Cultivation Techniques for Highquality and Highyield Spring Peanut in East Henan Province

Ruhong FU Jihua WU

Abstract［Objectives］ This study was conducted to explore the effects of different rootstocks on plant growth and fruit quality of watermelon.

［Methods］ The scions of two different watermelon varieties Qianmei No. 1, which has yellow flesh, and Qianli No. 1, which has red flesh, were grafted onto six different rootstocks of Lagenaria and Citrullus. Then, the seedling survival rate, growth potential and fruit quality in different rootstockscion combinations were investigated.

［Results］ Rootstock type showed an extremely significant correlation with the survival rate of grafted watermelon plants. The watermelons grafted onto Lagenaria rootstocks had higher survival rate and better fruit quality than those onto Cucurbita rootstocks. Among them, Jingxinzhenguan was more suitable for redfleshed watermelon variety, and bottle gourd rootstock was more suitable for yellowfleshed watermelon variety. There were significant correlations between rootstock type and the number of leaves, the number of branches and the length of main vine of grafted plants. The number of leaves per seedling of all grafted plants was higher than that of nongrafted plants. There were no significant differences in sugar content at the center part of flesh and total soluble sugar content of fruit between the grafted and nongrafted watermelons. In addition, the interaction between rootstock type and watermelon variety had no significant effect on vegetative growth and fruit quality of grafted watermelon.

［Conclusions］ This study provides a theoretical basis for the grafting of watermelon.

Key wordsWatermelon grafting; Vegetative growth; Fruit quality; Lagenaria rootstock



Received: September 2, 2018Accepted: October 20, 2018

Supported by the fund for Evaluation and Screening of Different Rootstocks for Watermelon (SQCX201703).

Jiali MENG (1987-), female, P. R. China, master, assistant professor, devoted to the research about new variety breeding and grafting of watermelon, Email: jiali1415118@163.com.

* Corresponding author. Email: jssyjry@163.com.

Grafting is now widely used in vegetable production. In watermelon production, it is adopted to prevent gummy stem blight, acute wilting, soilborne diseases, to improve plant resistance to low temperature and weak light, and the growth potential in latter growth period. The survival rate of grafted watermelon is increased to the maximum level when the procedure is carried out at seedling stage［1］.

There are many advantages of plant grafting in vegetable production. Compared with selfrooted seedlings, rootstocks have an established and healthy root system, which can improve plants ability to absorb water and nutrients from the soil［2-3］, promote the synthesis of endogenous hormones［4］, and reduce the intake of pollutants in the soil［5］. In addition, grafting can also improve plants resistance to stress, salt and floods［6-8］.

Previous studies have shown that grafting can improve the resistance of watermelon to Fusarium wilt［9］, increase the activity of defense enzymes such as phenylalanine ammonialyase (PAL), peroxidase (POD) and polyphenol oxidase (PPO), and the ability to scavenge reactive oxygen species and hydrogen peroxide in plants, improving the resistance of plants to diseases.

Previous studies have also shown that grafting can increase the yield of many fruit and vegetables such as tomato, watermelon, cucumber and eggplant. However, the impact of grafting on fruit quality is still controversial. It was reported that the singlefruit weight of watermelon could be increased by 30% by grafting onto gourd rootstocks［10-11］. However, grafting onto gourd and pumpkin rootstocks may lead to increased rind thickness of watermelon［11］.

In this study, the plant growth indices including the number of leaves per graft, the number of branches per graft and the length of main vine, and fruit quality indices including sugar content at center and side parts of fruit, rind thickness, longitudinal diameter and transverse diameter of grafted watermelons were measured, to evaluate the affinity between different rootstocks and watermelon scions, and to clarify the effects of different rootstocks on plant growth and fruit quality of watermelon. The results may provide a theoretical basis for the grafting of watermelon.

Materials and Methods

Materials

The experiment was conducted in a greenhouse at a base of Suqian Academy of Agricultural Sciences in the spring of 2016. The scions were Qianmei No.1, which has yellow flesh, and Qianli No. 1, which has red flesh. Six rootstocks: bottle gourd (Lagenaria), blackseed pumpkin (Citrullus), Japanese Xuesong (Citrullus), Jingxinzhen No. 4 (Citrullus), Jingxinzhen No. 2 (Citrullus) and Jingxinzhenguan (Lagenaria), were all purchased from local market. The seeds of the rootstocks are shown in Fig. 1.

