimes there's a process i call it heat and burp but effectively what you do is you you release some heat into the greenhouse so whether it's hot water in your pipes or turning a furnace on bro for a short time raises the temperature remember as air is warmer it can hold more moisture so it gathers more moisture out of the greenhouse because you've just injected energy with heat and then you open the vents and the combination of warm air rising and its higher capacity for moisture means that that warm moist air then quickly exits the vents and you've just effectively dried your greenhouse and then you close the vents and you button up for the night and the crop goes to sleep for the night and you don't have powdery mildew germinating on your crop this is a process i've been trying to help some growers learn for things like cannabis where powdery mildew is a constant problem that heat and burp strategy is absolutely critical for minimizing powdery mildew on a crop okay so let's go back to talking about potential energy i talked about gravitational potential energy i talked about physical potential energy in terms of water pressure and then i linked it to osmotic potential energy which is basically what drives the physical pressure in plants but we also have potential energy in water vapor and this is a introduction to what we call vapor pressure deficit which is um another interesting way of looking at moisture in the air of a greenhouse and as you get into growing you'll see that vpd or vapor pressure deficit is used quite a bit oh my phone yeehaw so what exactly is it so as air dries it reduces its water potential energy or vapor potential energy so remember with potential energy that diagram where you have water goes from high to low just like a waterfall goes from high gravitational to low gravitational or from a high osmotic potential to low osmotic potential so it goes from clear water to sugar water with water vapor it goes from high water vapor to low water vapor pressure so high water potential vapor potential energy to low water vapor potential energy so that downward hill flow is important to understand um and when we're talking about vpd we're talking about the difference between the plant and its environment so again goes from high to low uh and this is a cross section of a leaf and you can imagine inside the leaf is the spongy mesophyll right this is where all the photosynthesis is happening this is where the magic happens where sugars are made this is the factory and you have a lot of water being supplied by the xylem bringing water to these cells and they eventually evaporate it into the spongy mesophyll space uh sometimes called the interstitial space this these air spaces between the mesophyll accumulate high moisture content or high water vapor potential and the plant much like a greenhouse tries to vent that off just like we would burp heat and burp a greenhouse to reduce the moisture what the green what the leaf does it spends our stomata right so the stomata open creating a pore which releases that vapor potential energy into the surrounding environment so inside a leaf we have the spongy mesophyll you see these spaces that become very saturated with water vapor and that water vapor escapes through stomata into the environment just like a greenhouse vents water vapor out its vents um it's really helpful if you think of a leaf as a greenhouse because that sort of makes sense to us you know that our concept of venting it out how we assess how water will move from inside a leaf to outside is by looking at water vapor or vapor pressure deficit um from a growing perspective it's really important you manage a suitable vpd because if you don't have enough of a deficit the deficit being uh low vpd outside versus high vpd inside if your greenhouse is too humid air won't flow out and if you don't have evaporation happening in the mesophyll then water can't flow in because it's not evaporating so it's saturated and the entire plant shuts down and you don't have any growth so having high humidity or high vapor potential energy in your greenhouse will actually physically slow the movement of water through the plants which slows the movement of sugars shuts down everything every physical process in the plant so it just stalls i've seen this mistake uh with farms where they're trying to save energy in the winter by not venting warm air out because they don't want to lose the heat that they just paid to heat but unfortunately the humidity just builds and builds and builds in the greenhouse until there is absolutely no vapor pressure deficit and no water leaving the leaves of the plant which means it stalls and nothing grows so as the outside area is drier we get we get a larger deficit which means moisture inside wants to exit and you get more transpiration so it's important to understand that we need vpd vapor pressure deficit in order to get transpiration to happen okay that's the key one of the challenges we have in measuring vapor pressure deficit is that we can't easily there are maybe some scientific devices but in farms we don't have the ability to measure the humidity inside the mesophyll of a leaf that's very tricky but what we can do is we can measure the vapor pressure deficit of the environment relative to saturated air and so that vpd is calculated and estimated for what is happening in the leaf [Music] so most computer systems have output charts this is a chart from argus uh and there's these are vpd values that are in the entire chart and you'll notice that they're sort of it's a bit faded but there's uh sort of a pink yellow and green and then yellow and pink in terms of the different zones of the chart and hopefully it's intuitive but the green zone is where the plants are happiest that's where optimal transpiration is happening and outside that zone is the danger zone either too much vapor pressure deficit or too little i hadn't touched on this yet but if you have really really really high water stress the plants will panic and close their stomata because they don't want to lose more water than they can supply with their roots because that will start causing physical damage so there's such a thing as being too dry and such a thing as being too humid if you're looking at the graph you will notice that it's fairly broad um and you'll also notice that it uses