 So thanks for turning out this morning. I know this is right before the holiday. And in preparation for the final, Terry Curran is going to be holding office hours in the Bio 1B room just around the corner here. Next Monday, the last day of lecture from 10 to 12. And also on the day of the final from 10 to 12, the finals that evening. And I'll be holding my office hours continuously up until the day of the final as well, which are Monday, Wednesday, and Friday from 9 to 10 right after lecture time, even though there isn't a lecture after next Monday. But there is the possibility of a review session either Wednesday or Friday. So keep that in mind in terms of what time would work best for you, and we'll talk about that on Monday. All right, so today I'm going to address a couple of topics that are some of the most interesting topics. And plant physiology, and ones that are really pretty astonishing when you consider that plants are doing things here that we wouldn't necessarily expect in organisms that don't have circulatory systems or any sort of nervous system. And the first thing we're going to talk about is sap transport, which is basically just water or phloem sap transport. So both in the xylem and the phloem, water and minerals or sugar solution, organic nutrient solutions in the plant body. And this thing's not working again. Oh, and then we'll talk about how plants can perceive and respond to light as well. So a couple of topics, big topics today. But I just want to finish off with the nutrient discussion from last time to point out that last time I mentioned there are 17 different macro and micronutrients that are essential for plants that they have to obtain in order to survive and reproduce. And some of these are acquired from the air. Some of these have to be acquired from the soil. And I wanted to mention that some of these nutrients are mobile within the plant. The plant can translocate them from one region to another. And others are immobile that the plant can't translocate. And the main reason I'm bringing this up is that you can tell by the symptoms that a plant is suffering. If there's a deficiency of one of these essential nutrients, it's often pretty easy to determine whether it's one of the mobile or immobile nutrients. Because deficiencies of mobile nutrients are going to become evident from symptoms in the older tissues, all right? So plants are going to preferentially move nutrients from older tissues to actively growing tissues, which are the most precious parts of the plant, for the future of the plant. And so we're going to typically see the older tissues suffering if the nutrients are mobile. Whereas if they're immobile nutrients, we're going to see the symptoms in the younger tissues that are actively growing and using up resources and are not getting sufficient nutrients. And here's just a couple of examples. This might be valuable to you for your own house plants or plants in your yard. Mobile nutrient deficiency on the left and immobile nutrient deficiency on the right. So in this case, magnesium, which is mobile, has been moved out of these older leaves and up into the younger leaves here. So we see the symptoms in the older leaves. And in the case of iron, it's immobile, and we see the symptoms up here in the growing shoot tips in the young leaves. And this yellowing that you often see in unhappy plants is known as, well, it's referred to as chlorosis if it's the result of problems with regard to synthesis of chlorophyll. If it's a magnesium deficiency, magnesium is actually an essential element that is actually part of the chlorophyll molecule, whereas iron is a cofactor that's important, crucial, in the synthesis of chlorophyll. It's not actually part of the chlorophyll molecule. And I also just want to mention that different types of essential nutrient deficiencies have different symptoms. So it's not only can you help to isolate what the problem is based on whether the older or younger tissues are showing the symptoms, but also from some of these other features that I just want to, knowing these different symptoms is not important as much as to just point out that you can distinguish some of these nutrient deficiencies by specific symptoms. For example, this reddish tinge along the edges of the leaves and phosphate deficiency. This kind of burning along the edges of leaves with a potassium deficiency. And this dye back from the tips back down through the mid-rib region of the leaf and a nitrogen deficiency. But actually diagnosing what's wrong with a plant is pretty tricky. There are all kinds of potential problems and some of them look similar. So I'm not saying that this is the end all to figuring out what's wrong with your plants, but these different deficiencies do have distinctive symptoms. Okay, so that's all I want to really say about that. And now get into today's main topic, which is about water transport. And this is a pretty amazing situation. You may not realize it, but an average size tree is actually losing about a ton of water a day. So it's losing a tremendous amount of water to the environment. So plants have a tremendous need for water, not metabolically, but they're actually just losing this water inadvertently because in order to get the CO2 into their leaves that they need for photosynthesis, through those same pores in the stomata, water is evaporating. And that water loss through the stomata is what we call transpiration. So transpiration is just evaporative water loss from plants. And so how can you move a ton of water a day through a tree without expending any energy? I mean, that's a huge amount of work required to move water, to push water from the soil level