Effects of Temperature and Humidity Changes inside and outside Insect-proof Net in the Spring on the Spring Shoot Growth of Citrus Shatangju

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1. Institute of Fruit Tree Research, Zhaoqing University, Zhaoqing 526061, China; 2.College of Life Sciences, Zhaoqing University, Zhaoqing 526061, China

1 Introduction

The insect-proof net mulching cultivation technology is an effective means to prevent and control the citrus pests and diseases in the citrus production system, thus improving the growth of citrus trees. According to the current investment and profitability of insect-proof net mulching cultivation technology in China’s citrus production system, the use of insect-proof net for the prevention and control of citrus pests and diseases is expected to form a certain scale in the future for the citrus varieties and producing areas of high efficiency[1]. However, in the insect-proof net cultivation environment, the application of insect-proof net will affect the temperature and humidity in the net room, and the change of temperature and humidity in the environment is an important environmental factor that affects the normal growth of citrus[2]. The current research about the effects of insect-proof net on temperature and humidity is mainly reflected in the description of effects of insect-proof net on regular changes in the temperature and humidity of the citrus orchard[3-4]. However, there are few studies about the effects of insect-proof net on the growth performance of citrus trees through the regulation of temperature and humidity, as well as the correlation between the two. Thus, this experiment mainly studied the effect of insect-proof net mulching cultivation technology on the temperature and humidity of the greenhouse and the spring shoot growth of citrus Shatangju, in order to provide a theoretical basis for the promotion and application of insect-proof net mulching cultivation technology for citrus Shatangju.

2 Materials and methods

2.1ExperimentaltimeandsiteThe citrus Shatangju was triennial, and planted inside and outside the net room. The routine management was implemented after planting and some items were recorded, including the pests and diseases inside and outside net room, insecticide type and concentration, fertilization time, fertilizer types and concentrations. During January 2016 and April 2016, the test was carried out in the 40-mesh translucent insect-proof net of citrus base in Zhaoqing University and the open environment of base.

2.2TestmaterialsThree-year-old citrus Shatangju tree, provided by the citrus base of Zhaoqing University.

2.3Testmethods

2.3.1Test treatments and grouping. The test program set the treatment group and control group. The treatment group was 40-mesh net room, and the control group was the nearby citrus trees with no insect-proof net.

2.3.2Test instruments. L92 series temperature and humidity black box/recorder (Hangzhou Luge Technology Co., Ltd.); leaf area meter (British ADC); magnifying glass; tapeline.

2.3.3Temperature and humidity measurement. The tree crown automatic temperature and humidity recorder, installation position: the probe was placed on top of the canopy which was not covered with branches and leaves, 3 probes were uniformly installed in the control group and net room (3 replicates), 1 in the middle plants, 1 in the edge plants, 1 in between the middle and edge plants. Measurement: The humidity and temperature were measured every day at 8:00, 10:00, 12:00, 14:00, 16:00 and 18:00, respectively. The daily temperature and humidity were recorded at different time points, and the daily average temperature and humidity values were recorded. Finally, the monthly temperature and humidity were calculated and presented based on the average of 30 days.

2.3.4Determination of growth performance. After the spring shoots turned mature, with the tree as the unit, 3 trees were chosen, and 3 spring shoots were selected from each tree. The tapeline was used to measure the length of the spring shoots and the number of each spring shoot leaves. 3 mature leaves were randomly selected from each spring shoot, for measuring leaf area.

2.3.5Pest detection. The statistics about various pests were collected each month. 3 citrus Shatangju trees inside the net room and 3 citrus Shatangju trees outside the net room were selected for statistical analysis of various pests at the end of each month. Phyllocnisidae: 2 young shoots with length of about 2 cm were randomly chosen from each tree to investigate and estimate the amount of pests. Tetranychidae: 2 leaves infested with pests were randomly selected from each tree in five directions (east, south, west, north, middle), and it was marked with red nylon rope. The 1∶10 hand-held magnifying glass was used to investigate the number of pests on both sides of the leaves and estimate the amount of pests. Aleyrodidae: 2 leaves infested with pests were randomly selected from each tree in five directions (east, south, west, north, middle), and the number of pests on both sides of the leaves was investigated and the amount of pests was estimated.