Methods

Experimental design

The combinations of the six rootstocks and the two scions are shown in Table 1.

Field trial

The six rootstocks were sown on March 13, and the scions were sown on March 21, 2016. The scions were grafted onto the rootstocks as designed in Table 1, on March 29, when the rootstock seedlings grew to twoleaf stage, and the cotyledons of scion seedlings were unfolded. Thirty grafted seedlings were prepared for each rootstockscion combination. After chlorothalonil was sprayed, the seedlings were covered with plastic films, and placed in a shade netcovered low tunnel. The survived seedlings were then transplanted into a steelframe greenhouse, with 60 cm between seedlings in a row, and 5 m between the rows. The seedlings of different scionrootstock combinations were arranged following a random block design, with three replicates, and four plants in each replicate. Drip irrigation system was used in this greenhouse. The fruits of both grafted and nongrafted watermelons were harvested on June 17, 2016.

Measurement of survival rate of grafted watermelon seedlings

The survival rate of grafted plants of each rootstockscion combination was calculated when they were transplanted, using the formula:

Survival rate =Number of survived grafted seedlings /Total number of grafted seedlings ×100%



Fig. 1Seeds of the rootstocks used in this study

Table 1Combinations of rootstocks and scions in this study

Control

Rootstocks

Bottlegourd (1)Blackseedpumpkin (2)JapaneseXuesong (3)JingxinzhenNo. 4 (4)JingxinzhenNo. 2 (5)Jingxinzhenguan (6)

QianmeiNo. 1 (Y)Qianmei No. 1Bottle gourd (Y1)Qianmei No. 1Blackseed pumpkin (Y2)Qianmei No. 1Japanese Xuesong (Y3)Qianmei No. 1Jingxinzhen No. 4 (Y4)Qianmei No. 1Jingxinzhen No. 2 (Y5)Qianmei No. 1Jingxinzhenguan (Y6)

QianliNo. 1 (R)Qianli No. 1Bottle gourd (R1)Qianli No. 1Blackseed pumpkin (R2)Qianli No. 1Japanese Xuesong (R3)Qianli No. 1Jingxinzhen No. 4 (R4)Qianli No. 1Jingxinzhen No. 2 (R5)Qianli No. 1Jingxinzhenguan (R6)

Measurement of plant growth and development indices

Seven weeks after transplanting, the length of main vine, number of branches, and number of leaves per grafted seedlings were counted.

Measurement of fruit quality indices

Three ripe fruits were selected at random from each plot and weighed separately. Then, the watermelons were cut open to measure their longitudinal diameter, transverse diameter and rind thickness.

The sugar content at the center part and side part of each watermelon was measured using a handheld saccharimeter. The total soluble sugar content was determined with anthronesulfuric acid colorimetric assay. Titratable acid concentration was measured by titration. Vitamin C content was determined by molybdenum blue colorimetric assay.



Fig. 2Watermelons of different rootstockscion combinations

All data were analyzed for differences and their significance using SPSS Statistics 19 software.

Results and Analysis

Effects of different rootstocks on survival rate and vegetative growth of grafted watermelon seedlings

In most combinations, rootstock type showed a significant correlation with the survival rate, the number of leaves, the number of branches, and the length of main vine of grafted watermelon seedlings (P<0.05, 0.01 and 0.001), while watermelon variety and the interaction between watermelon variety and rootstock type showed no significant influences on vegetative growth of grafted watermelon seedlings. The data in Table 2 revealed that there was an extremely significant correlation between rootstock type and the survival rate of grafted watermelon seedlings. Among all treatments, nongrafted watermelon seedlings (the control) had the highest survival rate, up to 95.00%, followed by the seedlings grafted onto rootstock 1 (bottle guard, 78.30%), and seedlings grafted onto rootstock 2 (blackseed pumpkin), rootstock 4 (Jingxinzhen No. 4) and rootstock 6 (Jingxinzhenguan). The survival rates of seedlings grafted onto rootstock 3 (Japanese Xuesong) and rootstock 5 (Jingxinzhen No. 2) were the lowest (lower than 50%). The number of leaves per plant is linked to photosynthesis efficiency of plants. In general, more leaves indicate more chlorophylls that can participate in photosynthesis, and the plants can grow better. As shown in Table 2, grafting increased the number of leaves of watermelon seedlings, which was consistent with previous studies［12-13］. We also found that the number of leaves per seedling in all rootstockscion combinations was higher than that of the control group, which was 76.17 on average. The number of leaves of seedlings grafted onto Jingxinzhen No. 4 was the highest, up to 106.67. The number of branches and the length of main vine reflect plant vegetative growth. As shown in Table 2, the number of branches of seedlings grafted onto Jingxinzhenguan was the highest, up to 22.00, while there was no significant difference in the number of branches between other rootstockscion combinations and the control group. The main vine length of watermelon seedlings grafted on Jingxinzhen No. 2 was the highest, up to 238.83 cm, while there was no significant difference in this index between other rootstockscion combinations and the control group.

