Effect of Grafting Method, Graft Cover and Foliar Spray of some Mineral Elements on Persian Walnut Graft-take and Winter Survival Rate

Persian walnut (Juglans regia L.) is an important nut crop in Iran and many parts of the world. One of the major challenges of growing walnut is planting of non-grafted walnut trees in orchards, which leads to the reduction of yield, quality and productivity of walnut orchards. Compared to the other fruit trees, walnut grafting is difficult and even newly grafted walnut seedlings are vulnerable to fall or winter frost chilling, so that most of the seedlings are lost after subjecting to the cold winter. There are a few studies reporting successful grafting in outdoor conditions, however, final grafting take after winter has been usually ignored. Hence, increased walnut grafting success and improved tree growth after grafting through foliar nutrient application may lead to increased tolerance of chilling. Therefore, main goals of this research were to investigate the effect of some graft covers and role of foliar spray of calcium, boron and zinc on the reduction of frost damage in newly grafted seedlings under outdoor conditions.
This research was conducted at agricultural research station, Khoy city, west Azerbaijan province, during 2012-2014. In the first experiment, three methods of grafting including cleft, bark and V-shaped, and two kinds of graft covers including moist sawdust and superabsorbent plus cotton wool were investigated in terms of grafting success and quality of seedlings. In the second experiment, effect of the three above-mentioned grafting methods and two levels of foliar ¬spray including sequential spray of Ca (4 ppm), B and Zn (2%) (3 times during growth season) and control (no spray) were studied in terms of frost damage. The experiments conducted in factorial based on randomized complete block design with 10 trees in each plot. Data were collected 45 days after grafting take, final grafting take after one winter, subsequent scion growth length and diameter and concentration of Ca, B and Zn in the tissues of shoot tips as well as percentage of frost damage one year after grafting. The collected data were transformed by relevant methods and analyzed by GLM analysis using SPSS software.
According to the results obtained from the first experiment, significant differences were observed among grafting methods and grafting covers in terms of grafting success and scion growth. Cleft grafting with the grafting take of 47.4% after 45 days was ranked as the best method, followed by bark and V-shaped grafting methods with 40.0 and 35.0 %, respectively. Meanwhile, V-shaped grafting method finally showed the highest grafting take with 46.6%. The effect of grafting type was also significant for scion shoot length and diameter, with the highest scion growth obtaining from bark grafting method. Regarding the effect of cover types, significant differences were found between the two types of covers, so that the highest grafting take (75.5%) obtained from moist sawdust cover compared to the lowest grafting take (11.1%) from super absorbent plus cotton wool cover. The increase found in grafting success by sawdust cover was in agreement with the previous reports. This increase can be attributed to the buffering action of sawdust in absorbing xylem sap, provision of moist and aerated conditions suitable for better callus formation and subsequent scion growth without any wood rot symptoms around the graft area. The results of the second part of the research also revealed that percentage of frost to dieback of shoots varied statistically among the three grafting methods. The lowest frost damage (17.5%) was related to the cleft followed by V-shaped grafting method (20.0%). The highest frost damage (24.6%) was observed on scion woods grafted by bark grafting method. Results related to foliar spray showed that spray of Ca, B and Zn caused a significant reduction in frost damage percentage. In the sprayed plots, the average of frost damage was only 11.6% compared to the control plot with the damage rate of 29.4%. Moreover, foliar spray statistically increased the concentration of related elements in vegetative tissues of scion.
Based on the results, using V-shaped, cleft and bark grating methods covered with moist sawdust were effective for the increase of graft success percentage. Spraying of young grafted trees with Ca, B and Zn was also effective to reduce frost damage and is thus recommended in walnut nurseries. Moreover, there was a significant interaction between grafting method and spraying treatments in terms of balanced scion growth and higher contents of mineral in scion tissues, indicating that choosing an appropriate method of grafting and spraying can be effective in the reduction of frost damage. In this study, the lowest frost injury was observed with cleft grafting and spraying treatments due to relatively reduced scion growth as well as higher content of minerals in scion tissues.