Study on Variation Characteristics and Prediction Model of Stem Sap Flow of Jujube Under Different Injection Irrigation Models

Caili YU

Abstract [Objectives] This study was conducted to study the variation laws and characteristics of stem sap flow rate in jujube under the injection irrigation mode， to improve the growth quality of jujube trees.

[Methods] According to the characteristics of the growth environment and physiological characteristics of jujube Junzao trees in southern Xinjiang， eight different injection irrigation modes were designed. Through the variation of stem sap flow rate， the most suitable irrigation method for jujube Junzao trees in southern Xinjiang was explored. The variation law of stem sap flow rate of jujube and the effect of meteorological factors on stem sap flow rate of jujube were studied. The variation law of stem sap flow in jujube trees was analyzed， and multiple regression models and time series models were established to predict stem sap flow of jujube trees.

[Results] The results showed that the simulation results of the time series model were closer to the actual data， and the model could accurately predict the stem sap flow rate of Junzao.

[Conclusions] This study is suitable for the full utilization of water resources in the Tarim Basin in southern Xinjiang， and has important practical significance for analyzing the effect of water stress on the growth and development of jujube trees and precision irrigation.

Key words Stem sap flow rate; Correlation analysis; Regression analysis; Time series

Southern Xinjiang has a typical extreme arid desert climate， with the characteristics of long light time， arid and little rain. The annual average precipitation is only 20-80 mm， while the annual average evaporation is 2 000-3 000 mm. The climate is extremely bad. The southern region of Xinjiang is rich in light and heat resources and is an important production base for high-quality red dates in China. However， the growth of jujube trees often faces water stress under the natural conditions of widespread drought， which seriously affects the output and quality of red dates[1-2]. The soil moisture content is mainly distributed in the range of 30-50 cm. Injection irrigation not only has the characteristics of suppressing the change of soil moisture content and reducing soil evaporation， but also makes the moisture content of the soil profile change smoothly between 30 and 40 cm， providing a stable soil water environment for the water needs of red dates. Therefore， quantitatively describing the law of soil-jujube-atmosphere moisture migration in jujube orchards is of great practical significance for analyzing the effects of water stress on the growth and development of jujube trees and precision irrigation.

The crop growth model drives the localized crop growth model through spatial weather， soil， and farm crop management data to simulate the formation of crop roots， stems， leaves， flowers， fruits and other tissues and changes in morphological indicators such as volume， weight， number， etc.， and finally obtains crop phenology， vegetative growth， reproductive growth， yield formation and other crop growth information. After years of development， different series of system models have been formed， such as DSSAT， WOFOST， STICS and Chinese crop computer simulation optimization decision-making system CCSODS[3-5].

The methods and models for simulating water migration proposed by previous studies are often different due to different crops and experimental conditions[6]. In this study， we fully considered the dynamic process of soil-plant-atmosphere moisture migration and transformation， combined the simulation of soil water migration with the simulation of crop growth process， and established a coupled model of moisture migration and transformation and crop growth under drought conditions， which was then verified.

A sub-model of jujube soil moisture balance in southern Xinjiang was established， and the effects of water stress on the growth and development of jujube trees were evaluated through model simulation， providing a simulation method for precision irrigation.

Materials and Research Methods

General situation of the experiment field

The experimental area is located at the Water Irrigation Experimental Base of Tarim University on the southern edge of the Taklimakan Desert， 80°51′ east longitude， 40°37′ north latitude. The annual average temperature is 10 ℃. The soil is sandy loam， with good permeability and a soil density of 1.34 g/cm3. The water holding rate is 25%， and the groundwater depth is below 3 m.

Experimental design and determined items

The self-made injection irrigation equipment was composed of vertical pipes and horizontal pipes which were connected. The depth of the vertical pipes was determined according to the burying depth. The total length of the horizontal pipes was 200 mm， and the openings were at 20， 100 and 180 mm， and had an upward direction. Water was input from the vertical pipelines to the underground horizontal pipelines， and entered the soil through the openings of the underground horizontal pipelines. The water injection test was conducted on bare ground. The experiment was designed with 8 different water injection modes， each of which consisted of different water injection pressure， water injection volume per plant， and water injection pipe burying depth. The specific parameters are shown in Table 1. The variation rate of stem sap flow measured by probe 1 was stem sap flow 1， and the rest corresponded to stem sap flow 1， stem sap flow 2， stem sap flow 3， stem sap flow 4， stem sap flow 5， stem sap flow 6， stem sap flow 7， and stem sap flow 8， respectively. The planting of jujube trees in Xinjiang generally adopts the dwarf dense planting mode. The trunk diameter is generally 20-50 mm， the crown height is 1.0-1.5 m， and the root system is mainly distributed in the 80 mm soil layer. Considering the uneven size of jujube trees， the diameter of the bottom of the trunk was determined. The jujube stem sap flow data was measured every 1 h， and the meteorological data from April 2018 to September 2019 was measured simultaneously.

Results and Analysis

Diurnal variation characteristics of jujube stem sap flow

It can be seen from Fig.1 that the diurnal variation characteristics of jujube steam sap flow rate showed a "table" shape curve， which was related to the sunrise and sunset in southern Xinjiang， The variation rate of stem sap flow was very small and almost zero at night， but was greater and showed a single peak during the day. The stem sap flow rate of jujube trees was stable and low and had almost no change from 22：00 to 8：00 the next day; between 8：00 and 13：00， it increased significantly and almost changed linearly; during the 5 h from 13：00 to 18：00， the stem sap flow rate reached the maximum value and maintained a steady and dynamic variation; and between 18：00 and 22：00， the stem sap flow rate decreased significantly and almost linearly decreased to a lower state. The stem sap flow rate first increased largely， then kept a smooth and steady state and finally decreased sharply. The stem sap flow rate of jujube trees was active for up to 13 h during the day， and the cumulative stem sap flow during the day accounted for 89.67% of the whole days stem sap flow.