Agricultural Biotechnology2019

Table 2Effect of rootstocks on the survival rate and vegetative growth of grafted watermelon seedlings

Rootstocks

Survival rate∥%

QianmeiNo. 1QianliNo. 1Mean

Number of leaves per seedling

QianmeiNo. 1QianliNo. 1Mean

Number of branches per seedling

QianmeiNo. 1QianliNo. 1Mean

Length of main vine∥cm

QianmeiNo. 1QianliNo. 1Mean

Rootstock 176.70 cd80．00 bc78.30 ab67.33 d93.33 abc80.33 bc18.67 abc14.67 c16.67 b195.00 ab149.00 b172.00 b

Rootstock 270.00 cd76.70 cd73.30 b85.33 bcd83.00 cd84.17 bc16.67 abc17.67 abc17.17 b204.33 ab170.33 ab187.33 b

Rootstock 370.00 cd16.70 e43.30 c76.00 cd78.67 cd77.33 bc15.00 c17.00 abc16.00 b182.67 ab183.67 ab183.17 b

Rootstock 466.70 cd70.00 cd68.30 b108.33 a105.00 ab106.67 a19.00 abc20.67 abc19.83 ab176.33 ab195.67 ab186.00 b

Rootstock 530.00 e60.00 d45.00 c104.00 ab80.00 cd92.00 ab19.33 abc16.00 abc18.00 b238.33 a239.33 a238.83 a

Rootstock 680.00 bc66.70 cd73.30 b94.00 abc91.67 abc92.83 ab22.67 a21.33 ab22.00 a169.00 ab180.33 ab174.67 b

Control93.30 ab96.70 a95.00 a74.00 cd78.33 cd76.17 c15.33 bc17.00 abc16.17 b163.33 b177.33 ab170.33 b

Mean69.50 a66.70 a87.00 a87.14 a18.10 a17.86 a189.86 a185.10 a

Significance

Watermelon varietyNsNsNsNs

Rootstock type*******

Rootstock type×watermelon varietyNsNsNsNs

Effects of different rootstocks on fruit appearance indices of watermelon

As shown in Fig. 2, the waxy bloom on the surface of watermelons in combinations R2, R3, R4 and Y3 was less than that of the control. There was no significant difference in rind color between the grafted and nongrafted watermelons. The stripe pattern of watermelons also showed no significant difference between the control and the grafted watermelons except combination R2. Grafting increased the white veins (collections of dead/undeveloped flesh cells) in Qianli No. 1 watermelons in combinations R1 to R6, as well as in Qianmei No. 1 watermelons in combinations Y2, Y3 and Y6. And the white veins decrease the appearance and taste quality of watermelon flesh. But grafting increased the white veins in watermelon flesh in this study.

Our data also suggested that there were significant differences in singlefruit weight and transverse diameter between the two watermelon varieties, and between grafted and nongrafted watermelons. There was no significant correlation between the rind thickness of watermelon and rootstock type. The interaction between rootstock type and watermelon variety had no significant influence on the appearance of fruit. As shown in Table 3, the singlefruit weight of nongrafted Qianmei No. 1 was obviously lower than that of nongrafted Qianli No. 1. According to their longitudinal and transverse diameters, we found that the fruit of Qianmei No. 1 was nearly oval, and the fruit of Qianli No. 1 was round. The fruit grafted onto rootstock 4 was the largest, with a singlefruit weight of 4.36 kg, followed by that grafted onto rootstocks 1, 2, 3 and 6, and their singlefruit weight ranged between 3.52 and 3.72. The fruit grafted onto rootstock 5 was the smallest, with a singlefruit weight of 2.41 kg. Grafting had no significant influence on rind thickness of yellowfleshed watermelons, so there was no significant difference in rind thickness between grafted and nongrafted yellowfleshed watermelons. Grafting showed great influence on rind thickness of redfleshed watermelons. The rind thickness of combinations R1, R2, R3, R5 and R6 was obviously higher than that of the control (nongrafted Qianli No. 1), and only rootstock 4 did not increase the rind thickness of Qianli No. 1.

