Method for predicting stem water potential and system for predicting stem water potential

Abstract

To predict trunk water potential with high accuracy from the wilting condition of a leaf.SOLUTION: Provided is a Ψstem prediction method for obtaining a predicted value of the water potential (Ψstem) of a trunk based on the measured value of the amount of downward displacement of a leaf 100, the amount of downward displacement of the leaf 100 being measured by a laser rangefinder 200 whose direction is fixed, the relationship between the Ψstem and the amount of downward displacement of the leaf 100 being known, the laser range rangefinder 200 being directed obliquely upward toward the leaf 100, and being arranged in such a direction that the leaf 100 falls within the measurement range when the leaf 100 is displaced downward, and, based on the amount of downward displacement of the leaf 100 and the above relationship, the predicted value of the Ψstem of a plant 10 being obtained.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a technique for obtaining information related to water stress in plants.

Background Art

[0002] In plant cultivation, irrigation is important. The state in which a plant needs water is discussed in terms of the concept of water stress. The water potential (Ψstem) of a plant stem is known as an index for quantitatively evaluating water stress (see Non-Patent Document 1). Also, Non-Patent Document 2 and Patent Document 1 are known as techniques for estimating water potential.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

Non-Patent Document 2

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] By grasping the water potential (Ψstem) of the stem, it is possible to estimate the timing of irrigation for the target plant. However, as will be described later, the actual measurement of Ψstem is troublesome, and it is not practical to obtain real-time measurement data.

[0006] As a method for indirectly obtaining the Ψstem, a method utilizing the relationship between the degree of leaf wilting and the Ψstem described in Non-Patent Document 2 can be considered. Therefore, the present inventors measured the displacement of the tip portion of a leaf (the amount of downward displacement caused by wilting) of a peach and examined the relationship with the target Ψstem.

[0007] As a result, a correlation was confirmed between the displacement of the tip portion of the leaf and the Ψstem of the target plant. However, depending on the range of the Ψstem, it was difficult to predict the value of the Ψstem from the displacement of the tip of the leaf.

[0008] Specifically, for the peach that the present inventors observed, the Ψstem was in the range of approximately -25 to -5 (bar). Here, in the range of Ψstem = -15 to -5 (bar), a graph with the Ψstem on the horizontal axis and the displacement of the tip of the leaf on the vertical axis became almost horizontal, and it was difficult to predict the Ψstem from the displacement of the tip of the leaf.

[0009] If the Ψstem can be predicted, the timing of irrigation can be anticipated, which is extremely useful for crop growth. However, the above tendency is also expected in other plants, which has been a major problem in the practical application of this technology.

[0010] Against such a background, an object of the present invention is to accurately predict the stem water potential (Ψstem) from the degree of leaf wilting in a plant.

Means for Solving the Problems

[0011] The present invention is a method for predicting the stem water potential (Ψstem) for obtaining a predicted value of the stem water potential based on the actually measured value of the downward displacement amount of the leaf, wherein the downward displacement amount of the leaf is measured by a laser distance meter with a fixed orientation, and the relationship between the Ψstem and the downward displacement amount of the leaf is known. In the first stage of the measurement, The laser distance meter the leaf's From obliquely above a range within 20% from the tip of the leaf considering the length direction of the leafIt is a method for predicting the stem water potential that is directed in the direction and is arranged in a direction such that the leaf is within the measurement range when the downward displacement of the leaf occurs, and based on the downward displacement amount of the leaf and the relationship, obtains an estimated value of Ψstem.

[0012] In the present invention, examples of the distance measurement position by the laser rangefinder on the leaf include a mode in which the distance measurement position moves from the tip side to the base side of the leaf as the leaf descends. In the present invention, an example of the relationship between the displacement amount of the leaf based on the measured value of the laser rangefinder and Ψstem is a linear relationship.

[0013] The present invention includes a laser rangefinder with a fixed orientation for measuring the downward displacement amount of a plant leaf, and a storage unit that stores the relationship between Ψstem of the plant and the downward displacement amount of the plant leaf. In the first stage of the measurement, The laser rangefinder the leaf's is directed obliquely upward a range within 20% from the tip of the leaf considering the length direction of the leaf in the direction and is arranged in a direction such that the leaf is within the measurement range when the downward displacement of the leaf occurs, and can also be understood as a prediction system for the stem water potential that obtains an estimated value of Ψstem based on the measured value of the downward displacement amount of the leaf and the relationship.

