JP7698876B2 - Method and system for predicting water potential of stem - Google Patents

Description

The present invention relates to a technology for obtaining information related to water stress in plants.

Irrigation is important for plant growth. The state in which a plant needs water is discussed under the concept of water stress. The water potential of a plant's stem (Ψstem) is known as an index for quantitatively evaluating water stress (see Non-Patent Document 1). In addition, Non-Patent Document 2 and Patent Document 1 are known as techniques for estimating water potential.

PRINCIPLES OF SOIL AND PLANT WATER RELATIONS M.B. KIRKHAM Kansas State University(2005) Determining the timing of irrigation using the leaf wilting angle of Unshu mandarin as an indicator (Nakamoto et al., 1998 Kinki-Chugoku Agricultural Research Results Information)

JP 2019-193583 A

By understanding the water potential of the stem (Ψstem), it is possible to estimate the timing of watering the target plant. However, as will be described later, measuring Ψstem is tedious, and obtaining measured data in real time is not practical.

One possible method to indirectly obtain the Ψstem is to use the relationship between the degree of leaf wilting and the Ψstem, as described in Non-Patent Document 2. Therefore, the inventors measured the displacement of the leaf tips (the amount of downward displacement caused by wilting) of peaches and investigated the relationship with the target Ψstem.

As a result, a correlation was confirmed between the displacement of the leaf tip 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 leaf tip.

Specifically, the peaches observed by the inventors had a Ψstem in the range of approximately -25 to -5 (bar). Here, when Ψstem was in the range of -15 to -5 (bar), a graph with Ψstem on the horizontal axis and leaf tip displacement on the vertical axis became almost horizontal, making it difficult to predict Ψstem from the displacement of the leaf tip.

If the Ψstem could be predicted, the timing of irrigation could be predicted, which would be extremely useful for crop cultivation. However, the above tendency is also predicted for other plants, which has been a major problem in putting this technology to practical use.

In this context, the present invention aims to predict the stem water potential (Ψstem) of a plant with high accuracy from the degree of leaf wilting.

The present invention is a method for predicting the water potential of a stem, which obtains a predicted value of the water potential of the stem (Ψstem) based on an actual measurement of the amount of downward displacement of a leaf, wherein the amount of downward displacement of the leaf is measured by a laser range finder with a fixed orientation, and the relationship between the Ψstem and the amount of downward displacement of the leaf is known, and in an initial stage of measurement, the laser range finder is directed in a direction within a range of 20% from the tip of the leaf in the length direction of the leaf from diagonally above the leaf, and is positioned in an orientation such that the leaf falls within the measurement range when the downward displacement of the leaf occurs, and a predicted value of the Ψstem is obtained based on the amount of downward displacement of the leaf and the relationship.

In the present invention, the position of the leaf measured by the laser range finder may move from the tip of the leaf to the base as the leaf descends. In the present invention, the amount of displacement of the leaf based on the measurement value of the laser range finder may have a linear relationship with the Ψstem.

The present invention can also be understood as a system for predicting water potential of a stem, which includes a laser rangefinder, the orientation of which is fixed, that measures the amount of downward displacement of a leaf of a plant, and a memory unit that stores the relationship between the Ψstem of the plant and the amount of downward displacement of the leaf of the plant , and in the first stage of measurement , the laser rangefinder is directed in a direction within a range of 20% from the tip of the leaf in the length direction of the leaf from diagonally above the leaf, and is positioned in an orientation such that the leaf falls within the measurement range when the downward displacement of the leaf occurs, and a predicted value of the Ψstem is obtained based on an actual measurement of the amount of downward displacement of the leaf and the relationship.

According to the present invention, the water potential of a plant's stem can be predicted with high accuracy based on the degree of leaf wilting.