Table 3Effect of grafting on fruit appearance of watermelon

Rootstocks

Singlefruit weight∥kg

QianmeiNo. 1QianliNo. 1Mean

Longitudinal diameter∥cm

QianmeiNo. 1QianliNo. 1Mean

Transverse diameter∥cm

QianmeiNo. 1QianliNo. 1Mean

Rind thickness∥cm

QianmeiNo. 1QianliNo. 1Mean

Rootstock 13.23 b3.89 b3.56 ab19.27 ab19.57 ab19.42 abc17.77 bcde19.53 ab18.65 bcd0.87 ab1.17 a1.02 a

Rootstock 23.39 b3.66 b3.52 ab18.43 b18.20 b18.32 c17.03 e18.03 bcde17.53 cd0.93 ab0.92 ab0.92 a

Rootstock 33.64 b3.81 b3.72 ab19.67 ab19.93 ab19.80 abc18.27 bcde19.53 ab18.90 abc0.87 ab0.98 ab0.92 a

Rootstock 43.64 b5.08 a4.36 a19.83 ab18.03 b18.93 bc17.23 de17.47 cde17.35 d1.00 ab0.78 b0.89 a

Rootstock 51.81 c3.00 b2.41 c20.07 ab21.77 a20.92 a19.37 abc21.17 a20.27 a0.87 ab1.05 ab0.96 a

Rootstock 63.62 b3.56 b3.59 ab21.43 a19.67 ab20.55 ab19.13 bcd19.03 bcde19.08 ab0.87 ab1.10 ab0.98 a

Control3.11 b3.35 b3.23 bc21.30 a19.27 ab20.28 ab18.40 bcde18.43 bcde18.42 bcd0.93 ab0.80 b0.87 a

Mean3.20 b3.77 a20.00 a19.49 a18.17 b19.03 a0.90 a0.97 a

Significance

Watermelon variety*Ns*Ns

Rootstock type*****Ns

Rootstock type×watermelon varietyNsNsNsNs

Effects of grafting on fruit nutritional quality of watermelon

As shown in Table 4, there were significant differences in sugar content at center part and total soluble sugar content between the two watermelon varieties. For redfleshed Qianli No. 1, the sugar content at center part was 11.70%, and the total soluble sugar content was 58.90 mg/g, both higher than those of yellowfleshed Qianmei No. 1, whose sugar content at center part was 11.07%, and total soluble sugar content was 50.82 mg/g. However, there were no significant differences in sugar content at side part, titratable acidity and Vc content between the two varieties. Grafting only influenced sugar content at side part and Vc content, and had no significant effect on sugar content at center part, total soluble sugar and titratable acidity of the two watermelon varieties. The interaction between rootstock type and watermelon variety had no significant influence on fruit nutritional quality. Compared with nongrafted watermelons, The sugar content at side part of watermelons grafted onto rootstocks 4, 5 and 6 increased, ranging between 8.87% and 9.35%, and the Vc content of watermelons grafted onto rootstock 5 was generally low, only 1.63 mg/g, which was significantly lower than that of watermelons grafted onto other rootstocks (2.02-2.32 mg/g).

Table 4Grafting on fruit nutritional quality of watermelon

Rootstock

Sugar content at center part∥%

QianmeiNo. 1QianliNo. 1Mean

Sugar content at side part∥%

QianmeiNo. 1QianliNo. 1Mean

Total soluble sugar content∥mg/g

QianmeiNo. 1QianliNo. 1Mean

Titratable acidity∥mmol/100g

QianmeiNo. 1QianliNo. 1Mean

Vc content∥mg/g

QianmeiNo. 1QianliNo. 1Mean

Rootstock 110.50 a11.70 a11.10 a8.20 bc7.87 c8.03 b58.95 ab59.40 ab59.17 a1.18 ab0.86 b1.022.12 abc1.95 bc2.04 a

Rootstock 210.93 a12.00 a11.47 a7.87 c8.67 abc8.27 b48.84 bcd60.40 ab54.62 ab0.91 b1.08 ab1.00 ab2.05 bc2.39 ab2.22 a

