Effects of Nutrients on Growth and Yield of Shatian Pomelo Trees with Huanglongbing

Jun YE Wei ZHI Yupeng ZHANG Xiaoyuan CHEN Chongjian MA

Abstract [Objectives] In order to effectively alleviate Huanglongbing of Shatian pomelo trees， a nutrient preparation was developed and its effect on the growth and yield of Shatian pomelo was explored. [Methods] A field experiment was carried out with four treatments： control group， nutrient solution group， agent group， and nutrient solution+agent group， and the liquids were infused into the fruit trees through the trunk using a hanging bag. [Results] Through field trials， it was found that the nutrient solution treatment of fruit trees significantly increased the leaf chlorophyll content and nitrogen content， which increased by 17% and 12% compared with the control group， respectively， reaching （70.80±1.57） and （5.53±0.1） mg/kg， respectively; and the combined application of the nutrient solution and powder agents greatly alleviated the symptoms of Huanglongbing in Shatian pomelo plants， and significantly improved the bacterial disease index， photosynthetic intensity and yield， which increased by 152%， 158% and 213.53% respectively， reaching （371.00±7.22）， （406.67±7.09） and 23.15 kg/hm2. [Conclusions] The use of the nutrient solution and agents greatly alleviated the symptoms of Huanglongbing of Shatian pomelo plants， and significantly improved the chlorophyll content， nitrogen content， bacterial disease index and photosynthetic intensity. Meanwhile， the number of fruit per plant and the weight of single fruit were better than other treatment groups and the control group， the technical operation was simple and the effect was obvious. This provides a certain reference basis for the prevention and control measures of Huanglongbing in Shatian pomelo plants.

Key words Shatian pomelo; Huanglongbing; Nutrient; Fruit quality

Shatian pomelo （Citrus grandis var. shatinyu） belonging to Citrus of Rutaceae， also known as Xiangluan， Huluan and Wendan， is currently the largest and most productive pomelo variety in China[1]. Shatian pomelo is rich in nutrients and has a high medicinal value[2]. It has a high vitamin C content. The vitamin C content of every 100 ml of juice is about 110 mg[3]. It also has the effects of regulating qi， dispelling coagulation， nourishing the lung and clearing the intestine， replenishing blood and invigorating the spleen， and is recognized by the medical profession as the most therapeutic fruit[4]. However， due to the rampant spread of Huanglongbing， the planting area and yield of Shatian pomelo have been seriously affected， which hinders the development of the citrus industry including Shatian pomelo[5].

Huanglongbing can harm all cultivars of citrus plants including oranges， tangerines， grapefruits and citrons. In view of the fact that no feasible treatment for Huanglongbing has been found， the current measures to prevent and control the disease are mainly plant quarantine， cultivation of disease-free seedlings， timely excavation of diseased plants， and control of the vector insect such as Asian citrus psyllid[6-7]. Huanglongbing can have a serious impact on the vegetative growth and fruit quality of citrus plants. The diseased plants show complex and diverse symptoms， including gradual decline in growth， reduced yield， deformed diseased fruit， soft peel， small and sour fruit， short economic life， reduced plant disease resistance or even death[8]. The disease symptoms of leaves in different growing seasons are different， and the disease is manifested as mottled yellowing， uniform yellowing， and nutrient-deficient yellowing[6]. As the incidence of disease intensifies， diseased trees are prone to early fruit drop[9].

In this study， with diseased Shatian pomelo trees as the material， the combination of a self-made nutrient solution and agents was infused into the fruit trees through the trunk using a hanging bag， so as to observe the effect of the self-made nutritional preparation on alleviating the disease of Shatian pomelo.

Materials and Methods

Nutrient solution preparation

The following agents were added into 1 000 ml of distilled water： 0.22 g of ammonium phosphate， 1.05 g of potassium nitrate， 0.16 g of ammonium sulfate and ammonium nitrate， and 0.01 g of ferrous sulfate.

Agent preparation

The antimicrobial agent， antacid agent， and synergist were mixed in proportion.