up to the leaves of a tall tree. But plants can do this, and we'll talk about how. So there are three main mechanisms. First, we should talk about in terms of how fluids can actually move inside of plants. And these are, first off, most obvious one, passive transport. So when we talk about transport in plants, we're talking about movement across cell membranes. So the term transport in the way it's used, plant physiology, refers to movement across a membrane. In this case, just passive diffusion of substances across a membrane from areas of higher concentration to lower concentration. So down the concentration gradient. So that's a slow process. It's effective. It requires no energy on the part of the plant, but it's pretty slow. Another way is active transport. And this is actually pumping a solute across a membrane against its concentration gradient. So from lower concentration to higher concentration. And that doesn't happen passively. That requires energy input on the part of the plant. So this requires an expenditure of energy with the use of ATP. And finally, the most important one to consider for today is bulk flow. And bulk flow is not moving across a membrane. It's movement of a fluid due to a difference in pressure between two locations. And those locations we're gonna be talking about are two opposite ends of a tube. So either the xylem or the phloem. So bulk flow, well let's first just talk about, we have to, in order to understand this, we have to talk about osmosis and water potential. So osmosis again is passive transport of water. So it's movement of water passively through a differentially permeable membrane, say a cell membrane. And osmosis is a passive process where water is gonna be moving in both directions across a membrane, as shown here, the water molecule. You can see the oxygen and the two hydrogens here. It's going in both directions. But the rate of movement is gonna be from the area of lower solute concentration or lower dissolved substance concentration to higher dissolved substance concentration or solute concentration. And that's because that there are more water molecules that are free on the side of this membrane where there are fewer dissolved solutes than there are on the other side where there's a higher concentration of dissolved solutes. So you can see these water molecules bound up with the solutes here. So basically it's just a matter of passive movement that's occurring at a more rapid rate to the right because there's more free water molecules here. So it's just a basic physics here. And you can see here where we have equilibration then of the concentration of solutes on either side of this membrane with more water now on this side than this side. So that's just something to keep in mind in terms of the importance of solute concentration in terms of the direction in which water's gonna move. So moving across a membrane then, again, there can be passive transport or active transport across a membrane. And passive transport can happen in a couple of ways. We can have simple diffusion across the membrane from higher concentration of the solute to lower concentration. Or we can have facilitated diffusion and this involves some helper proteins that are in the cell membrane. This is a channel protein that helps to facilitate the movement of a particular solute through the membrane forming a selective channel here. It doesn't require any energy on the part of the plant to move that through at a faster rate, but this channel protein facilitates that movement. Another type is a carrier protein like this that alternates between a couple of stable states that combine to a particular solute and then release it to the inside. Again, in the direction of higher concentration to lower concentration of that solute. So it's still passive and doesn't require energy. Whereas here we have active transport where we're going from a lower concentration to a higher concentration. That's always gonna require energy and again involves some proteins, helper proteins in the membrane and the use of cellular energy in the form of ATP. Okay, so we just talked then about what determines the direction of water movement when it comes to solute concentration. So water's gonna move from areas of lower solute concentration to areas of higher solute concentration just because water's freer to move in that direction. But another consideration in terms of what dictates the movement of water is physical pressure. So clearly water's gonna move from areas of higher, where water's under higher pressure to areas of lower pressure. That's a little even more obvious situation. And physical pressure turns out to be the most important factor to consider when it comes to the movement of water through a plant. Although solute concentration also comes into the picture when we talk about phloem. We'll get to that in a moment. Okay, so in a typical plant cell we have a higher solute concentration inside the cell than we do in the surrounding area of the environment surrounding that cell. And you might wonder how can that be maintained based on osmosis we'd expect the water concentration to be equilibrate to the same or the solute concentration to equilibrate based on water movement into the cell like we see here. Well the thing to consider is the plant cell then is also surrounded by a cell wall which is a firm structure that presents resistance to further cellular expansion. Once that cell membrane is tied up against that cell wall so the pressure, the positive pressure that's present in that cell wall exerts then causes an osmotic equilibrium to be achieved here at a higher solute concentration inside the cell and outside. And