Effects of Intelligent Irrigation on Photosynthetic Characteristics of Citrus Leaves and Fruit Quality

Taiqing HUANG, Yanfei HUANG, Dan LU, Yaoxin LIU

Abstract [Objectives] This study was conducted to improve the water use efficiency and fruit quality of citrus， and realize the automatic irrigation and standardized production in citrus orchards.

[Methods] With Orah as the research object， the effects of intelligent irrigation management in citrus orchards on citrus leaf chlorophyll content， photosynthetic characteristics and fruit quality were investigated by comparing with conventional farmer management.

[Results] The smart irrigation measure in citrus orchards significantly increased the SPAD value of leaves at the maturation stage of citrus， and simultaneously significantly improved the photosynthetic rate， transpiration rate and stomatal conductance at the flower bud differentiation stage， as well as the photosynthetic rate， transpiration rate， stomatal conductance and intercellular CO2 concentration at the maturation stage. However， the effects on the photosynthetic characteristic indexes in the rainy season were not significant. Compared with conventional experience management， the smart irrigation management measure of citrus orchards increased the edible rate and juice rate of citrus by 4.53 and 3.69 percentage points， respectively， and increased soluble solids， total sugar， vitamin C and sugar-acid ratio by 16.75%， 20.86%， 24.10% and 13.17%， respectively.

[Conclusions] The smart irrigation management fully met the water demand for citrus growth due to timely irrigation， significantly improved the photosynthesis indicators of citrus leaves during drought， and significantly improved the quality of citrus.

Key words Photosynthetic characteristics; Citrus quality; Smart irrigation; SPAD value; Citrus orchard

Received： May 17， 2022  Accepted： July 18， 2022

Supported by Guangxi Key Research and Development Project （GK AB1850024）.

Taiqing HUANG （1986-）， male， P. R. China， associate researcher， master， devoted to research about soil ecology and high-value agriculture.

*Corresponding author.

Citrus fruit is one of the main fruits in Guangxi， making Guangxi the most important citrus producing area in China. Citrus is a high-water and fertile crop， and the scientific management of water has a direct impact on the yield and quality of citrus and the economic benefits of orchards. Although Guangxi has a tropical and subtropical monsoon climate with abundant rainfall， the rainfall is concentrated in the months from March to September， accounting for more than 80% of the annual rainfall， and seasonal droughts will occur during the rest of the year， when irrigation is required to meet the needs of citrus growth. Traditional irrigation often adopts drip irrigation， sprinkler irrigation and other forms only based on experience， resulting in untimely irrigation and excessive or delayed irrigation， which not only leads to waste of water resources， but also causes environmental pollution， increases the cost of citrus planting， and also affects citrus yield and quality. Some studies have shown that water can significantly affect the photosynthetic rate， transpiration rate， stomatal conductance and intercellular CO2 molar concentration of citrus leaves， but moderate water deficit will not affect the photosynthetic indicators of citrus leaves， and can save 15% of irrigation water[1-2]. Mild water deficit can increase the yield of citrus to 5.0% and the water use efficiency to 5.5%[3].

With the rapid development of big data and Internet+， the intelligent control system of agricultural Internet of things has gradually been applied. Chi[4] designed an intelligent agricultural greenhouse management system using ZigBee wireless sensor network as the networking technology， which can realize data collection and remote control in the agricultural production process. Zeng[5] developed an automatic irrigation system integrating water and fertilizers based on programmable logic controller （PLC） and human-machine interface （HMI）. Hai et al.[6] developed the precise irrigation of orchards based on the LPWAN Internet of Things， and realized real-time monitoring and precise irrigation by collecting orchard environmental information. Pan et al.[7] designed a set of distributed orchard remote environmental monitoring system based on the ZigBee short-distance wireless transmission technology and 3G/4G router technology， which achieved long-distance data transmission and wide coverage. Zhao[8] collected the environmental information （soil temperature， moisture， air temperature and humidity， light intensity） of dragon fruit orchards through the Internet of Things technology， and then classified the collected data with the decision tree method， and the fitting accuracy of the model was over 99%. Yang et al.[9] adopted the artificial intelligence technology to upload the data monitored by node sensors to the server through the wireless network， and then allowed experts to make decisions. Shen et al.[10] carried out water irrigation experiments on kiwifruit， Nanfeng mandarin oranges and early-ripening pears， and found that modern water-saving irrigation can not only improve crop quality and yield， but also improve crop soil environment. The investment of automatic control water-saving irrigation equipment can be recovered in 4-6 years， and the net benefit is between 14 790-23 745 yuan/hm2. There have been many studies on the integrated design and application of water and fertilizers based on the Internet of Things and water-saving irrigation experiments， but there are few reports on the intelligent water management based on the Internet of Things on the photosynthetic characteristics and fruit quality of citrus. Therefore， in this study， the effects of intelligent management on the chlorophyll content， photosynthetic characteristics and fruit quality characteristics of citrus were investigated， aiming to provide a theoretical basis for the application of intelligent moisture management based on the Internet of Things to Guangxi citrus.