Advantages of the Invention

[0014] According to the present invention, the stem water potential can be predicted with high accuracy from the degree of wilting of the leaves in a plant.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0016] 1. First Embodiment (Overview) FIG. 1 is a conceptual diagram of the embodiment. In FIG. 1, a leaf 100 to be measured, a laser distance meter 200, and a data processing device 300 are shown. FIG. 2 is a drawing substitute photograph actually taking a picture of the measurement state shown in FIG. 1.

[0017] Here, a peach 10 is selected as the target plant, and its leaf 100 is the object of measurement. Here, an example of a peach is shown, but the type of the target plant is not particularly limited.

[0018] The laser distance meter 200 is a distance meter using the principle of light wave distance measurement using laser light. As the laser distance meter 200, commercially available equipment is used. The laser distance meter 200 measures the displacement downward from the leaf 100 obliquely upward in a state where the position and posture are fixed. The laser distance meter 200 is fixed to a pan-tilt 201, and the pan-tilt 201 is fixed on a leg 202 such as a tripod. The position and posture of the laser distance meter 200 are adjustable.

[0019] The data processing device 300 is composed of a PC (Personal Computer). The data processing device 300 includes a CPU, a storage device, an input / output interface, a communication device, and a GUI (Graphical User Interface), and can perform the same operations and operations as a normal PC. As the data processing device 300, a form of preparing a dedicated arithmetic processing device and a form of using a data processing server installed at a remote location are also possible.

[0020] Based on the measurement values obtained by the laser rangefinder 200, the data processing device 300 calculates (predicts) the water potential (Ψstem) of the trunk of the plant to be measured (in this example, the peach tree 10). The relationship between the measurement values obtained by the laser rangefinder 200 and Ψstem has been acquired in advance and is stored in a storage unit (such as a hard disk device or a semiconductor memory) within the data processing device 300.

[0021] It can be understood that the data processing device 300 includes a Ψstem prediction unit 301 that predicts Ψstem based on the measurement values obtained by the laser rangefinder 200, and a storage unit 302 that stores the data necessary for the calculation for this prediction.

[0022] The laser rangefinder 200 is connected to the data processing device 300 via wireless LAN or wired communication. The measurement values of the laser rangefinder 200 are input into the data processing device 300. Based on this input, Ψstem is calculated by referring to the data obtained in advance.

[0023] (Measurement method) At the start of measurement, the leaf 100 tries to make the water potential (Ψstem) of the trunk of the plant (in this case, the peach tree 10) having the leaf 100 maximum (saturated state). In this example, the measurement is started from a state where sufficient irrigation has been performed.

[0024] At the start of measurement, the laser rangefinder 200 is placed at a position diagonally above the leaf 100. In FIG. 1, θ is appropriately 45° to 75°. When θ < 45°, when the tip of the leaf 100 descends, the optical axis may deviate from the leaf, increasing the possibility that the distance cannot be measured. When θ > 75°, the possibility of not being able to accurately measure the displacement of the leaf 100 increases.

[0025] In the first stage, the posture of the laser rangefinder 200 is adjusted so that the measurement point is at the tip portion of the leaf 100. That is, the optical axis of the laser rangefinder 200 is made to coincide with the tip of the leaf 100. The measurement is performed with the position and posture of the laser rangefinder 200 fixed.

[0026] When the peach leaf 10 (leaf 100) wilts and the tip of the leaf 100 descends, it is necessary to determine the posture of the laser distance meter 200 so that the measurement laser light of the laser distance meter 200 does not deviate from the leaf 100. That is, as the wilting progresses, the tip of the leaf 100 descends. Of course, the leaf 100 itself also gradually moves downward. At this time, the measurement laser light from the laser distance meter 200 must continue to hit the leaf 100.

[0027] For this purpose, in the example of FIG. 1, the posture of the laser distance meter 200 at the start of measurement is adjusted so that the assumed movement path of the tip of the leaf 100 (usually a loose arc) when wilting occurs and the optical axis of the laser distance meter 200 are included in the same plane. Briefly, assume a vertical plane including the branch at the root of the leaf 100, and make the optical axis of the laser distance meter 200 substantially coincide with this vertical plane. The separation distance between the leaf 100 and the laser distance meter 200 is appropriately about 15 cm to 50 cm.