also that gives a tergidity to the cell that is the cell becomes firm. And that's really important for the structural support of the plant. I think if you've forgotten to water your plants so you know what happens, they become flaccid, the cells become flaccid or limp where the plant will droop, the leaves will droop, the stem will droop, plants in big trouble especially if this goes all the way to the point where the cell membrane actually pulls away from the cell wall, cell becomes plasmalized and this is a serious situation. We don't, you know, if you don't water your plant pretty quickly after it wilts, it's gonna be history. So this is some basic physics of cells that are important to consider in terms of some of the principles we're just talking about. Are there any questions about any of this so far? Okay, so now it's important to introduce this concept of water potential. And what water potential is is really just the potential energy of water. So the freedom of water to move and we're just talking about the freedom of water to move across a permeable membrane, differentially permeable membrane that water can move across but not solute so readily. And it's gonna be moving from areas of lower solute concentration to higher solute concentration because of that freedom, greater freedom in the side of the membrane with lower solute concentration. And there are a couple of things to consider with regard to water potential which is represented by the Greek capital letter Psi here. And it's measured in megapascals, which is the MPA here. But basically the situation with regard to water potential in plants, we can boil it down to two components that are by far the most important. And that has to do with the solute concentration inside of the tissues. And the component of water potential that has to do with solute concentration is indicated here by S. And the other consideration is pressure on the solution, either positive pressure or negative pressure in the way of surface tension. And that's the P component here. And these two components together then largely make up what we call the water potential overall. And in many cases, water potential will be negative. We're dealing with oftentimes negative water potentials in plants. So it's important to keep in mind that if water potentials are negative, then higher equals less negative. So that's one way people get a little confused by these equations because they forget that these are negative numbers to oftentimes in plants. And the less negative, the higher the water potential or the higher the free energy of the water, the easier it is for the water to move. So again, the higher the solute concentration, the lower the water potential. So the less freedom the water has to move when there's more solute molecules binding up the water. The higher the pressure on the solution, the higher the water potential, more readily the water can move when it's under high pressure, as you know. So here you can see in a turgid cell, this is the one example of this. This is a turgid cell, a flaccid cell in a plasmalized cell where the cell membranes come up as shrunken away from the cell wall. You can see only in the turgid cell do we have a positive pressure potential because of that cell wall exerting that positive pressure, that resistance on the cell membrane. That's the one component here that's positive in this particular set of examples. So have a look at this slide later in more detail and digest this a little bit more, but this is the nuts and bolts of it. It boils pretty intuitive, you think about it in these terms. So are there any questions about water potential so far? Yeah, right, yeah, so right. So the freedom of the water to move or even if you're dealing with an open tube, if you apply pressure at one end, it's gonna move in the other direction, yeah. Or if you grab a hold of a straw and start sucking on a straw in water, water's gonna move up due to negative pressure, the tension, yeah. All right, so xylem transport then is a pretty amazing situation, especially when you consider something like a giant sequoia or a coast redwood that has to be moving, that's moving water hundreds of feet up this plant body to the leaves. And again, here about a ton of water can move through this typical tree in a day and we can ask how that happens without the plant really exerting any energy. The one possible solution to that problem is the plant might be pushing the water up from the roots. This actually happens in plants. Plants do exert some positive pressure at the level of the roots that can move water through a plant and you've probably seen this before if you've gone out in the morning, early in the morning, and see little droplets on leaves in symmetrical positions on the leaves, like out here at the tips. That's not due. That's water that's being extruded out of the leaf due to positive pressure. And so what happens is at nighttime, the plant's stomata close up. Plants are stimulated to open in light and they close up in darkness. That prevents water loss at night when there's no possibility of photosynthesis, except in a few specialized groups like cacti that we won't talk about, which actually do open their stomata at night. But typically, stomata are closed at night and at that same time, the plant's actively loading minerals into the xylem. Plants need these essential nutrients and they often have to be loaded against the concentration gradient and we can end up with minerals in the xylem that result in a higher solute concentration in the tray kids and vessel elements than there is in the surrounding cells. And so as we talked about, that's gonna mean water's gonna move in to