Materials and Methods

Experimental locations and materials

This study was carried out in the Lijian Scientific Research Base （23°14′N， 108°02′E） of Guangxi Academy of Agricultural Sciences in Wuming District， Nanning City， Guangxi. The area belongs to the subtropical monsoon climate zone， with suitable light temperature and abundant rainfall， which is suitable for the growth of citrus. The soil physical and chemical indicators were as follows： pH 5.84， organic matter 1.6%， total nitrogen 0.071%， total phosphorus 0.0152%， total potassium 1.26%， hydrolyzable nitrogen 56.78 mg/kg， available phosphorus 24.32 mg/kg and available potassium 92.50 mg/kg. The citrus variety tested was Orah， which is a late-maturing citrus variety， with a row spacing of 3.0 m×2.5 m and a planting density of 1 275 plants/hm2. The orchard soil was cultivated once a year， and mainly fertilized 3 times， that is， the germination fertilizer， the blossom-falling and fruit-promoting fertilizer and the shoot-promoting and fruit-strengthening fertilizer. The applied fertilizers were organic fertilizer， compound fertilizer， urea， potassium sulfate， calcium magnesium phosphate fertilizer， zinc sulfate heptahydrate and water-soluble boric acid， etc. The test period was from February 2021 to February 2022， 12 months totally， including a complete growth cycle from the differentiation of citrus flower buds to maturity.

Experimental design

Two treatments were set up in this experiment， namely smart moisture management （SWM） and traditional moisture management. The smart water management is a remote smart management measure with the help of the Internet of Things platform and automated water and fertilizer integrated irrigation facilities. Specifically， moisture and temperature sensors were arranged in the experimental area， mainly to collect data of the 20 and 40 cm soil layers， and to analyze them on the client host through data communication transmission， and through the preliminary research foundation and model construction， the soil moisture conditions suitable for citrus growth were determined. And when the soil moisture content reached the threshold， an irrigation command was sent to the equipment through communication transmission， and the battery valve switch was automatically triggered to automatically irrigate the citrus. In this study， the traditional water management model was used as the control （CK）， that is， local workers with experience in citrus planting and management were hired for management， and artificial irrigation was adopted for the irrigation of the orchard. In other words， the irrigation time and the amount of irrigation were determined according to personal routine experience in the control. The experiment was carried out in a manner of wide field trial， in which the area of each adjacent large field was 666.7 m2. Weeds in all fields were uniformly controlled with chemical herbicides， and the same fertilization and pest management was adopted.

Sample collection and determination

Sample collection

Citrus leaf samples were collected at the flower bud differentiation stage， young fruit stage， fruit expansion stage and fruit maturation stage of citrus， respectively. The nutritious spring shoots of the year that grew evenly were collected uniformly each time， and the 2nd to 3rd leaves from the top of the shoots were collected. Multiple fruit trees were selected in the plot， and each fruit tree was sampled in the east， south， west， north and upper， middle and lower parts. In mid-January 2022， citrus fruit samples were collected. Fruit trees with medium growth were selected， and the fruit was collected in the middle of the tree height from four directions of east， south， west and north as samples for analysis and determination. The leaf samples were washed after collection， subjected to deactivation of enzymes in an oven at 105 ℃ for 30 min， then dried at 65 ℃ to constant weight， and crushed and stored. The total nitrogen， phosphorus， and potassium contents of the leaves were measured. Fruit samples were stored in a refrigerator at 4 ℃ immediately after collection， and various quality indicators were determined as soon as possible.

SPAD value determination

When the leaves were collected at the four stages of flower bud differentiation， young fruit， fruit expansion and maturation， nine healthy fruit trees with similar growth were randomly selected for each treatment as the test objects. For fully developed mature functional leaves on three sunny branchlets without pests and diseases， the relative content of chlorophyll in leaves， namely SPAD value， was measured by an SPAD-502 Plus chlorophyll meter （Konica-Minolta， Japan）， and its reading range was 0-99.9， dimensionless.