FIG. 1 is a schematic diagram of an embodiment. 2 is a photograph corresponding to FIG. 1 . 13 is a graph showing the relationship between the amount of leaf displacement and Ψstem. 13 is a graph showing the relationship between the amount of displacement (amount of descent) of the leaf tip obtained from an image and Ψstem. 13 is a graph showing the relationship between leaf angle and Ψstem obtained from an image. FIG. 13 is a diagram showing a model for verifying the relationship between the measurement value by a laser range finder and Ψstem. 7 is a graph predicting the relationship between the measurement value by the laser range finder and Ψstem based on the model in FIG. 6.

1. First embodiment (overview)
Fig. 1 is a conceptual diagram of an embodiment, showing a leaf 100 to be measured, a laser range finder 200, and a data processing device 300. Fig. 2 is a drawing substitute photograph of the measurement state shown in Fig. 1.

Here, a peach 10 is selected as the target plant, and its leaves 100 are the target of measurement. Here, a peach is shown as an example, but the type of target plant is not particularly limited.

The laser rangefinder 200 is a rangefinder that utilizes the principle of optical distance measurement using laser light. A commercially available device is used as the laser rangefinder 200. With its position and orientation fixed, the laser rangefinder 200 measures the downward displacement of the leaf 100 from diagonally above. The laser rangefinder 200 is fixed to a pan head 201, which is fixed onto legs 202 such as a tripod. The position and orientation of the laser rangefinder 200 are adjustable.

The data processing device 300 is composed of a PC (personal computer). The data processing device 300 is equipped with a CPU, a storage device, an input/output interface, a communication device, and a GUI (graphical user interface), and can operate and be operated in the same way as a normal PC. The data processing device 300 can also be configured to use a dedicated arithmetic processing device or to use a data processing server installed in a remote location.

The data processing device 300 calculates (predicts) the water potential (Ψstem) of the stem of the plant being measured (in this example, peach 10) based on the measurement value obtained by the laser range finder 200. The relationship between the measurement value obtained by the laser range finder 200 and the Ψstem is obtained in advance and stored in a memory unit (e.g., a hard disk drive, a semiconductor memory, etc.) in the data processing device 300.

The data processing device 300 can be understood to include a Ψstem prediction unit 301 that predicts the Ψstem based on the measurement value by the laser rangefinder 200, and a memory unit 302 that stores data necessary for the calculation to perform this prediction.

The laser range finder 200 is connected to the data processing device 300 via wireless LAN or wired communication. The measurement value of the laser range finder 200 is input to the data processing device 300. Based on this input, Ψstem is calculated by referring to the data acquired in advance.

(Measuring method)
At the start of the measurement, the leaf 100 strives to maximize (saturate) the water potential (Ψstem) of the stem of the plant (in this case, the peach 10) that has the leaf 100. In this example, the measurement starts from a state where the plant is sufficiently irrigated.

When measurement begins, the laser range finder 200 is placed at an angle above the leaf 100. θ in FIG. 1 should be between 45° and 75°. If θ<45°, there is an increased possibility that the optical axis will deviate from the leaf when the tip of the leaf 100 descends, making it impossible to measure the distance. If θ>75°, there is an increased possibility that the displacement of the leaf 100 will not be measured accurately.

In the first stage, the attitude of the laser rangefinder 200 is adjusted so that the measurement point is at the tip of the leaf 100. In other words, the optical axis of the laser rangefinder 200 is aligned with the tip of the leaf 100. Measurements are performed with the position and attitude of the laser rangefinder 200 fixed.

When peach 10 (leaf 100) wilts and the tip of leaf 100 drops, it is necessary to determine the position of laser range finder 200 so that the measurement laser light of laser range finder 200 does not deviate from leaf 100. In other words, as the leaf 100 wilts, the tip of the leaf 100 drops. Of course, the leaf 100 itself also gradually moves downward. At this time, it is necessary for the measurement laser light from laser range finder 200 to continue to hit leaf 100.

For this reason, in the example of Figure 1, the attitude of the laser range finder 200 at the start of measurement is adjusted so that the expected path of movement of the tip of the leaf 100 when wilting occurs (usually a gentle arc) and the optical axis of the laser range finder 200 are included in the same plane. For simplicity, a vertical plane is assumed that includes the branches at the base of the leaf 100, and this vertical plane and the optical axis of the laser range finder 200 are approximately aligned. Note that a distance of approximately 15 cm to 50 cm is appropriate between the leaf 100 and the laser range finder 200.