2.4StatisticalanalysisData were statistically analyzed using SAS (SAS 8.1, Inst. Inc., Cary, NC). The data were tested for normality and homogeneity of variance using Shapiro-Wilk andF-test. After the data were tested by normality and homogeneity of variance, the significance test was conducted on the temperature, humidity and number of citrus pests by the repeated measurement analysis of variance. The Lsmeans/PDIFF statements were used for multiple comparisons. The significance test was conducted on the growth performance of spring shoots byttest, withP<0.05 (significant difference) andP<0.01 (highly significant difference) as criterion.

3 Results and analysis

3.1Thehumiditychangesinsideandoutsideinsect-proofnetSeen from Fig. 1, from January to April, the ambient humidity gradually increased (P<0.01), but in the case of high humidity in February to April, there was no difference in the humidity inside and outside the net room (P>0.05). In January, the humidity inside and outside the net room was lower than in the rest of the months, but the humidity inside the net room was higher than the humidity outside the net room, and there was a significant difference (P<0.05).

3.2Thetemperaturechangesinsideandoutsideinsect-proofnetSeen from Fig. 2, from January to April, the ambient temperature gradually increased (P<0.01), and at the same time, the temperature inside the net room was significantly higher than the temperature outside the net room (P<0.05). However, there was no interaction between the effects of insect-proof net and different months on the net room temperature (P>0.05).

Note: The average of the data,n=3; different letters of a, b in the same row of data indicated significant difference (P<0.05).

Fig.1Effectsofinsect-proofnetonhumidity

Note: The average of the data,n=3; different letters of a, b in the same row of data indicated significant difference (P<0.05).

Fig.2Effectsofinsect-proofnetontemperature

3.3Effectofinsect-proofnetonthenumberofAleyrodidaeSeen from Fig. 3, in January to April, there was an extremely significant increase in the number of Aleyrodidae inside and outside the insect-proof net (P<0.01), but the insect-proof net extremely significantly reduced the number of Aleyrodidae in the citrus orchard (P<0.01). Through the interaction, it showed that the larger the number of Aleyrodidae, the more significant the role of insect-proof net in reducing the number of Tetranychidae in the citrus orchard (P<0.01).

Note: The average of the data,n=3; different letters of a, b in the same row of data indicated extremely significant difference (P<0.01).

Fig.3Effectsofinsect-proofnetonthenumberofAleyrodidae

3.4ThechangeinthenumberofPhyllocnisidaeinsideandoutsidetheinsect-proofnetFrom Fig. 4, it was found that the number of Phyllocnisidae inside and outside the insect-proof net gradually increased in January to April (P<0.01), and the insect-proof net played a role in extremely significantly reducing the number of Phyllocnisidae in the citrus orchard (P<0.01). At the same time, the insect-proof net’s reduction of the number of Phyllocnisidae in the citrus orchard had a time effect (P<0.01).

Note: The average of the data,n=3; different letters of a, b in the same row of data indicated extremely significant difference (P<0.01).

Fig.4Effectsofinsect-proofnetonthenumberofPhyllocnisidae

3.5ThechangeinthenumberofTetranychidaeinsideandoutsidetheinsect-proofnetAs can be seen from Fig. 5, in the spring, the insect-proof net could reduce the extremely significant increase in the number of Tetranychidae inside and outside the insect-proof net caused by the increase in the number of the month (P<0.01). At the same time, the insect-proof net reduced the number of Tetranychidae, and it increased with time (P<0.01).

Note: The average of the data,n=3; different letters of a, b in the same row of data indicated extremely significant difference (P<0.01).

Fig.5Effectsofinsect-proofnetonthenumberofTetranychidae

3.6Thechangeintheperformanceofspringshootsinsideandoutsidetheinsect-proofnetFrom Table 1, it can be seen that the insect-proof net in the spring had a very significant role in increasing the length and leaf area of spring shoots (P<0.01). The insect-proof net tended to increase the number of leaves in spring shoots, but the difference was not significant (P=0.075).

