Jinli LIU, Qiuxiang YU, Lixian CUI, Jing LIU, Zhongkui BAI

Changli Institute of Pomology, Hebei Academy of Agriculture and Forestry Sciences, Changli 066600, China

Abstract [Objectives] To accelerate the improvement of walnut varieties and change the cultivation status of low yield and poor quality. [Methods] Through experimenting on the optimization technology of walnut trees, this paper studied the suitable top grafting period and scientific top grafting protection measures of walnut trees in Eastern Hebei, and established the top grafting technology system of walnut. Moreover, this paper explored the impact of different time and scientific management and protection measures on the survival rate and growth of bare scion top grafting of walnut trees. [Results] The grafting time had a great impact on the growth of new shoots, and there were significant differences in the survival rate among different grafting periods. The highest grafting survival rate was 96.97% on April 27 and the lowest grafting survival rate was 66.67% on May 4; the average survival rate was 78.10%. Among them, the survival rate of top grafting was the highest on April 20 and April 27. The growth of new shoots grafted in the early stage was greater than that in the late stage. The growth of new shoots and branches on April 8 and April 20 were the largest, with lengths of 160.3 and 143.7 cm; the thickness was 1.72 and 1.43 cm; the branch lengths were 79.8 and 63.4 cm; the branch thickness was 0.98 and 0.82 cm; the growth index was also the highest on April 8 and April 20 at 10, 20, 30 d after grafting, which was significantly different from other time periods. [Conclusions] Walnut trees with early grafting in Eastern Hebei can quickly restore their crown growth and had higher yield, thus this technology can be widely applied in Eastern Hebei or Yanshan area.

Key words Walnut, Top grafting, Survival rate, Growth

1 Introduction

Eastern Hebei is suitable for the growth of high-quality walnut for its sufficient sunlight and great temperature difference between day and night, it has a great potential in the industrialization development of walnut industry, and the local walnut varieties are well-known for the big and easily-taken kernels, light color and strong flavor[1]. As modern people have deepened their understanding of health and fitness, and attached more importance to the life quality improvement, walnut (JuglansregiaL.) had become more popular among consumers as a kind of healthy food[2-3], thus farmers have had rising enthusiasm for growing walnut trees[4-5], more new quality resources have been introduced, as a result, mixed varieties and variety selection have become critical in the industry development, because the overall quality of walnut products have been influenced by the inconsistent maturation periods of walnuts, uneven quality, difficulty in precision management and farming[6-11]. To accelerate the upgrading of walnut varieties[12-13], and the popularization of superior varieties, change the low yield and quality of local walnut trees, it is critical for the industrialization of walnut to explore the optimal top grafting technology for Eastern Hebei. Top grafting of walnut trees has been reported frequently, but top grafting of walnut has been largely influenced by local geographical conditions, climate and standard technical measures. Therefore, exploring a suitable top grafting technique for the local area is significant for accelerating the improvement of local walnut varieties, and increasing local farmers’ income. This paper targeted at the practical problems in local walnut product, such as low survival rate of walnut grafting, short growing season of bud grafting in northern China, slow growth of crown, limited growth of new treetop, great precipitation in summer, easy water accumulation around buds, devoted great efforts in solving these problems, and attempted to explore the top grafting of walnut bare scions through experiments, and to achieve the goal of bearing fruits and high yield as soon as possible.

2 Materials and methods

2.1 Study plotThe study plot is located in the Shigezhuang Walnut Orchard of Changli Institute of Pomology, Liangshan Town in northeast Hebei. It is in the southeastern range of Yanshan Mountains and enjoys warm temperate and semi-humid continental climate, thus there is sufficient sunlight, distinctive four seasons, long autumn and frost-free period, and small hydrothermic coefficient. Annual mean temperature is 11.3 ℃ and annual mean precipitation is 712.7 mm, local soil is sandy loam. Line spacing of walnut trees in the study plot is 4 m×5 m.

2.2 Material selectionEight-year strong walnut trees of seeding propagation and low yield in Shigezhuang Walnut Orchard were selected, only main branches with a diameter above 4 cm were chosen for the top grafting, and the scions were taken from the 1-year strong developmental branch from walnut trees in the base, with fully-growing buds and without any plant disease or insect pest.

2.3 Technical methodsThe grafting experiments set different time treatments, applied bark whip grafting, and grafted 15 trees each time with 2-3 fully-growing buds on each scion, and 1-2 scions for each grafted branch. Twenty or 30 d before bud sprouting, 1-year fully-growing branches were picked up for wax sealing, and then stored in refrigeration house at the temperature of 0-5 ℃ after being wrapped tightly with high-quality pvc sheets, then the scions stayed in dormant state. Then, the scions were taken out 2-3 d before grafting and cut into sections about 20-30 cm, then immersed fully into water for absorbing water totally to facilitate the germination of the scions and separation from the bark, and again the scions were sealed with wax. In this process, the scions must be smeared with vaseline if the wax was gone to avoid water loss. The scions were grafted to the trees in different periods from the early April to the middle May. Growth conditions of the grafted scions were observed 10, 20, 30 d after grafting.

