Journal APS Oct 2017

W alnut

223

on the grafting dates in both years along with nursery conditions. We used only one grafting method (whip-tongue) to avoid the effects of different grafting methods.  A common rule for walnut grafting is that walnut requires high temperature for callus formation (Ebrahimi et al., 2007; Gandev, 2009; Dehgan et al., 2010; Soleimani et al., 2010). We examined the relationship between environmental factors and graft success (Table 1). In 2013 we found a strong negative correlation between mean temperatures, 15 days and one month after grafting; delaying the grafting time of walnut decreased the graft success under our conditions. Poor graft success may be due to higher temperatures on later grafting dates, which affected the growth of rootstocks. There was also a strong negative correlation between growth stage of rootstocks and graft success (p<0.01). The reduced graft success may have been due to excessive carbohydrate utilization during rapid shoot growth  Karadeniz (2005) reported a positive correlation between graft success and relative humidity; however, in our study the correlation between relative humidity and graft success was not significant at the 5% level. This may be due to the parafilm application to scion wood and graft union after wrapping plastic tape. We also did not evaluate the effects of light in the study; different light intensities should be examined at equal rootstock growth stages, temperature and relative humidity conditions.  Walnut can be grafted indoors and outdoors. Indoor grafting is done generally in the winter or early spring. In the case of indoor grafting, dormant rootstock is used. But in the scenario of outdoor grafting, the growth stages of rootstocks depend on the grafting date. Indoor grafting provides a stable temperature for a particular time period, but for outdoor grafting daily temperatures obviously fluctuate. Ramos (1998) reported that walnut callus formation starts outdoors when the daily temperatures reach 21-24°C. Therefore, it is important to

which also had the highest mean temperature one month following grafting.  In 2013, graft success ranged from 60.0- 100.0% in the open field and 33.3-98.3% in the greenhouse (Table 2). In the open field highest graft success was obtained from grafting on 5 April (100.0 ±0.0%). However, in the 50% shaded greenhouse graft success was best on 15 March which was 20 days earlier than the open field regardless of grafting date (98.3 ± 1.7 %). Results between 50% shaded greenhouse and open field were similar to 2012. Also, rootstock growth stage depended on grafting date. The longest rootstock shoots were measured on 15 May for both nursery conditions. Similar to 2012, graft success and growth stage of rootstock were negatively correlated (Table 1). Rootstock shoot length increased from bud break (15 March) to 30-40 cm on 15 May. In the 50% shaded greenhouse optimal success occurred when rootstocks were grafted at bud break on 15 March (100.0 ± 0.0%). However, in the open field 5April was the optimal date for success when rootstock’s had 10- to 20 cm-long shoots. In both nursery conditions lowest graft success occurred on 15 May which also had the highest mean temperature one month following grafting similar to 2012.  In both years, graft success was poorly correlated with relative humidity. Correlation between graft success and mean temperature 15 days and one month following grafting changed depending on the year. In 2013, but not 2012, graft success was significantly and negatively correlated with mean temperature, 15 days and one month following grafting. Discussion  We examined possible factors that affect graft success such as growth stage of rootstocks at grafting time, mean temperature, fifteen days and one month following grafting and relative humidity one month following grafting. We observed high graft success in both nursery conditions in this study. Graft success changed depending