The variation rates of stem sap flow under different injection irrigation modes from June 13 to June 23 are shown in Fig.2. It could be seen from the figure that the variation law of stem sap flow rate was inconsistent between day and night. During the day， the magnitude of stem sap flow 4 was the largest， followed by stem sap flow 6， stem sap flow 1， stem sap flow 7， stem sap flow 3， stem sap flow 5， stem sap flow 2， and finally stem sap flow 8. At night， stem sap flow 5 showed the largest amplitude， followed by stem sap flow 4 and stem sap flow 6， and the differences of the remaining several kinds of stem sap flow were not very obvious. From the perspective of the stem sap flow rate throughout the day， the variation amplitude of stem sap flow 4 was the largest， so the fourth type of water injection irrigation could be considered.

Relationship between jujube stem sap flow with meteorological factors

Jujube tree growth and development are not only related to the soil environmental factors， but also related to local meteorological factors. As shown in Fig.3（a）， the stem sap flow rate was negatively correlated with relative humidity. Due to the strong solar radiation during the day and the large temperature difference between day and night in southern Xinjiang， the relative humidity is relatively small， but is relatively large at night. When the relative humidity is high， there is a greater water potential gradient between trees and the soil， and the root system can easily absorb water from the soil to compensate for the lack of water in trees. It can be seen from Fig.3 （b）， Fig 3（c） and Fig 3（d） that the stem sap flow rate of jujube trees had a positive correlation with the changes of solar radiation， temperature， wind speed， especially with the change of solar radiation. The correlation between the two was the strongest， and the peaks of the two were basically the same. It can be seen from Fig.3（c） that the stem sap flow rate changed with the change of air temperature， but the peaks and troughs of the two appeared at different times. Generally speaking， the peaks and troughs of air temperature lagged behind the peak of stem sap flow rate. From Fig.3（d）， it can be seen that the variation laws of stem sap flow rate and wind speed were not very significant. Therefore， as the sun rises， the transpiration of jujube trees gradually increases， and the stem sap flow rate increases with the increase of solar radiation intensity， the rise of temperature， and the decrease of relative humidity， and vice versa.

It indicates that under normal water supply conditions and moderate drought stress， solar radiation， air temperature and relative humidity are the main environmental factors that affect the change of stem sap flow rate， and under severe drought stress， the degree of soil water deficit affects the stem sap flow rate of jujube plants and weakens the effect of relative humidity on its stem sap flow rate.

We analyzed the correlation between the stem sap flow rate of jujube trees with meteorological factors. The results are shown in Table 1. The stem sap flow rate had the most significant correlation with solar radiation， with a correlation coefficient of 0.956， followed by temperature and relative humidity， with correlation coefficients of 0.783 and -0.624， respectively， and the correlation coefficient with wind speed was 0.564， indicating that the stem sap flow rate of jujube trees was affected by meteorological factors such as solar radiation， temperature， relative humidity and wind speed， and the effects ranked as solar radiation>temperature>relative humidity>wind speed.

Agricultural Biotechnology2020

Prediction model of stem sap flow in jujube based on regression analysis and time series analysis

Through the establishment of the correlation regression model， the environmental factors could be used to predict the change of the stem sap flow rate in jujube trees under drip irrigation conditions， and thus provide a certain theoretical basis for the rational irrigation of jujube trees under drip irrigation.

y=15.921+0.163*c+0.8778*a+ 0.117*b+ 0.416*d， R-squared=0.936， Prob（F-statistic）=0.000， where y is the stem sap flow rate， a is solar radiation intensity， b is temperature， c is relative humidity of the air， and d is wind speed.

LOG（y）=[AR（1）=1.613 436 308 19， AR（2）=-0.616 312 405 724， MA（1）=-0.261 188 533 62，UNCOND，ESTSMPL="1 264"]， R-squared=0.976， Prob （F-statistic）=0.000， where y is the stem sap flow rate.

Results and Conclusions

When the plants suffer from water stress or the meteorological conditions change suddenly， the predicted value of the stem diameter change obtained by the principal component regression model is somewhat different from the measured value， but the overall change trend is consistent.

Red dates grow fastest at the beginning of flowering， and it is necessary to supplement water in a timely and effective manner during this period. Through experimental comparison， it is concluded that under the conditions of the same site conditions and agronomic measures， the growth rate of red dates under injection irrigation is significantly faster than those under drip irrigation， which suggests that injection irrigation is beneficial to the absorption and transportation of water by the roots of red dates[7]. Compared with drip irrigation， the fruit setting rate and yield of red dates are significantly increased. Judging from the variation law of soil moisture throughout the growth period， the soil moisture content of the root zone of jujube under drip irrigation conditions is more drastic than that under injection irrigation， and especially in July， the lack of water is bound to affect the growth of red dates. Drip irrigation in July and August can no longer meet the needs of the growth of red dates. At this stage， moisture retention is very important， and other irrigation methods must be applied simultaneously to achieve a high yield. However， there is no ground evaporation under injection irrigation， and the soil moisture changes relatively slowly throughout the growth period. Jujube trees can make full use of soil moisture under injection irrigation， which can also meet the needs of red date growth without the need for other auxiliary irrigation measures.  As a new type of water-saving technology， injection irrigation has significant water-saving benefits and can be widely used in the root irrigation of fruit trees in southern Xinjiang. Multiple regression method was applied to establish correlation regression models of stem sap flow rate in jujube trees and meteorological factors， and multiple regression equations were obtained. After regression coefficient and correlation coefficient tests， the regression coefficients reached the extremely significant level.

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