the xylem from an area of lower concentration of solutes to higher concentration of solutes, just due to that freedom of water to move in that direction. That's gonna create a positive pressure inside the xylem at the roots and that's gonna push water up the xylem. And that can actually be effective for up to about five meters. So up to like 20 feet, we can move water in that way. And it can be extruded out here in this sort of situation. But typically when the stomata are open, the amount of evaporative water loss that's happening from the plant, the amount of transpiration, vastly offsets the pressure filled up from the roots because water's being lost from the leaves at a tremendous rate, much faster than it can be pushed up. And also, as I mentioned, this pushing only works for about five meters and beyond that the water column's too heavy. And so this is basically the pull, the plants are not pushing water up, they're pulling water up. And that's not the result of active transport or some sort of energy expenditure on the plant. This is due to simple physics. So what's happening is that we have solar energy on the leaf. We have very dry air surrounding the plant leaf. The concentration of water in the atmosphere is so much less than inside the leaf. There's a huge water potential gradient here. And water is evaporating readily through these pores that have to be open for photosynthesis to occur to let in CO2. And as that evaporation occurs from the cells on the inside of the leaf, there's surface tension that starts to build up in the cell walls of the leaf cells. And that surface tension, which is negative pressure, is transmitted by the cohesive force or cohesive cohesion of water molecules all the way down the xylem. So we have negative pressures building up inside the leaf transmitted to the xylem, to the water that's in the tracheids and vessel elements. And that results that the cohesion of water molecules is extremely strong cohesive force. And the water placed under that kind of negative tension is gonna be pulled up. If you think about sucking up air through a straw, that's what we're talking about, essentially. And so transpiration then, the evaporative water loss from the plant from these inner leaf cells increases the surface tension of water on the cell walls. And this tension, which is the same thing as negative pressure, is transmitted through the water column in the xylem as a result of the cohesion of water molecules. And there's a contribution to that by the adhesion of water molecules to the cell walls of those tracheids and vessel elements. So we not only have the cohesion of the water molecules to one another, we also have the adhesion of the water to the cell walls. And that pull, then that pull that we get from that negative pressure is what's response, basically is what we call this transpiration, cohesion tension mechanism that moves water up the plant. So this is driven by solar energy, not by any energy expenditure on the part of the plant. Are there questions about this then? Okay, so here we see again the route that water takes. We've already talked about this a bit. So we have the root hairs here on the outside of a tap root that we have the water coming in apoplastically or symplastically, although it has to be symplastic to get through the endodermis into the xylem. So we've talked about, and it moves up the xylem to the veins of the leaf and then enters the mesophil where photosynthesis is occurring and finally as vapor leaves through the stomates. And about 95% of the water that's lost by a plant is being lost through the stomata. So the cuticle is really effective and the paradigm in all is really effective at preventing water loss, but even these tiny openings in the leaves are sufficient to let out a huge amount of water given that really strong gradient, the really steep gradient between very low water potential in the atmosphere and the water potential in the leaf. And so this water potential gradient here drives that movement up by this pull from the leaves transmitted from the leaves all the way down to the soil. Okay, so obviously it's gonna be a really important thing for plants to regulate the opening of the stomata since they're losing almost all their water through these stomates. And if conditions aren't really good for photosynthesis or if the plant's under drought stress, best to keep these stomata closed. And the functioning of the stomata works in a pretty simple way. These guard cells are two of them that flank each one of the pores in the stomata and a stomate, a stoma. And you can see how they have thicker cell walls on the side facing the pore than they do on the outside here. So when the turgor of these cells increases, they're gonna bow outward. You can see that they're not quite so bowed here when they're closed. And as these things increase in turgor, this thicker cell wall here is gonna prevent this wall from expanding as much as this wall. And the thing bows outward. Also, these cellulose microfiberals in the cell walls are oriented in this direction, which also promotes that kind of bowing of these cells. All right, so how do they actually regulate this? There's some signaling that the plants, the plants can actually sense light. We're gonna talk about how they do that in a minute. But they can sense various things. They sense light, they can sense the darkness, they can sense drought stress. And when the guard cells are stimulated to actually become turgid for the stomates to open, for example, under high CO2 conditions, when there's light present primarily, we have a proton pump situation operate where the plant is gonna actively pump protons out of the