Table1Effectoftheinsect-proofnetonthespringshootsofcitrus

ItemsThegrouphavingnoinsect⁃proofnetThegrouphaving40⁃meshinsect⁃proofnetSEMPvalueLengthofspringshoots∥cm20．1823．660．510．01Leafnumberofspringshoots10．8312．560．440．07Leafareaofspringshoots∥mm22890．113347．3364．400．01

4 Discussions

With the rise of temperature and humidity in the spring, the citrus pests and diseases have started to gradually recover. In recent years, due to the change of planting and cultivation system, the renewal of varieties and the change of climate, the citrus pest species, distribution and harm also have corresponding changes. However, the main pests are still pest mites, scale insects, Aleyrodidae, aphids and Phyllocnisidae[5-6]. In this experiment, we can also clearly see that from January to April, the main pests on citrus such as Aleyrodidae, Tetranychidae and Phyllocnisidae gradually increased inside and outside the net room with the month. The insect-proof net mulching technology is one of the important measures to implement the virus-free seedling breeding of fruit trees such as citrus, which is mainly used for controlling the occurrence and spread of the pests serving as a vehicle for viral transmission such as citrus aphids and citrus psyllids, in order to protect the safe production of virus-free seedlings of fruit trees[7]. In this experiment, there was no citrus psylla in the experimental environment in which this experiment station was located, so we mainly reflected the role of insect-proof net mulching technology in controlling the citrus pests through the number of Aleyrodidae, Tetranychidae and Phyllocnisidae inside the insect-proof net. We found that the number of Aleyrodidae, Tetranychidae and Phyllocnisidae inside the net room was significantly smaller than the number of Aleyrodidae, Tetranychidae and Phyllocnisidae outside the net room, indicating that the 40-mesh insect-proof net can be used as an effective means to reduce the number of citrus pests in the insect-proof net.

Spring is the citrus spring shoot germination, budding, flowering and fruit dropping period. During this period, the environmental factors are closely related to the status of citrus, spring shoots germination rate, flowering rate and physiological fruit falling rate, thus affecting the current year’s output, quality and economic benefits[8]. Compared with the outside of net, the inside of net is a more specific growth environment, and the insect-proof net causes the changes in the temperature and humidity inside the net. The temperature and humidity will not only affect the light conditions, citrus photosynthesis, and transpiration in the net room, but also regulate the growth, development and other physiological processes of citrus fruits, leaves and branches[9-10]. In addition, the temperature and humidity also affect the synthesis, transport of carbohydrate and microbial propagation in the insect-proof net[11]. Therefore, the change of temperature and humidity in the insect-proof net is an important environmental factor that affects the growth performance of citrus. After establishing the mulching insect-proof net, it was found in the experiment that in January to April 2016 (spring), the humidity was low in January, while the humidity was high inside and outside the net room in February, March and April due to sufficient rainfall. Consistent with the trends of humidity, the temperature inside and outside the net room also gradually increased with the month. In the same month, in January, it was found that the relative humidity of air inside the 40-mesh insect-proof net was obviously higher than the relative humidity of air outside the insect-proof net. In February to April, there were not big changes in the relative humidity of the air outside net and the relative humidity of the air inside net. In different months, the temperature inside the insect-proof net was significantly higher than the temperature outside the insect-proof net. The humidity difference change in different months inside and outside the insect-proof net was significantly different, the lower the humidity outside the net, the greater the difference in the humidity inside and outside the insect-proof net. The temperature difference was consistent in different months for the insect-proof net, and the net room temperature regulation by the insect-proof net was not affected by the time. The research pointed out that the insect-proof net could impede the transpiration of citrus Shatangju leaves and the heat dissipation in the net room[12]. This obstruction had a buffer effect on the decrease of the temperature and humidity in the insect-proof net, which might be the main reason for the results of the test.

In this experiment, we found that the number of pests increased as the relative temperature and humidity of air increased. The insect-proof net can increase the humidity and temperature inside the insect-proof net in the early spring dry conditions, but when the ambient humidity gradually increased, the insect-proof net had no effect on the humidity. Citrus is a subtropical evergreen tree, and likes a humid climate. The growth of spring citrus shoots is closely related to the citrus production performance[13]. In this experiment, the insect-proof net cultivation technology could increase the length of spring shoots of citrus Shatangju, the number of leaves on spring shoots and the leaf area of spring shoots, indicating that the insect-proof net could promote the growth of spring shoots of citrus. The low temperature and humidity outside the net and many pests are not conducive to the growth of spring shoots of citrus Shatangju, so the role of insect-proof net in improving the temperature and humidity inside the net room in the spring and reducing the number of pests in the net room, might be an important reason for the insect-proof net to improve the production performance of citrus trees.

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