relative humidity and temperature to calculate the vpd and so our wet bulb sensor gives us our current relative humidity and then with the temperature that's measured with a thermometer the vpd is calculated and estimated for the leaf i have had farms that struggle with powdery mildew because they don't understand uh vpd and they don't use vapor relative humidity they just use uh i mean they don't use vpu they just use relative humidity and one of the troubles here let me back that up they use vpd but they don't use relative humidity the main reason is because let's say you on your computer system say look i'm going to set my vpd to be 0.9 and the computer will open and close the vents and modulate the greenhouse to manage a 0.9 vpd all right now as the temperature fluctuates during the day it's not hard to see how your greenhouse would be 22 degrees during the day and you might float it down to 16 at night right that's sort of your day-night temperature look at what happens to your 0.92 so here's a point this is pretty close 0.93 0.93 0.91 0.94 [Music] now the computer is managing your vpd within the specification that you assigned but there's a problem look at the relative humidity to maintain a vpd of 0.9 at 16 degrees celsius we're going down to 45 relative humidity and if you remember some of my chats with you and maybe in your pathology with respect to powdery mildew powdery mildew does two things above 75 relative humidity the spores will germinate and grow hyphy below 50 relative humidity powdery mildew will sporulate and produce more spores so if you're fluctuating between high and low humidity not only are you growing your powdery mildew but then you're encouraging it to form spores so that it spreads even more and you have a bigger and bigger problem all the time so it's these wild fluctuations that we have to be very careful with and one of the reasons why you can't use vpd alone you have to monitor relative humidity as well to make sure you're not going into a danger zone so keep that in mind that might solve you uh some headaches with powdery mildew so maintaining a suitable vpd vapor pressure deficit whoops trying to erase that um we'll keep water moving through your plant if we don't have enough deficit pla the water stops and the plant stops growing and likewise the other direction if we have too extreme a vapor pressure deficit the plant will shut down its stomata in a self-preservation mode and you also stop and usually the plants overheat at that point because there's no evapotranspiration to cool the leaves so you get heat stress you get root collapse a lot of bad things happen physiologically um let's talk briefly about condensation um condensation is not necessarily a bad thing it removes moisture from the air we talked about this when we talked about shade but condensation can also cause problems in terms of water and the ground which encourages algae and maybe shore flies and fungus gnats it can drip from your covering onto seedlings and wash out seedlings there's all sorts of challenges associated with with uh condensation but when we use curtains we can use them to manage uh humidity so i'd like to think of energy curtains as a tool for managing vapor pressure deficit so for example you could if you're heating a greenhouse you can steer the vapor pressure deficit by opening and closing the curtains depending on the light levels time of day and stuff like that but uh just like when i talked about curtains by opening the curtains in the morning we let that humid air come up hit the greenhouse glass condense collect that condensation that's gone it's out of the air and you have what we call an active environment where the plants wake up and start to transpire as the sun comes up so energy curtains are a tool you can use in monitoring and managing vapor pressure deficit i want to talk a little bit about the boundary layer i mentioned this once before but let's talk again about it um airflow is super critical especially when we're talking about humidity because we don't often see what it is uh where the actual high humidity is when we look at the greenhouse you walk in you experience the environment for sure that is absolutely something that you can perceive but when you look at the microscopic level of a plant there is actually a layer close to the surface where the gases are sort of stuck to the leaf we call that a boundary layer and you can have high humidity in that boundary layer which will impact water moving out through the stomata so we need to be able to get rid of that or influence that boundary layer to encourage water to flow out of the leaves [Music] on this picture up here you see all the trichomes that are sticking up off the leaf and just like in a forest they create an environment where air flows over the leaf right and it may not actually flow into and between the trichomes as effectively so you get localized high humidity so some plot some crops that have a lot more trichomes are more susceptible to that and require more air flow across the leaf surface to influence the boundary layer just like a field of corn uh this is just showing how the wind is blowing over the corn and the wind blows across but in between the stalks you have a different environment much more humid um again influencing things like calcium movement through the tissues because they rely heavily on transpiration if you grow too densely and there's not enough air flow between the plants you can have calcium deficiencies in the lower parts of the plants so spacing when you seed the crop is important for air flow so if you get too greedy and you plant too many plants in a fixed space you don't get enough airflow between the tissues and the leaves and you end up with calcium deficiencies because you have inadequate transpiration all right that's enough uh hopefully you've followed that humidity is an important topic vapor pressure deficit is an important topic matt will cover this one more time in the winter term with your greenhouse computer or controls class make sure you understand humidity [Music] on uh just business notes i remind you all that there are quizzes being added every week now for all three of my classes um so keep an eye on that and uh those will remain open until the end of the term but don't wait i would just do them all you can alright talk to you later