[0028] As time passes from the state where the water potential (Ψstem) of the trunk is maximum, with the transpiration of water from the leaf 100, the value of Ψstem decreases. Along with this, the leaf 100 wilts, and the leaf 100 succumbs to gravity and is displaced downward. As a result, the leaf 100 moves away from the laser distance meter 100, and the displacement amount (downward displacement amount) is measured according to the principle of laser distance measurement.

[0029] As will be described in detail later, when measuring the displacement of the leaf 100 in the state of FIG. 1, even if the aim is initially set at the tip of the leaf 100, as the leaf 100 descends, the measurement point gradually moves from the tip of the leaf 100 to the center (the root direction as seen from the tip). Therefore, the system in FIG. 1 does not always measure the displacement of the tip of the leaf 100.

[0030] (Regarding the acquisition of data to be performed in advance) As described above, the relationship between the measurement values obtained by the laser distance meter 200 and the Ψstem is acquired in advance. An explanation of this acquisition is provided below. First, a sample plant 10 (a peach in this case) is selected. Next, the above-described measurement is performed on the leaves 100 of this plant 10. At the start of the measurement, the leaf 100 endeavors to have the maximum (saturated state) water potential (Ψstem) of the trunk of the plant (a peach 10 in this case) that has the leaf 100. In this example, the measurement is started from a state where sufficient irrigation has been performed.

[0031] Note that it is possible to use a minimum of one leaf as the object of measurement by the laser distance meter 200. Of course, it is also possible to select multiple leaves and adopt an average value.

[0032] On the other hand, other leaves of the same target plant are selected as sample leaves for measuring the water potential (Ψstem) of the trunk. A plurality of sample leaves are prepared. Here, the displacement of the leaf 100 by the laser distance meter 200 is measured, and at the timing when the measurement value is obtained, the water potential (Ψstem) of the trunk is measured using the above sample leaves.

[0033] What is measured here is the water potential of the trunk called Ψstem. Since the water potential of the trunk cannot be directly measured, here, in the case of the leaf and the tree, the value of the water potential of the leaf in a state where the contained moisture has reached an equilibrium state is measured and acquired as the water potential Ψstem of the trunk. In this case, since the moisture contained in the tree and the leaf has reached an equilibrium state, the water potential of the leaf and the water potential of the trunk can be evaluated as the same.

[0034] There are various methods for measuring Ψstem, and dedicated measuring devices are also commercially available. Here, Ψstem was measured by the following method.

[0035] First, in each of a plurality of sample leaves, the leaf is sealed with an aluminum vapor deposition bag to suppress transpiration of water from the leaf. Then, when the stage where water movement between the leaf and the tree has ceased is estimated, the branch with the leaf is cut off and placed in a sealed chamber (pressure chamber), and the air pressure inside the sealed chamber is increased with the cut portion of the branch protruding outside the sealed chamber.

[0036] When the pressure inside the sealed chamber is increased, air pressure is applied to the leaf, and water oozes out from the cut end of the branch exposed outside the sealed chamber. From the air pressure in the state where this water oozes out, the water potential (Ψstem) of the trunk of the target plant is obtained.

[0037] Suppose that water oozes out from the cut portion of the above branch under the condition that the pressure inside the sealed chamber is low. In this case, since the water in the leaf was relatively sufficient, water oozed out with a little pressurization. In this case, Ψstem becomes a relatively large value. That is, it is evaluated that the water content in the tree is relatively high.

[0038] On the other hand, suppose that water oozes out from the cut portion of the above branch under the condition that the pressure inside the sealed chamber is high. In this case, since the water in the leaf is relatively less, water does not ooze out unless the pressure is increased. In this case, Ψstem becomes a relatively small value. That is, it is evaluated that the water content in the tree is relatively low.

[0039] Ψstem is an index indicating the pressure of water in the tree of the target plant. When Ψstem is small, it indicates that the plant lacks water and is in a state of wanting water. Conversely, when Ψstem is the maximum value (saturation value), it is interpreted that the plant has sufficient water.

[0040] That is, the value obtained by attaching a minus sign to the value of the pressure in the above sealed chamber becomes Ψstem, and Ψstem is obtained by the above method.

[0041] The maximum value of Ψstem is almost determined by the type of plant. For example, in the case of a peach tree, the maximum value of Ψstem is about -3 bar (-0.3 MPa). Note that, precisely, even for the same plant, there may be some differences depending on the variety.