Experimental design

The field trial was carried out in Renhua County， Shaoguan City， Guangdong Province. 20 diseased Shatian pomelo trees were selected and divided into 4 treatments， namely the control group （CK）， the nutrient solution group， the agent group， and the nutrient solution+agent group， each of which included 5 tress. The CK group was free of any treatment. For the nutrient solution group， the prepared nutrient solution was filled into bags， which were hung on the trees and infused into the trees through the trunk. As to the agent group， the agents were dissolved in pure water， obtaining the preparation， which was filled into bags and infused into the trees through the trunk. For the nutrient solution+agent group， the agents were dissolved in the nutrient solution in proportion， obtaining a liquid， which was filled into bags and infused into the trees through the trunk. After the treatment started， the chlorophyll content， nitrogen content， and bacterial disease index in the leaves in middle of the tree body were measured. The photosynthetic intensity was measured when the growth was vigorous in September， and the yield was measured at the harvest in November.

Shatian pomelo quality determination

The chlorophyll content and nitrogen content were measured with a plant nutrition analyzer. The bacterial disease index was measured with a plant disease analyzer. Photosynthesis was measured with a photosynthesis analyzer. The yield was determined by the weight of single fruit and the number of fruit per plant.

Data processing and Analysis

The SPSS 18.0 statistical analysis software was used for the difference test.

Results and Analysis

Effects of different treatments on chlorophyll content

It can be seen from Table 1 that with the growth process， the chlorophyll content of each group gradually increased， and most of them reached a peak in mid-to-late July. After that， the content gradually decreased， but the reduction remained at a normal level. For example， in the control group， the chlorophyll content was （56.26±4.55） on March 19， which increased to （62.53±2.05） on July 23， with an increase of 6.27 units， and then slightly decreased to （60.50±1.58） on October 27， with a decreased of 2.03 units. Meanwhile， by comparing the treatment groups， it was found that on October 27， the chlorophyll content in the nutrient solution group was （70.80±1.57）， which increased by 10.30 units compared with the CK group （60.50±1.58）. The difference between the two groups was significant， which meant that the nutrient solution significantly increased the chlorophyll content in the leaves of Shatian pomelo. The agent group and the nutrient solution+agent group showed the values of （65.30±3.05） and （66.47±4.88）， respectively， which were not significantly different from the CK group.

Effects of different treatments on leaf nitrogen content

As one of the essential elements for plant growth， nitrogen is a limiting factor in many ecosystems. Shatian pomelo trees infected with Huanglongbing have reduced nitrogen content in their leaves， resulting in weakened photosynthesis， reduced leaf toughness， and weakened chemical defense capabilities， thereby reducing yield and increasing the risks of dieback and other diseases. It can be seen from Table 2 that in the two measurements on March 19 and May 18， the leaf nitrogen content of each group subjected to the bag hanging treatment was basically significantly different from that of the CK group， and the leaf nitrogen content of the nutrient solution+agent group was the highest， and was 0.84 and 1.00 mg/kg higher than the CK group respectively， reaching （5.47±0.15） and （5.23±0.40） mg/kg， respectively. Meanwhile， in the middle and late stages of the growth process， the leaf nitrogen content in the nutrient solution group was significantly higher than that in the CK group. For example， on October 27， the leaf nitrogen contents in the nutrient solution group and the CK group were （5.53±0.12） and （4.93±0.15） mg/kg， respectively， and there was a significant increase of 0.60 mg/kg between the two groups， while the agent group and the nutrient solution+agent group were （5.20±0.10） and （5.23±0.35） mg/kg， respectively， which were not significantly different from the CK group. It indicated that the use of the nutrient solution for the diseased Shatian pomelo trees significantly increased the nitrogen content in the leaves， thereby enhancing its photosynthesis， improving the leaf toughness and chemical defense capability， and delaying leaf yellowing.