Determination of photosynthetic indexes in leaves

During the flower bud differentiation stage， young fruit stage， fruit expansion stage and maturation stage of citrus growth， a portable photosynthesis instrument （instrument model Li-6400， Li-COR， USA） was used to measure the net photosynthetic rate （Pn）， transpiration rate （Tr）， stomatal conductance （Gs） and intercellular CO2 concentration （Ci） of leaves. Specifically， a day with sufficient weather and light during the above-mentioned period of citrus growth was chosen， and the indexes were measured from 10：00 to 11：00 in the morning. For each treatment， 12 mature leaves in four directions of east， south， west and north were selected for measurement.

Determination of leaf nutrition indexes

Total N was determined by sulfuric acid-mixed accelerator-distillation method. Total phosphorus was determined after H2SO4-H2O2 digestion by molybdenum antimony anti-colorimetric method. Total potassium was determined after H2SO4-H2O2 digestion with a flame photometer.

Determination of fruit sample indexes

The collected fruit samples were weighed first， and then the citrus seeds and flesh were separated and weighed separately to calculate the edible rate. The flesh was mashed with a tissue masher for the determination of fruit quality. The content of titratable acid was determined by NaOH titration， and citric acid was used as the calculation reference. The content of VC was determined by 2，6-dichloroindophenol titration method. The content of total sugar was determined by boiling water extraction-anthrone colorimetry. The content of soluble solids was measured directly with a sugar meter. The sugar-to-acid ratio was the ratio of total sugar to titratable acids.

Data processing

In this study， Excel 2019 and SPSS 25 software were used to summarize， analyze and plot the data， and Duncans new multiple range method was used for significance test.

Results and Analysis

Effects of different irrigation methods on chlorophyll content of citrus leaves

It can be seen from Fig. 1 that the SPAD value of citrus leaves under the smart irrigation measures was lower than that of the CK at the flowering and young fruit stages of citrus， and the value was reduced by 1.60% and 1.32% in two periods， respectively， but there were no significant differences between the two. However， in the fruit expansion stage and maturation stage， the SPAD values of citrus leaves under the smart irrigation measure were higher than those in the CK， increasing by 1.98% and 8.28%， respectively， and the leaf SPAD value of the smart irrigation measure at the maturation stage was significantly higher than that of the CK.

Effects of different irrigation methods on photosynthetic characteristics of citrus leaves

Photosynthesis is the basis for the formation of plant yield and quality. About 90%-95% of the dry matter accumulation comes from photosynthetic products in leaves[11-12]. Transpiration rate represents the amount of water transpired by plant leaves per unit leaf area in a certain period of time， and is an important physiological indicator of water metabolism， which is closely related to photosynthesis. Studies have shown that the greater the transpiration rate of plants， the less the dry matter accumulation[13]， and the stomatal conductance and intercellular CO2 concentration， which indicate the degree of stomatal opening， directly affect the photosynthesis and transpiration of plant leaves. From Table 1， it can be seen that in the flower bud differentiation stage， the photosynthetic rate， transpiration rate， stomatal conductance and intercellular CO2 concentration of citrus leaves in the smart irrigation treatment were higher than those in the CK， by 18.21%， 16.11%， 20.65% and 2.68%， respectively， and the photosynthetic rate， transpiration rate and stomatal conductance among them increased significantly. At the young fruit stage， the photosynthetic rate and transpiration rate of the smart irrigation treatment increased by 5.09% and 4.49%， respectively， while the stomatal conductance and intercellular CO2 concentration decreased by 5.22% and 0.93%， respectively， compared with the control， but the differences were all not significant. At the fruit expansion stage， the photosynthetic rate， transpiration rate， stomatal conductance and intercellular CO2 concentration of citrus leaves in the smart irrigation treatment were higher than those in the CK， increasing 5.15%， 5.19%， 12.09% and 5.83%， respectively， and the difference in intercellular CO2 concentration reached a significant level. At the citrus ripening stage， the photosynthetic rate， transpiration rate， stomatal conductance and intercellular CO2 concentration of citrus leaves in the smart irrigation treatment were significantly higher than those in the CK， increasing by 23.16%， 33.05%， 12.20% and 12.59%， respectively. It could be seen that in the maturation period and flower bud differentiation period of the dry season， the smart irrigation treatment could better highlight its promoting effect on improving the photosynthesis of citrus.