[0042] Measure Ψstem using the above-mentioned sample leaves in various situations of the target plant (in this case, a peach tree). Specifically, measure Ψstem in the above-mentioned state in a plurality of states where different values are obtained as the measurement values of the laser distance meter 200.

[0043] Here, take the displacement amount of leaf 100 measured by the laser distance meter 200 as the vertical axis, and the measured value of Ψstem at the timing when this displacement amount is obtained as the horizontal axis, and plot the value of Ψstem. In this way, the graph of FIG. 3 is obtained. The displacement amount of leaf 100 is the displacement amount of the leaf on the optical axis 211 of the laser distance meter 200 in FIG. 1.

[0044] The displacement amount on the vertical axis of FIG. 3 is the displacement amount based on the start of measurement. When leaf 100 moves away from the laser distance meter 100, the displacement amount increases.

[0045] In the graph of FIG. 3, a linear correlation is seen between Ψstem and the displacement amount of the leaf measured by the laser distance meter 200. Also, it can be seen from FIG. 3 that similar data can be obtained regardless of the measurement time.

[0046] It has been found that the water potential of the leaves measured directly without being covered with an aluminum vapor deposition bag is stable throughout the tree before dawn, but fluctuates greatly at other times. This is because photosynthesis and transpiration of water from the leaves vary depending on the position of the leaves to be measured within the tree. In contrast, the water potential of the trunk is not affected by the fluctuations in the water potential of each leaf and can be stably measured even during the day. In the graph of FIG. 3, the relationship between the displacement amount of leaf 100 measured by the laser distance meter 200 and the water potential (Ψstem) of the tree is stably obtained regardless of the time. That is, the relationship between the displacement amount of leaf 100 measured by the laser distance meter 200 and Ψstem is stably obtained.

[0047] In the graph of Fig. 3, there is a range where the displacement amount on the vertical axis is negative. This means that upward displacement of leaf 100 was observed after the start of measurement. After the plant is considered to have sufficiently absorbed water, it is possible that a further increase in Ψstem is observed due to factors such as temperature change, humidity change, and change in the state of photosynthesis. The range where the displacement amount on the vertical axis is negative is considered to be related to this phenomenon.

[0048] (Example of processing) Obtain the graph of Fig. 3 in advance and store the data in the storage unit 302 of the data processing device 300. Then, measure the downward displacement amount of leaf 100 by the laser distance meter 200. The downward displacement amount is obtained as the change amount of the distance measured by the laser distance meter 200. This downward change amount is applied to the graph of Fig. 3 to predict the value of Ψstem on the horizontal axis of Fig. 3. This process is performed in the prediction unit 301 of the data processing device 300.

[0049] (Discussion) Hereinafter, the factors for obtaining the highly linear data shown in Fig. 3 will be discussed. Fig. 4 is a graph in Fig. 1 where the displacement amount downward from the tip of leaf 100 is detected from an image instead of the laser distance meter 200, and this is used as the vertical axis. The horizontal axis of Fig. 4 is Ψstem obtained at the timing when the value on the vertical axis was acquired.

[0050] The tip of leaf 100 wilts and displaces downward as the water content decreases (which causes a decrease in Ψstem). This displacement amount is plotted on the vertical axis of Fig. 4. The displacement of the tip of leaf 100 follows a trajectory (path) that draws a loose arc, but in the case of Fig. 4, the moving amount in the linear direction is calculated from the image analysis.

[0051] As is clear from Fig. 4, in the range of Ψstem = -10 to -5 bar, the dependence on the moving amount of Ψstem is not clearly shown. This means that even if the moving amount of the tip of leaf 100 is calculated from the image, Ψstem cannot be accurately predicted in the above range.

[0052] Figure 5 is a graph with the vertical axis representing the change in the angle of leaf 100 caused by withering, rather than the movement amount of the tip of leaf 100, detected from the captured image. The horizontal axis is the same as in Figure 4. A similar trend to that in Figure 4 can also be read in the case of Figure 5.

[0053] Figures 4 and 5 are not consistent with Figure 3. Hereinafter, this point will be considered. First, in the first stage of measuring the displacement of leaf 100 using the laser light in Figure 1, the aiming of the laser rangefinder 200 is aligned with the tip of leaf 100. Also, the laser rangefinder 200 is directed at leaf 100 from obliquely above.

[0054] Here, when leaf 100 is displaced downward as it withers, the measurement point of the laser rangefinder 200 gradually moves from the tip of leaf 100 toward the base (center) of leaf 100. This is explained using the model in Figure 6. Figure 6 shows the state where the tip of leaf 100 gradually descends as it withers.