Rootstock 310.67 a11.30 a10.98 a7.80 c8.53 abc8.17 b63.29 ab55.86 ab59.58 a1.04 ab1.17 ab1.10 ab2.14 abc1.90 c2.02 a

Rootstock 411.33 a12.13 a11.73 a9.73 a8.97 abc9.35 a40.55 cd69.27 a54.91 ab1.09 ab1.14 ab1.12 ab2.10 abc2.54 a2.32 a

Rootstock 511.57 a11.63 a11.60 a8.73 abc9.23 abc8.98 ab37.09 d51.85 bc44.47 b1.31 a1.12 ab1.22 a1.31 d1.93 bc1.63 b

Rootstock 611.90 a11.93 a11.92 a9.40 ab8.33 abc8.87 ab50.65 bc60.90 ab55.77 ab0.97 ab1.04 ab1.00 ab2.06 bc2.04 bc2.05 a

Control10.57 a11.23 a10.90 a8.10 bc8.63 abc8.37 b56.34 ab54.62 b55.48 ab0.99 ab0.84 b0.91 b1.94 bc2.11 abc2.02 a

Mean11.07 b11.70 a8.55 a8.60 a50.82 b58.90 a1.07 a1.04 a1.96 a2.12 a

significance

Watermelon variety*NS*NSNS

Rootstock typeNS*NSNS**

Rootstock type×watermelon varietyNSNSNSNSNS

Conclusions and Discussion

Grafting has now widely used in vegetable production around the world［14］. Our results showed that Lagenaria rootstocks gave higher survival rate and better fruit quality than Citrullus rootstocks. And rootstock 6 (Jingxinzhenguan) was more suitable for yellowfleshed Qianmei No. 1, and rootstock 1 (bottle gourd) was more suitable for redfleshed Qianli No. 1. A similar conclusion has been drawn in previous studies［15-19］.

The survival rate of grafted seedlings mainly depends on the compatibility (affinity), the ability of callus formation and vascular connection between rootstock and scion［1, 20-21］. Jeffree［22］ reported that the affinity between rootstock and scion depends on cell recognition between them. Our results showed that Lagenaria rootstocks have a better affinity to watermelon scions than Citrullus rootstocks.

In addition, our results also showed that the number of leaves per seedling on rootstock 4 (Jingxinzhen No. 4) was obviously higher than that on all other rootstocks, and the number of leaves of nongrafted seedlings was the smallest. According to the plant growth indices including the number of leaves, the number of branches and the length of main vine, all the grafted seedlings except these on rootstock 3 showed higher growth potential than nongrafted seedlings. Similar results have been reported in previous studies［23-26］. Salehimohammadi［27］ showed that the welldeveloped root system of rootstocks increased plants ability to absorb water and nutrients from the soil, and finally increased the biomass of the scion. The increased number of leaves and length of main vine of grafted plants in our study proved the high compatibility between the rootstocks and the scions［13, 28-29］. The length of main vine, the number of leaves and the number of branches reflect the vegetative growth of grafted watermelon plants, but the balance between vegetative growth and reproductive growth is more important. Excessive vegetative growth may inhibit reproductive growth, which is not conducive to the development of watermelon fruit.

It has been reported that mRNA molecule transfer between rootstock and scion is the main reason causing the variations of plants after grafting. The mRNA molecules in rootstock can be transported long distances via the phloem to scion, and then transcribed into cDNA, integrated into chromosomes of scion cells, regulating or interfering with gene expression of scion cells, thereby changing leaf shape, flowering period, fruit quality and other traits of watermelon. Our data showed that grafting had no significant influence on sugar content at center part and total soluble sugar content of watermelon, which was consistent with previous studies［11, 30-31］. Contrary to our findings, several studies showed that grafting watermelon onto gourd and pumpkin rootstocks resulted in a decrease in soluble solid content of watermelon fruit, and the effect was more obvious in pumpkin rootstocks［28, 32-34］.

At present, there are still many issues that need to be clarified in watermelon grafting, such as the mechanism by which rootstocks influence fruit quality, the molecular mechanism for the metabolism of sugar, acids and volatile substances in watermelon fruit, and the reasons causing thick rind and decreased flesh quality after grafting. Moreover, the mechanism for the formation of disease resistance of grafted watermelon, the transport of mRNAs and small RNAs from rootstock to scion, and how to induce the resistance of grafted plants to diseases and stresses all need to be addressed in future work.

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