It can also be seen from Table 1 that the basic laws of photosynthetic rate and transpiration rate in various growth stages of citrus in the CK were young fruit stage>fruit expansion stage>flower bud differentiation stage>maturation stage， while the basic laws of stomatal conductance and intercellular CO2 concentration were young fruit stage>flower bud differentiation stage>fruit expansion stage>maturation stage. In the smart irrigation treatment， the basic laws of photosynthetic rate， transpiration rate and intercellular CO2 concentration in various growth stages of citrus were as follows： young fruit stage>flower bud differentiation stage>fruit expansion stage>maturation stage， while the order of stomatal conductance was flower bud differentiation stage>young fruit stage>fruit expansion stage>maturation stage. The photosynthesis of citrus leaves was the strongest in the high-temperature rainy young fruit stage， and the weakest in the maturation stage with low temperature and drought.

Effects of smart irrigation and citrus quality

Table 2 shows the quality characteristics of citrus under different irrigation management methods. The edible rate and juice rate of citrus under the smart irrigation management were significantly higher than those of the CK， by 4.53 and 3.69 percentage points， respectively. The soluble solids， total sugar， vitamin C and sugar-acid ratio of citrus under the smart irrigation management were significantly higher than those in the CK， increasing by 16.75%， 20.86%， 24.10% and 13.17%， respectively. The titratable acid content of the CK was lower than that of the smart irrigation treatment， but the difference was not significant.

Discussion

Citrus is a subtropical and tropical evergreen fruit tree， which has a large demand for water and fertilizers. This study showed that the smart irrigation measure could significantly improve the chlorophyll content and photosynthetic characteristics of citrus in the dry season， but the differences were not significant in the rainy season when the soil water content was high. It might be related to the water content of the orchard soil. In the rainy season， the water content of the soil in the province was relatively high due to the rainfall in the various treatment measures， and could meet the needs of citrus growth without additional irrigation. The soil conditions were basically the same， and the growth conditions of citrus were basically the same， so there were no significant differences in the physiological indicators. In the dry season， the smart irrigation system could detect the changes of soil moisture in real time， and knew what soil moisture level is conducive to the growth of citrus， and could irrigate in time， so the soil moisture conditions conducive to the growth of citrus were always maintained， while the traditional artificial irrigation had problems such as untimely irrigation or insufficient irrigation. Studies have shown that when water is deficient， on the one hand， the content of abscisic acid will increase， thereby reducing stomatal conductance and affecting the absorption of carbon dioxide， and on the other hand， it will also affect the output of photosynthetic products， hinder leaf growth， and then affect leaf area expansion; and when the water shortage is serious， the photosynthetic mechanism will be damaged[14]. Soil nutrient content affects the growth and development， metabolism and physiological process of plants[15]， and then affects the photosynthesis of plants. N limitation will reduce the sensitivity of plant photosynthetic organs， resulting in reduced leaf area and blocked chlorophyll synthesis， which inhibit the formation of photosynthetic products. P and K deficiency will reduce plant respiration and carbon metabolism， and decrease light use efficiency and electron transfer rate. Mg can activate ribulose diphosphate carboxylase， stabilize CO2 concentration and enhance plant photosynthetic capacity， and Ca is used for the formation of cell walls and other organs and affects chlorophyll content， transpiration and photosynthetic rate[16]. Nutrient elements such as C， N and Fe are the main factors affecting plant water use efficiency and chlorophyll content[17]. In the dry season， if citrus crops are replenished with water in time， it will improve the supply of soil nutrients and microbial activity， and promote the absorption of nutrients by the citrus root system， so the photosynthesis of leaves is improved. On this basis， smart irrigation management can naturally improve the quality of citrus fruits.

Smart irrigation uses big data and the Internet of Things technology to automatically collect， analyze and wirelessly transmit information on orchard soil moisture and meteorological data and further realize precise and real-time remote control of orchards[18]， and simultaneously makes decisions and performs precise irrigation combining with expert knowledge. The citrus plantations built based on the Internet of Things have a high initial investment cost， but its effect is remarkable in water saving， production increase and efficiency improvement. The cost can be recovered in 3 to 5 years， and the economic benefits are high[19].

Conclusions

The smart irrigation management measure could significantly improve the SPAD value of citrus leaves during drought， as well as photosynthetic characteristic indicators such as photosynthetic rate， transpiration rate， stomatal conductance and intercellular CO2 concentration due to precise control of citrus irrigation timing and amount， and finally could improve the edible rate， juice rate， soluble solids， total sugar， vitamin C， sugar and acid and other indicators of citrus fruit， and improve the quality of citrus. The irrigation method is a better scientific water management method for citrus orchards.

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