[0055] Here, considering the structure connected as trunk ⇒ branch ⇒ leaf, the downward movement of the withering leaf is not uniform throughout, but is relatively large at the tip of the leaf and relatively small at the base of the leaf. As a result, as shown in Figure 6, the measurement point of the laser rangefinder 200 gradually shifts from the tip of the leaf toward the central part (the direction of the base of the leaf).

[0056] Hereinafter, according to the above model, the relationship between the displacement amount and Ψstem when measuring the displacement of the leaf with a laser rangefinder according to the model in Figure 6 is predicted from the data in Figure 4.

[0057] Here, the following model is assumed. (1) The measured part of the laser rangefinder gradually shifts toward the central part of the leaf as the tip of the leaf descends. (2) The tip of the leaf droops, but the base does not.

[0058] The curve of the leaf drooping condition in Fig. 6 was created using the curve drawing tool in spreadsheet software. Also, it was assumed that the tip of the leaf moves on an arc but drops vertically. Further, the laser distance meter was configured to measure from diagonally above.

[0059] Assuming the model in Fig. 6, the change in distance in the optical axis direction of the measurement laser light was made to correspond to the change in the vertical direction of the leaf tip. The procedure is as follows.

[0060] First, from the approximate curve of the plotted points in the graph of Fig. 4, the values of the displacement distances of the leaf tip corresponding to the values of Ψstem of -7.5, -10, -12.5, -15, -17.5, -20 bar, which were 12.3, 16.9, 27.8, 44.9, 68.2, 97.7 pixels, were read. Note that the minimum value of Ψstem at which the displacement distance of the leaf tip becomes 0 pixels was -3.45 used from the plotted points of the same graph. Next, the change in the position of the leaf in the optical axis direction of the laser measurement light corresponding to the displacement of the leaf tip in the model of Fig. 6 was read from Fig. 6, and its length was obtained from the ratio of the apparent length on the drawing.

[0061] The amount of displacement of the leaf position in the optical axis direction of the laser measurement light thus obtained becomes the displacement distance measured by the laser light. Then, with Ψstem on the horizontal axis and the displacement distance measured by the laser light on the vertical axis, the graph of Fig. 7 was created.

[0062] Fig. 7 is an estimated graph obtained by generating data from the actual measurement data in Fig. 4 according to the model in Fig. 6 on the premise of the assumptions in (1) and (2) above. In Fig. 7, a high linearity is obtained in the relationship between the horizontal axis and the vertical axis, similar to Fig. 3.

[0063] Considering that Fig. 4 is the actual measurement value, the estimated graph in Fig. 7 accurately reflects the actual situation, and the validity of the assumptions in (1) and (2) above and the model in Fig. 6 can be inferred.

[0064] Summarizing the findings obtained from the above considerations simply, it is as follows. "As Ψstem decreases, the tip of the leaf descends. When a laser rangefinder is directed at the tip of the leaf from an obliquely upward direction, as the tip of the above leaf descends, the measurement site gradually moves from the tip of the leaf towards the base. There is a strong correlation close to linearity between the change in the measured distance and the change in Ψstem at this time. Using this correlation, Ψstem can be obtained from the measured values related to the displacement of the leaf. For example, in the case of a peach, in the range of Ψstem > -10 bar where Ψstem cannot be obtained by focusing on the amount of movement or the angle change at the tip of the leaf, Ψstem can be obtained from the measured values with high resolution."

[0065] 2. Second Embodiment The initial aiming position of the laser rangefinder does not have to be exactly at the tip of the leaf. In this case, the aiming of the laser rangefinder in the first stage should be at a part as close as possible to the tip of the leaf (of course, making sure not to deviate from the leaf). Specifically, it is preferable that the aiming at the stage of starting the measurement is within a range of 20% or less from the tip in the length direction of the leaf.

[0066] As a machine for measuring the downward displacement amount of the leaf, a smartphone with a distance measurement function can also be used. In this case, pay attention to the fact that the direction of the optical axis is fixed and the measurement point gradually moves from the tip side to the base side as the leaf descends. Regardless of the downward state of the leaf, when the tip of the leaf is set as the measurement point, as shown in Figure 4, the relationship between the measured value (horizontal axis) and Ψstem (vertical axis) becomes non-linear.