Effects of different treatments on tree bacterial disease index

When Shatian pomelo is infected with Huanglongbing， its own disease resistance， leaf toughness and chemical defense capacity will be greatly reduced， which makes it susceptible to other diseases， such as bacterial， fungal and virus infection， which can cause trees with Huanglongbing to die at an accelerated speed. In this study， a plant disease detector was used to measure the tree bacterial disease index， quickly analyze and determine the type of disease， and accurately diagnose the source of plant infection. When the bacterial disease index is greater than 420， it means that the plant grows normally and has no adverse symptoms; and when the value is less than 249， it means that the plant grows poorly and is in a sick state. Since the tested fruit trees in this study were all Shatian pomelo trees suffering from Huanglongbing， their values should be less than 249 at the time of initial testing. It can be seen from the results in Table 3 that on May 18， the bacterial disease index of the nutrient solution group and the CK group were close， being 138.67±3.51 and 142.33±1.15， respectively， without a significant difference， indicating that their growth status was basically the same; the agent group and the nutrient solution+agent group were significantly different from the CK group， 46.33 and 80.33 higher than the CK group， respectively， but both were less than 249， indicating that these two groups were also infected. The bacterial disease index of Shatian pomelo in the CK group was always less than 249 during the whole measurement process， indicating that it was always in a diseased state. The index of other three treatment groups was less than 249 at the initial measurement， and the values gradually increased as it grew， indicating that its growth and development status had gradually improved. The index of the agent group and the nutrient solution+agent group reached （254.33±3.51） and （280.00±8.66）， respectively when measured on July 23， which were both greater than the pathological value of 249， while the nutrient solution group only increased from the initial （142.33±1.15） on October 6 by （122.67） to （265.00±4.35）， which indicates that the agents had a greater effect on the treatment of bacterial diseases. Through the comparative analysis between the treatment groups， the bacterial disease index of all Shatian pomelo trees was less than 249 at the initial inspection， indicating that they were infected; in the mid-term inspection， other three treatment groups were significantly better than the control group; and in the later inspection， all the bag hanging treatment groups showed a bacterial disease index greater than 249， and had significant differences therebetween， and the bacterial disease index of the nutrient solution+agent group reached （371.00±7.22）.

Effects of different treatments on the photosynthetic intensity of trees

Leaf photosynthesis mainly uses solar energy to fix carbon dioxide in the photosynthetic part of leaves， synthesize organic matter， and transport it to the tree body for tree growth， development and fruit ripening. When normal plants undergo photosynthesis， it is normal that their photosynthesis intensity is greater than 505. Once the value is lower than 311， it indicates that there is a problem with plant growth， and the photosynthesis is poor or hardly can be performed. Since the tested fruit trees in this study were all Shatian pomelo trees suffering from Huanglongbing， their photosynthetic intensity should be less than 311 in the initial test. Through the analysis of Table 4， it can be seen that with the growth of fruit trees and the maturity of the leaves， the chlorophyll content in the leaves increased， and the photosynthetic intensity of each treatment group basically showed an upward trend， but the growth rate of each group was different. The photosynthetic intensity increased by 12.34， 75.67， 212.66 and 170.67 in the various groups， respectively， and the largest increase appeared in the agent group. Meanwhile， the photosynthetic intensity of each treatment group was significantly higher than that of the CK group， and there were also significant differences between the groups. On October 6， the nutrient solution+agent group had the largest photosynthetic intensity， which was （406.67±7.09）， which was 249 higher than that of the CK group. In the initial measurement， the photosynthetic intensities of all treatment groups were less than 311， that is， the trees were all diseased; and in the measurement on July 23， the photosynthestic intensities of the agent group and the nutrient solution+agent group were both greater than 311， respectively， of （321.00±2.64） and （395.00±9.53）. In summary， it indicated that the use of the agents effectively promoted the growth and development of the branches and leaves of Shatian pomelo， expanded the photosynthetic area of the leaves， and improved and enhanced the photosynthesis of diseased Shatian pomelo trees.

Effects of different treatments on yield

The results in Table 5 showed that there were significant differences in the number of fruit between the treatment groups. The number of fruit in the nutrient solution+agent group was significantly higher than that of other groups， reaching （23.33±1.53）， which increased by about 13 per plant compared with the control group; and the single fruit weights and single fruit sizes of the various groups were not significantly different， but were all higher than the control group. The nutrient solution+agent group had the largest single fruit weight， which was （0.99±0.03） kg/fruit， which was 0.3 kg/fruit higher than the control group. It indicated that the nutrient solution or the agent treatment of diseased Shatian pomelo plants increased the weight and fruit size of single fruit. From the perspective of yield， the yield of the nutrient solution+agent group was the highest at 23.15 kg/hm2， which was 15.79 kg/hm2 higher than that of the CK group， followed by the agent group， and there were significant differences in the yield between the various groups. In general， the nutrient solution and agent bag hanging treatments of Shatian pomelo plants suffering from Huanglongbing had better fruit yields than the control group， and the mixed use of the nutrient solution and agents had a great effect on the improvement of Shatian pomelo fruit quality.