When time passes from when the stem's water potential (Ψstem) is at its maximum, the value of Ψstem decreases as water evaporates from the leaves 100. As a result, the leaves 100 wilt and succumb to gravity, displacing downward. As a result, the leaves 100 move away from the laser rangefinder 100, and the amount of displacement (downward displacement) is measured using the principles of laser ranging.

As will be described in detail later, when measuring the displacement of the leaf 100 in the state shown in FIG. 1, even if the tip of the leaf 100 is initially targeted, the measurement point gradually moves from the tip of the leaf 100 to the center (towards the base as viewed from the tip) as the leaf 100 descends. Therefore, the system shown in FIG. 1 does not constantly measure the displacement of the tip of the leaf 100.

(Regarding advance data acquisition)
As described above, the relationship between the measurement value by the laser range finder 200 and the Ψstem is acquired in advance. This acquisition will be described below. First, a sample plant 10 (in this case, a peach) is selected. Next, the above-mentioned measurement is performed on the leaf 100 of this plant 10. Note that at the start of the measurement, the leaf 100 strives to maximize (saturate) the water potential (Ψstem) of the stem of the plant (in this case, the peach 10) that owns the leaf 100. In this example, the measurement is started from a state where the plant has been sufficiently irrigated.

The laser range finder 200 can measure a minimum of one leaf. Of course, it is also possible to select multiple leaves and use the average value.

On the other hand, another leaf of the same target plant is selected as a sample leaf for measuring the stem water potential (Ψstem). Multiple sample leaves are prepared. Here, the displacement of the leaf 100 is measured by the laser rangefinder 200, and the stem water potential (Ψstem) is measured using the sample leaf at the time when the measurement value is obtained.

What is measured here is the water potential of the stem, called Ψstem. Because the water potential of the stem cannot be measured directly, the value of the leaf water potential is measured when the moisture content of the leaves and tree has reached equilibrium, and this is obtained as the water potential of the stem, Ψstem. In this case, since the moisture content of the tree and leaves has reached equilibrium, the water potential of the leaves and the water potential of the stem can be evaluated as being the same.

There are various methods for measuring the Ψstem, and dedicated measuring devices are commercially available. Here, we measured the Ψstem using the following method.

First, for each of the multiple sample leaves, the leaf is sealed in an aluminum-coated bag to suppress evaporation of water from the leaf. Then, when the movement of water between the leaf and the tree has stopped, the branch with the leaf is cut off and placed in a sealed chamber (pressurized chamber), and with the cut part of the branch exposed outside the sealed chamber, the air pressure inside the sealed chamber is increased.

When the pressure inside the sealed chamber is increased, air pressure is applied to the leaves, causing water to seep out from the cut ends of the branches exposed outside the sealed chamber. The water potential of the stem of the target plant (Ψstem) can be calculated from the air pressure at which this water seeps out.

Let's say that moisture seeps out from the cut part of the branch when the pressure inside the sealed chamber is low. In this case, the moisture inside the leaf is relatively sufficient, so even a small amount of pressure causes moisture to seep out. In this case, Ψstem will have a relatively large value. In other words, the moisture content inside the tree is evaluated to be relatively high.

On the other hand, suppose moisture seeps out from the cut part of the branch under conditions of high pressure inside the sealed chamber. In this case, since there is relatively little moisture in the leaves, moisture will not seep out unless the pressure is increased. In this case, Ψstem will have a relatively small value. In other words, the moisture content inside the tree is evaluated as being relatively low.

Ψstem is an index that indicates the water pressure within the tree of the target plant. When Ψstem is small, it indicates that the plant does not have enough water and needs more. Conversely, when Ψstem is at its maximum value (saturation value), it is interpreted as the plant having enough water.