leaf, and that sets up a membrane potential in the guard cells where we have a positive charge on the outside of the membrane, a negative charge on the inside of the membrane, and that's gonna allow for ready movement of potassium ions, which are positively charged through that membrane. So there's a negative charge on the inside of the membrane, a positive charge on the outside of the membrane, and that results in the potassium ions flowing into the cell. So we get a higher solute concentration inside the cytoplasm with the potassium ion buildup. Water moves in through osmosis, and we end up with these guard cells becoming turgid and opening the stomates. Okay, so that's the simple process by which this operates. So you might think about what happens when drought stress gets really severe. So in a really major drought, or even during times of the day when the amount of water being lost by the plant can't be, the roots just can't take water up as fast as it's being lost by the leaves. That happens all the time in plants on a hot summer day, for example, or during conditions of freezing stress. What can happen is that air will come out a solution in the water column of the xylem and under extreme tension, and that will result in a break of this water column. Air can expand relatively easily, and we break this cohesive force of the water molecules by a pocket of air, and you no longer get any pull here. So one nice way in which plants can combat this is that they have pits, as we've already talked about, that connect these trachea elements, the tracheids and the vessel elements, these areas of thin primary cell wall with pores in them that the water can move through. And so here you can see now the water's no longer moving through this particular channel because an air pocket's here, but it can be diverted into this other channel here. So the negative tension is pulling the water in this direction. But if this gets too severe, if this is carried too far, the plant can suffer irreparable damage to xylem and potentially suffer too much water stress and be killed. And so what you typically see, we've already talked about early wood and late wood, how the early wood has wider diameter to the tracheids and vessel elements. So the situation is that as the trachea element becomes wider, the potential for this process called cavitation or formation of an air bubble increases. So we have less surface area of secondary wall in contact with the water and the adhesive forces of water to those cell walls is not gonna be as, we're not gonna have as much adhesion holding the water column together. Because remember it's not just cohesion, it's also adhesion of the water to the walls of the inside of those trachea elements. And so what you often see is that in the late season wood, remember it looks darker because those elements are narrower in diameter and it makes a lot of sense for the plant to produce narrower trachea elements like vessels or tracheids during that time of year when there's gonna be more potential for drought stress. And the same goes if you look at a group of plants that's radiated into different environments, you can often see like I study a group in Hawaii that's done a lot of ecological shifts between wet habitats and dry habitats. And if you look at the wood, you can see that the wood and the plants from dry wet habitats has much broader, they have much broader trachea elements than the ones in dry habitats. So these are evolutionary shifts that are made to in the course of plant evolution. So desert plants often have really narrow trachea elements where you have more adhesive force holding the water column together. And this is just a little diagram here to point out that there actually are irrigation systems that are set up to measure the sap flow in the xylem and to gauge the amount of basically negative pressure in the xylem to make sure that doesn't get beyond a certain level before the plants are watered. So these are really high tech irrigation systems that prevent cavitation in plants. Okay, so are there questions about xylem transport then before we move on to phloem transport? So think about phloem transport now. In terms of xylem transport, typically we're moving water from the roots to the leaves. But in phloem transport, we're typically moving water from the leaves to the roots or at least moving from the leaves to some other part of the plant. So this can't operate in the same way as xylem transport. And the thing to recall about phloem transport is that we're gonna be moving from the source to the sink. So the source of the sugar to the source of the soluble sugar to the sink. And a sink is just a place where something is deposited. So in this case where the sugar is being utilized or concentrated or stored, for example, in a storage organ. So this is a typical situation during a plant's life where the roots are sinks, the roots are not producing sugars, but they need sugar. They have to metabolize, they have to undergo a respiration which requires energy, they need sugar. And the reproductive parts of the plant often are not undergoing much photosynthesis either and they need a lot of energy. So the leaves are the producers, the source, and often the reproductive parts and the roots of the sink. Now sometimes the roots might actually be the source. So think about storage roots like carrots or beets. Early in the growing season, we're gonna have starch concentrated there that's gonna be then converted to soluble sugar and the roots are gonna be for a temporary period at least a source for other parts of the plant. So there is some, it's not always the situation that the leaves are the source and other