[0067] 3. Third Embodiment It is also possible to have a form where the process of predicting Ψstem is performed by a server. In this case, for each of a plurality of different plants, data corresponding to Figure 3 is acquired in advance and stored in the server. For example, data of Figure 3 related to peaches, data of Figure 3 related to apples, data of Figure 3 related to pears, data of Figure 3 related to grapes, etc. are acquired in advance and stored in the server.

[0068] The user measures the displacement of the withering leaf 100 as shown in Fig. 1 using the laser distance meter 200, and sends the measured value to the above server via the Internet. At this time, the type of the target plant is specified.

[0069] The server that has received the measured value refers to the data corresponding to Fig. 3 of the target plant, and obtains Ψstem of the plant from the leaf displacement amount. This becomes the predicted value of Ψstem.

[0070] For example, assume that the user wants to know Ψstem for a pear. In this case, the displacement of the pear leaf is measured using the laser distance meter shown in Fig. 1. This measured value is sent to the above server via the Internet together with the fact that the target is a pear. The server that has received this data accesses the data corresponding to Fig. 3 related to the pear, and predicts Ψstem of the pear based on the received measured value. In this case, by sending the measured value to the server, the predicted value of Ψstem can be obtained in real time.

[0071] 4. Fourth Embodiment In the prediction of Ψstem using the graph of Fig. 3, the reliability of the absolute value of the displacement amount on the vertical axis is important. That is, if the starting point (origin) of the displacement is different when obtaining the reference data in Fig. 3 and when actually performing the measurement, the reliability of the predicted value of Ψstem decreases.

[0072] As a countermeasure against this problem, there is a method of making the irrigation state at the start of measurement as similar as possible, but here another method will be described.

[0073] First, obtain the data in Fig. 3. This is the same as in the case of the first embodiment. Next, in actual measurement, measure the displacement amount corresponding to the vertical axis in Fig. 3 at two or more points. Next, fit these two or more points, and set a straight line having the same slope as the graph in Fig. 3. Then, obtain the correction value γ of the value on the vertical axis necessary to match this graph with the graph in Fig. 3.

[0074] This correction value γ becomes a value such as +10 mm or -5 mm. Add this correction value γ to the measured value α during actual measurement, apply α + γ to the vertical axis of FIG. 3, and predict the value of Ψstem on the horizontal axis.

[0075] According to this method, even if there is a difference between the irrigation state at the time of acquiring the data of FIG. 3 obtained in advance and the irrigation state at the time of actual measurement, the difference can be corrected. Of course, it goes without saying that it is preferable to make the irrigation states as the same as possible during actual measurement and when acquiring the data of FIG. 3 obtained in advance.

Explanation of Signs

[0076] 10… plant, 100… leaf, 200… laser rangefinder, 201… pan-tilt, 202… leg, 300… data processing device (PC).

Claims

1. A method for predicting the stem water potential (Ψstem) to obtain a predicted value of the stem water potential based on the measured value of the downward displacement amount of the leaf, comprising: the downward displacement amount of the leaf is measured by a laser rangefinder with a fixed orientation; the relationship between Ψstem and the downward displacement amount of the leaf is known; in the first stage of measurement, the laser rangefinder is directed in a direction within 20% from the tip of the leaf in the length direction of the leaf from obliquely above the leaf, and is arranged in such a direction that the leaf is within the measurement range when the downward displacement of the leaf occurs; A method for predicting the stem water potential to obtain a predicted value of Ψstem based on the downward displacement amount of the leaf and the relationship.

2. The method for predicting the stem water potential according to claim 1, wherein the distance measurement position of the laser rangefinder on the leaf moves from the tip side to the root side of the leaf as the leaf descends.

3. The method for predicting the stem water potential according to claim 1 or 2, wherein there is a linear relationship between the displacement amount of the leaf based on the measured value of the laser rangefinder and Ψstem.

4. A laser rangefinder with a fixed orientation for measuring the downward displacement amount of a plant leaf, a storage unit storing the relationship between the Ψstem of the plant and the downward displacement amount of the plant leaf, comprising: in the first stage of measurement, the laser rangefinder is directed in a direction within 20% from the tip of the leaf in the length direction of the leaf from obliquely above the leaf, and is arranged in such a direction that the leaf is within the measurement range when the downward displacement of the leaf occurs; A system for predicting the stem water potential to obtain a predicted value of Ψstem based on the measured value of the downward displacement amount of the leaf and the relationship.

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