In other words, the Ψstem is the pressure value of the sealed chamber above plus a negative value, and the Ψstem can be calculated using the method described above.

The maximum value of Ψstem is mostly determined by the type of plant. For example, in the case of peaches, the maximum value of Ψstem is about -3 bar (-0.3 MPa). To be precise, there may be some differences depending on the variety even within the same plant.

The above-mentioned Ψstem measurement using the sample leaf is performed in various conditions of the target plant (in this case, a peach). Specifically, the above-mentioned Ψstem measurement is performed in multiple conditions in which different values are obtained as the measurement value of the laser rangefinder 200.

Here, the displacement of the leaf 100 measured by the laser range finder 200 is plotted on the vertical axis, and the measured value of the Ψstem at the timing when this displacement was obtained is plotted on the horizontal axis, and the value of the Ψstem is plotted. In this way, the graph in Figure 3 is obtained. The displacement of the leaf 100 is the displacement of the leaf on the optical axis 211 of the laser range finder 200 in Figure 1.

The displacement amount on the vertical axis in Figure 3 is the displacement amount based on the start of measurement. If the leaf 100 moves away from the laser range finder 100, the displacement amount becomes larger.

The graph in Figure 3 shows a linear correlation between Ψstem and the amount of leaf displacement measured by the laser range finder 200. Figure 3 also shows that similar data is obtained regardless of the measurement time.

It has been found that leaf water potential measured directly without covering the leaves with aluminum-evaporated bags is stable over the entire tree before dawn, but varies greatly at other times. This is because photosynthesis and water transpiration from the leaves differ depending on the position of the leaf being measured within the tree. In contrast, the water potential of the trunk is not affected by fluctuations in the water potential of each leaf, and can be measured stably even during the day. In the graph of Figure 3, the relationship between the displacement of the leaf 100 measured by the laser rangefinder 200 and the tree's water potential (Ψstem) is obtained stably regardless of the time. In other words, the relationship between the displacement of the leaf 100 measured by the laser rangefinder 200 and the Ψstem is obtained stably.

In the graph of Figure 3, there is a range where the displacement on the vertical axis is negative. This means that an upward displacement of the leaf 100 was observed after measurement began. After the plant appears to have absorbed sufficient water, it is possible that a further increase in Ψstem may be observed due to factors such as changes in temperature, humidity, and the state of photosynthesis. The range where the displacement on the vertical axis is negative is thought to be related to this phenomenon.

(Example of processing)
The graph in Fig. 3 is obtained in advance, and the data is stored in the memory unit 302 of the data processing device 300. Then, the amount of downward displacement of the leaf 100 is measured by the laser range finder 200. The amount of downward displacement is obtained as the amount of change in distance measured by the laser range finder 200. This amount of downward change is applied to the graph in Fig. 3, and the value of Ψstem on the horizontal axis of Fig. 3 is predicted. This process is performed by the prediction unit 301 of the data processing device 300.

(Consideration)
The factors that enable the extremely linear data shown in Fig. 3 to be obtained will be considered below. Fig. 4 is a graph in which the vertical axis represents the amount of downward displacement of the tip of the leaf 100 detected from an image, rather than from the laser range finder 200 in Fig. 1. The horizontal axis of Fig. 4 represents the Ψstem obtained at the timing when the value on the vertical axis was obtained.

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

As is clear from Figure 4, in the range of Ψstem = -10 to -5 bar, the dependence of Ψstem on the amount of movement is not clearly evident. This means that even if the amount of movement of the tip of the leaf 100 is calculated from the image, it is not possible to accurately predict Ψstem in the above range.

Figure 5 is a graph in which the change in the angle of the leaf 100 caused by wilting is detected from the captured image, rather than the amount of movement of the tip of the leaf 100, and this is plotted on the vertical axis. The horizontal axis is the same as in Figure 4. The same tendency as in Figure 4 can be seen in Figure 5.