Conclusions and Discussion

According to reports， Huanglongbing can cause uneven mottled yellowing or zinc-deficient yellowing of leaves， and the leaves become smaller， and has a reduced chlorophyll content in the leaves; and the fruit becomes smaller， and has poor flavor， and the peel is smooth， dull， unevenly colored[11-12]. In this study， the leaves of the diseased Shatian pomelo showed yellow-green mottled yellowing and were small， which was in line with the reports. The root system of Shatian pomelo fruit infected with Huanglongbing is easy to rot， and the xylem becomes black， which in turn causes the root nutrient absorption to be blocked or even the roots unable to absorb nutrients， making the fruit trees unable to maintain normal growth and development. Leaf synthesis of chlorophyll needs to absorb a large amount of iron， which is an important enzyme prosthetic group and activator in the synthesis process. Once the fruit tree is infected， the absorption of nutrients will be blocked， which will inevitably affect the synthesis of chlorophyll in the leaves and cause the content to decrease. Under normal circumstances， more than 50% （up to 70%） of the nitrogen in the leaves will be absorbed into the chloroplast to participate in photosynthesis[13-14]. Diseased Shatian pomelo fruit trees are blocked in terms of nutrient absorption and chlorophyll synthesis， which means that the nitrogen content is simultaneously reduced， while infusing nutrient solutions into trees through the trunk endows diseased fruit trees with the nutrients needed for growth and development， the synthesis of chlorophyll in the leaves can thus continue， and the content can be increased; and the content of chlorophyll increases and is absorbed into the chloroplast， and nitrogen participating in photosynthesis also increases， which is basically consistent with the data results in this study.

The first step of photosynthesis is the absorption of light energy by the chloroplast， and this absorption is achieved by the chlorophyll in the chloroplast. Due to the infusion of fruit trees， the infusion of nutrient solutions or antibacterial agents， the harmful organisms in diseased fruit trees are killed， and the nutrients are supplemented. As a result， the senescence of the leaves of the tree body is delayed， the number increases， the chlorophyll content increases， and the photosynthesis intensity is greatly improved.

Huanglongbing is mainly caused by Candidatus Liberibacter spp.， which shows symptoms on the leaves， fruits， flowers， roots， seed coats and stems of fruit trees. In this study， the nutrient solution or agents were infused into the diseased fruit trees to make the fruit tree show certain resistance. According to the experimental data， the bacterial disease index of fruit trees was significantly higher than that of the control CK group after the nutrient solution or agents were infused， and the bacterial disease index of the nutrient solution+agent group was significantly improved compared with the CK group. It might be because the infusion of nutrients or antibacterial agents could effectively improve the disease resistance of the Shatian pomelo tress and accelerate the restoration of normal growth. Meanwhile， with the supplement of nutrients， the fruit trees could use the supplemented nutrients to promote growth and development， bloom and fruit setting， and more nutrients were transferred to the fruit， thereby increasing yield.

After a Shatian pomelo plant is infected with Huanglongbing， the leaves of the plant become mottled， yellow and smaller， the content of chlorophyll is reduced， and the photosynthetic intensity is weakened， which has a great impact on the number of fruit， individual size， single fruit weight and taste quality. In this study， a nutrient solution was prepared for the bag hanging treatment on diseased fruit trees. It was found that the nutrient solution treatment of fruit trees significantly increased the leaf chlorophyll content and nitrogen content， which increased by 17% and 12% respectively compared with the control group， reaching （70.80±1.57） and （5.53±0.12） mg/kg， respectively; the combined application of the nutrient solution and agents greatly alleviated the symptoms of Huanglongbing of Shatian pomelo plants， and the bacterial disease index， photosynthetic intensity and yield were significantly improved， and were increased by 152%， 158% and 213.53%， respectively， reaching （371.00±7.22）， （406.67±7.09） and 23.15 kg/hm2， respectively. Infusing nutrients into Shatian pomelo trees with Huanglongbing through the tree trunk enhanced the vigor of Shatian pomelo trees with Huanglongbing， increased leaf chlorophyll content， enhanced photosynthesis， enhanced leaf toughness， and increased disease resistance of fruit trees to a certain extent; and to a certain extent， it improved yield and fruit quality， and delayed and reduced the phenomena of leaf yellowing， dieback and yield reduction caused by Huanglongbing， and the operation was simple. This study provides a certain reference for the prevention and control of Huanglongbing in Shatian pomelo plants.

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