parts of the sink. There's some variation in that in different situations. All right, so this model, this pressure flow model explains flow and transport and there's a lot of evidence for this model that's accumulated to this point. So again, the movement of the flow of sap, the sugar solution, organic nutrient solution is gonna be from the area of higher sugar concentration to lower sugar concentration. That's the way it has to be or the plant's not gonna be able to survive. It has to be able to do that. So think about this little setup over here where we have this tube. This is an open tube, but at the ends there is a semi-permeable membrane on both ends that allows in water but doesn't allow solutes to escape. And these beakers are just full of pure water. So we could think about these beakers then as being the xylem, which is transporting water and minerals. It's adjacent to the phloem. Remember, a facular tissue, the xylem and phloem are separate, but they're adjacent. And so if we have a higher solute, say sugar concentration here at the source, then we do here at the sink where it's being removed. Just by simple osmosis, water is gonna be moving through this, at a high, moving into this, through this semi-permeable membrane into the solution here at a higher rate than it's gonna be moving in here. Because we have a really steep concentration gradient from pure water to highly concentrated solute, whereas there's much less concentrated solute here compared to the water. So the water moving in here is gonna create positive pressure, and that's gonna translate to a net movement of water in this direction from the area of higher concentration to lower concentration, taking the sugar with it. So this is the bulk flow that occurs in phloem is the result of this situation where we have osmosis from the xylem moving into the sieve tubes of the phloem in the area where there's higher sugar concentration at a higher rate, I should say, than down at the sink, and it's gonna be a net movement of water in this direction. And that's exactly what this all says here. So water pressure will build up within the source and result in bulk flow from source to sink. And here's what this looks like in a cartoon form, at least in the actual plant. So here's the companion cell of a phloem. Here's the sieve tube with the different sieve tube elements. And so we have sugar being loaded into the sieve tube here, into the open channel of the sieve tube at the source where photosynthesis is occurring. And we have sugar being removed at the sink, say down in the roots, where the sugar's being utilized. And this can be active or passive. There can be both active or passive loading and unloading going on. And then we have water then moving from the xylem from one of the tracheary elements into the sieve tube and creating this positive pressure results in this net movement in this direction. And then as the solutes are removed, as the sugar is removed down at the sink, the water is gonna be moving out of this tube and moving back into the xylem. So this is a pretty snazzy way in which plants move their sugars around where they're needed. And it operates without requiring a lot of regulatory. I mean, basically this mechanism alone can explain how they do this. Any questions about that? Okay, so another point to make is that the sink and the source and the sink are often very close together. So if you look at a typical plant here, the sources we can see here in green, the sink again in purple. And you can see that these upper leaves are the sources for the growing shoot tip, whereas the lower leaves are the source for the roots. And so the sources are often close to the sink. It's not as if the whole, all of the sources are contributing equally to each sink or each place where the sugar's being utilized. So that's just another aspect to think about with regard to the physics of this. So in a nutshell then, bulk flow explains xylem transport and phloem transport. That is, we're having movement through a tube essentially due to pressure in both cases. But in the case of the xylem, it's negative pressure. It's tension that's pulling the water column up. Whereas in the case of phloem, it's positive pressure that's pushing the phloem solution from source to sink. So positive pressure here, I'm sorry, negative pressure here resulting in a pull and positive pressure here resulting in a push. So there's two very different ways in which the xylem and phloem are moving solution through the plant by bulk flow that basically involves open channels here in both cases. So any questions about transport, that's all I really wanna say about transport in plants. So the next topic is how do plants actually sense light? They don't have a nervous system, but they can. And they have, there's a lot more that's been learned about this in recent years. There are ways that plants can detect light using a couple of different types of photoreceptors. And these different types of photoreceptors are really not two necessarily very similar classes of compounds, but they break out in the way they're classified by the light that they actually absorb, that they detect. And so there are blue light receptors that detect light of short wavelength in this range. And these blue light receptors are important for stomata opening for the detection of light with regard to stomates opening this proton pump situation we talked about where potassium is moved in. And they also help plants orient to light and during their growth. And so if you lay a plant on its side and shine a light directly above it, these blue light receptors in part are responsible for the plant being able to reorient towards