Figures 4 and 5 are not consistent with Figure 3. This point will be considered below. First, in the first stage of measuring the displacement of the leaf 100 using the laser light in Figure 1, the laser range finder 200 is aimed at the tip of the leaf 100. Also, the laser range finder 200 is directed at the leaf 100 from diagonally above.

When the leaf 100 is displaced downward as it wilts, the measurement point of the laser range finder 200 gradually moves from the tip of the leaf 100 toward the base (center) of the leaf 100. This will be explained using the model in Figure 6. Figure 6 shows how the tip of the leaf 100 gradually descends as it wilts.

Considering the structure connected from trunk to branches to leaves, the descent of the leaves as they wilt does not occur uniformly across the plant, but is relatively greater at the leaf tips and relatively smaller at the leaf bases. As a result, as shown in Figure 6, the measurement point of the laser rangefinder 200 gradually shifts from the leaf tips to the center (towards the leaf base).

Below, we use the data in Figure 4 to predict the relationship between the amount of displacement and Ψstem when leaf displacement is measured using a laser range finder according to the model in Figure 6, based on the model above.

Here, the following model is assumed.
(1) The measured part of the laser rangefinder gradually shifts to the center of the leaf as the tip of the leaf descends.
(2) The tips of the leaves droop, but the base does not.

The curve showing the degree of leaf drooping in Figure 6 was created using a curve drawing tool in a spreadsheet software. The tip of the leaf moves on an arc, but is assumed to hang down vertically. The laser range finder was also set up to measure from diagonally above.

Assuming the model in Figure 6, the change in distance in the optical axis direction of the measurement laser light corresponds to the change in the vertical direction of the leaf tip. The procedure is as follows.

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

The resulting displacement of the leaf position in the direction of the optical axis of the laser measurement light is the displacement distance measured by the laser light. The graph in Figure 7 was created by plotting the Ψstem on the horizontal axis and the displacement distance measured by the laser light on the vertical axis.

Figure 7 is an estimated graph obtained by generating data from the measured data in Figure 4 according to the model in Figure 6, based on the assumptions (1) and (2) above. In Figure 7, a high degree of linearity is obtained in the relationship between the horizontal and vertical axes, just like in Figure 3.

Considering that Figure 4 shows actual measurements, the estimated graph in Figure 7 accurately reflects the actual situation, and the validity of the assumptions (1) and (2) above and the model in Figure 6 can be inferred.

The findings from the above investigation can be briefly summarized as follows:
"As the Ψstem drops, the tip of the leaf drops. If a laser rangefinder is pointed at the tip of the leaf from diagonally above, the measurement part will gradually move from the tip of the leaf towards the base as the tip of the leaf drops. There is a strong, almost linear correlation between the change in measurement distance and the change in Ψstem. Using this correlation, it is possible to calculate the Ψstem from the measurement values related to the displacement of the leaf. For example, in the case of peaches, the Ψstem can be calculated from the measurement values with high resolution in the range of Ψstem > -10 bar, where the Ψstem cannot be calculated when focusing on the amount of movement or angle change of the leaf tip."

2. Second embodiment The target position of the laser range finder in the initial stage does not have to be exactly at the tip of the leaf. In this case, the target of the laser range finder in the initial stage is as close to the tip of the leaf as possible (without, of course, straying from the leaf). Specifically, the target at the start of measurement is preferably within 20% of the tip of the leaf in the length direction.

A smartphone with a distance measurement function can also be used as a device to measure the downward displacement of the leaf. In this case, it should be noted that the direction of the optical axis is fixed, and the distance measurement point gradually moves from the tip to the base as the leaf descends. If the tip of the leaf is used as the distance measurement point, regardless of the state of leaf descent, the relationship between the measurement value (horizontal axis) and Ψstem (vertical axis) becomes nonlinear, as shown in Figure 4.

3. Third embodiment It is also possible to perform the process of predicting Ψstem on a server. In this case, data corresponding to FIG. 3 is acquired in advance for each of a plurality of different plants and stored in the server. For example, data on peaches in FIG. 3, data on apples in FIG. 3, data on pears in FIG. 3, and data on grapes in FIG. 3 are acquired in advance and stored in the server.