3 Results and analysis

3.1 Influence of different treatments on the survival rate and growth of grafted treesAccording to Table 1-2, survival rate of walnut trees showed significant difference in different time treatments, the highest survival rate of bark whip grafting was up to 96.9%, the lowest was 66.67%; the mean survival rate was 78.10%. Survival rate of grafting in different stages also varied greatly, top grafting on April 20 and April 27 had the highest survival rate, mainly because this period was the early growth stage of walnut trees that trees consume less nutrients, tree stocks and scions had fully-growing tissues, temperature and humidity were suitable for the parenchymal cell division and the healing of calluses. However, temperature rose after May, branches and leaves grew larger, the aboveground parts of trees consumed a lot, tree bleeding increased, and the survival rate decreased obviously. Scion growth of the early grafted trees was greater than that of the late grated ones, the new top developed fully, and some new tops had new branches. According to the investigation, the mean growth of new top was 138.68 cm, the thickness was 1.48 cm; the most branches were 7, and the mean growth was 64.6 cm, the mean thickness was 0.84 cm, the largest branch length was 96.6 cm, and the thickness was 1.06 cm, these figures laid a solid foundation for the restoration of tree crowns, and early fruit bearing and high yield.

Table 1 Investigation on survival rate of walnut trees at different grafting stages

Table 2 Investigation on growth at different grafting stages

3.2 Influence of different treatments on growth of walnut trees grafted in different periodsAccording to Table 3, new shoots of the grafted walnut trees grew fast, top grafting in different periods showed great difference in growth, among which growth indexes of trees grafted on April 8 and April 20 was the highest 10, 20, 30 d after grafting, and were followed by those grafted on April 27, May 4 and May 15, these indexes showed significant differences. The figures indicated that grafting time had great effect on bark whip grafting, too early or too late grafting were not good for the growth of new tops. And the new tops grew fast 20 d after grafting.

4 Key technical measures

Bare-scion top grafting of walnut trees requires: (i) Cut section must be 10-15 cm long, the cut part accounts for 3/4 of the scion thickness, particularly the upper part is thicker than the lower part, the tongue-shaped section has a gentle slope, and the cut section length of the tree stock must be 0.5 cm longer than the scion cut section, the scion must have a 1-cm long part left above the grafted section to facilitate the co-growth of stock and scion and improve the survival rate. (ii) The stocks must have many buds, and the buds are removed every 15 d to facilitate the germination of scion buds. (iii) When the new tops grow to 50 cm long, it is ready for pinching to ensure the safe overwintering of branches. (iv) Walnut trees have higher liquid compression, hollow and thick tubes, which easily cause bleeding, therefore, "bleeding troughs" are needed on the lower part of northern side of trees before top grafting, three 12-cm long and 0.5 cm wide troughs 30 cm above the ground. The troughs must be as deep as the xylem of the stocks to facilitate the drainage of bleeding sap, avoid its negative impact on survival rate, and improve the survival rate of the grafted trees.

5 Discussions

Bark whip grafting during the leaf-growing stage in spring helps the fast tree shaping and high survival rate, good growth momentum of trees contributes to the early yielding[14]. After top grafting of walnut trees, stocks and scions grow into an integrated conduction system, the newly-germinated top grows fast because of the strong root systems of the tree stock so that it can absorb sufficient minerals and water from soil for its growth. According to the top grafting investigation of bare scions, the grafting period had great effect on the growth of new tops[15], the new top of scions grafted in the suitable periods grew fast, had smaller pith, developed fully. Moreover, 3 key facts were summarized for the top grafting, (i) the suitable top grafting periods must be grasped, the beginning of top grafting is the period when temperature reaches 10 ℃ and above, buds of walnut trees started to germinate, in this period trees have less bleeding, and the best period is that the temperature rises to 13-17 ℃ (blooming of male flowers to leaf-growing period), the trees have the least bleeding. (ii) Fully-growing thick scions should be chosen, the thickness > 1.2 cm, straight and smooth with full buds, bright colors and no diseases. (iii) For the wax sealing of scions, the cut must be wrapped tightly to avoid water loss of scions, and as the new top grows to 30 cm long, it must be tied in time to avoid wind damage. Top grafting of bare scions saves labor, materials and time, and it is also an easily-operated technique for promotion.