the light, as you can see here in high speed. So plants can do this because they have this ability to detect the source of the light and grow in response to that. And better understood are the phytochromes. And these are light receptors that detect light in the long wavelength into the spectrum. So the red and far red lights are detected. And I'm showing two sets of arrows here because there are two forms of phytochromes. And these are interchangeable. They're isomeric forms of the same molecule. So red and far red light will convert these forms to the alternative form. And we have a balance of what's called red phytochrome and far red phytochrome in the plant. And it's this balance between the two forms that allows the plant to really get a sense of what kind of environment it's in, whether it's in a shady situation, whether it's in a situation where germination for, well, yeah, so the plants can actually detect the quality of the light by the relative amounts of red and far red phytochrome. And it's the inner convertibility of the phytochrome to these two forms that adjust a balance between the forms of phytochrome that allow the plant to get a sense of the kind of light situation it's in and to grow out a shade, for example. Also, seed germination is closely tied to the forms of phytochrome that are present. So seeds that have a light requirement for germination, phytochrome is what's responsible for that light detection. And we can talk a lot about phytochromes, and I'm gonna come up here again in a minute, but that's a really important group of different light photoreceptor, photoreceptors of plants. So circadian rhythms, these are things you're probably all familiar with, and these photoreceptor molecules are in part responsible for plants being able to set their circadian rhythms, that is, their daily rhythms in terms of their physiological activity during different times of the day and night as a result of these phytochromes. So circadian rhythms, these are human circadian rhythms here shown over the course of a day. These are endogenous rhythms. These are not strictly responses to the external environment. So for example, if you go into a dark room and stay inside of total darkness for a long period of time, this doesn't break down, but the actual timing of these activities changes. And so the environment helps to set the biological clock. It doesn't absolutely dictate these rhythms. So anybody that's experienced jet lag knows what I'm talking about, is that your body gets thrown completely out of whack. You have to get up in the middle of the night to urinate because normally you're conserving, your kidneys don't process fluid as quickly at night while you sleep, so your sleep's not interrupted, but if you have jet lag, it is interrupted, and that's one of the reasons it's hard to sleep. But eventually, if you get outside, especially you can readjust, plants are exactly the same way. These phytochromes help them adjust to these, keep these circadian rhythms in check. But if you put a plant in darkness, it'll still go through its circadian rhythms, but the timing of these activities, all these physiological functions will get thrown off a bit. So the biological clocks, basically photoperiodism, physiological response in relation, in response to the relative length of night and day is something that plants experience. And their photoperiodic responses with regard to flowering that are particularly interesting and well understood. So that's what I want to focus on is what is it that triggers the onset of flowering? It's a photoperiodic response in many plants. There are some plants that you'd plant them and they'd grow and eventually they get to a mature size and they just flower and it doesn't matter what photoperiod that is. That is the relative length of light and dark. There are other plants though, for which the photoperiod is crucial to dictating when they flower. And if the appropriate photoperiod doesn't arise, they won't flower and they're obligately photoperiodic plants. So these photoreceptors like phytochrome play a critical role in detecting the photoperiod in plants. And this is seen in various experimental studies. So we categorize plants into two groups with regard to photoperiodism. There are short day plants that flower when the night length increases past a critical threshold. And there are long day plants that flower when night length decreases below a critical threshold. So these names short day and long day actually are old names that precede the time when we actually understood it's the actual length of the night of uninterrupted darkness, not the length of the day that dictates this photoperiodism in plants. So it's night length, not day length that's really critical to consider. So in a short day plant, here you can see that short day plants then typically flower in the late spring, early summer. And there are a lot of different examples, a lot of grasses are this way, and some tobacco like you see here in this example. So as the night period which is dark here gets past this critical threshold shown here as 14 hours, the plant then is stimulated to flower. And if on the other hand in a long day plant which typically flower in late summer or fall, a lot of sunflowers after a you do this, composites, you can see here that as the night time decreases below a critical threshold, the plants flower. And photo receptors such as phytochrome are responsible for this. And I'll get into the details of this. We're just going to go into a little detail about this next time before moving on to growth regulation and hormones for the last part of the course. So have a great holiday and see you next Monday.