As shown in Figure 1, the user measures the movement of the leaf 100 due to wilting using the laser distance meter 200, and sends the measurement value to the server via the Internet. At this time, the user specifies the type of plant to be targeted.

The server that receives the measurement value refers to the data corresponding to Figure 3 for the target plant, and calculates the Ψstem of the plant from the amount of leaf displacement. This is the predicted value for the Ψstem.

For example, suppose a user wants to know the Ψstem of a pear. In this case, the displacement of the pear's leaves is measured using a laser range finder as shown in Figure 1. This measurement value is sent to the above-mentioned server via the Internet along with the fact that the target is a pear. The server that receives this data accesses the data related to pears corresponding to Figure 3, and predicts the Ψstem of the pear based on the received measurement value. In this case, by sending the measurement value to the server, a predicted value of the Ψstem can be obtained in real time.

4. Fourth embodiment When predicting Ψstem using the graph in Fig. 3, the reliability of the absolute value of the displacement on the vertical axis is important. In other words, if the starting point (origin) of the displacement is different between when the reference data in Fig. 3 is obtained and when the actual measurement is performed, the reliability of the predicted Ψstem value decreases.

One way to deal with this issue is to keep the irrigation conditions as consistent as possible when measurements begin, but here we will explain another method.

First, the data in Figure 3 is obtained. This is the same as in the first embodiment. Next, in the actual measurement, the displacement amount corresponding to the vertical axis in Figure 3 is measured at two or more points. Next, fitting is performed to these two or more points to set a straight line with the same slope as the graph in Figure 3. Then, a correction value γ for the vertical axis value required to make this graph coincide with the graph in Figure 3 is obtained.

This correction value γ is, for example, a value of +10 mm or -5 mm. This correction value γ is added to the measured value α during actual measurement, and α + γ is applied to the vertical axis of Figure 3 to predict the value of Ψstem on the horizontal axis.

With this method, even if there is a difference between the irrigation conditions when the pre-obtained data in Figure 3 is acquired and the irrigation conditions 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 conditions as similar as possible at the time of actual measurement and when the pre-obtained data in Figure 3 is acquired.

10...plant, 100...leaf, 200...laser range finder, 201...platform, 202...leg, 300...data processing device (PC).

Claims (4)

A method for predicting stem water potential, which obtains a predicted value of stem water potential (Ψstem) based on an actual measurement of downward displacement of leaves, comprising:
the downward displacement of the leaf is measured by a laser range finder with a fixed orientation;
The relationship between Ψstem and the downward displacement of the leaf is known.
In a first step of the measurement, the laser range finder is directed in a direction within a range of 20% from a tip of the leaf in a longitudinal direction of the leaf from an obliquely upward direction of the leaf , and is disposed in such an orientation that the leaf falls within a measurement range when the downward displacement of the leaf occurs;
A method for predicting stem water potential, comprising obtaining a predicted value of the Ψstem based on the downward displacement of the leaf and the relationship. The method for predicting water potential of a stem according to claim 1, wherein the distance measurement position on the leaf measured by the laser distance meter moves from the tip side of the leaf to the base side as the leaf descends. The method for predicting stem water potential according to claim 1 or 2, wherein there is a linear relationship between the leaf displacement amount based on the measurement value of the laser range finder and the Ψstem. A laser range finder whose orientation is fixed and which measures the downward displacement of the leaves of a plant;
a memory unit that stores a relationship between the Ψstem of the plant and the downward displacement amount of the leaves of the plant,
In a first step of the measurement, the laser range finder is directed in a direction within a range of 20% from a tip of the leaf in a longitudinal direction of the leaf from an obliquely upward direction of the leaf , and is disposed in such an orientation that the leaf falls within a measurement range when the downward displacement of the leaf occurs;
A system for predicting stem water potential that obtains a predicted value of the Ψstem based on the actual measured value of the downward displacement of the leaves and the relationship.

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