STUDY ON THE INFLUENCE OF GRAFTING PERIOD AND ENVIRONMENT CONDITIONS UPON WALNUT GRAFTING IN CONTROLLED CONDITIONS (PLASTIC GREENHOUSE)

The study was conducted during 2014-2017 to obtain planting material by grafting the walnut directly in solarium conditions. The grafting method used was chip budding. The two grafting periods were August (sleeping bud grafting) and April (growing bud grafting). The best results were obtained in the case of walnut grafting in April, when the temperature conditions, the relative humidity of the air and the intensity of the light are close to the optimal, in this case grafting success percent is between 39 and 57%. In the case of walnut grafting in August, the best results were obtained with the use of the grafted branches wrought one month before grafting (grafting success 24%). Cuvinte cheie: nuc, altoire, metoda de altoire, perioada de altoire, solar.


Introduction
The quality of the planting material used for the establishment of walnut plantations is essential for their efficiency.
Unlike other tree species, walnut grafting is a technological sequence that requires special conditions to achieve good results.
Grafting, using improved copulation method and forcing the material into forcing rooms after grafting, remains one of the most used technologies but with quite large production costs (Achim, G., Botu, I., 2001).
Under conditions of walnut grafting in the field, favorable environmental conditions are not recorded until certain periods of the year, but the results obtained are not satisfactory (Karadeniz, 2003).
Success of grafting is influenced by several factors such as grafting, temperature, atmospheric humidity, phenolic compounds, hormonal and nutritive conditions of grafted varieties, and grafting time (Mitrovic, 1995;Mehmet et al., 1997).
Environmental conditions, during and after grafting, play a very important role in the formation of walnut callus (Avanzato and Atefi, 1997).Optimally, the temperature should be maintained at about 27 o C after grafting for better callus formation and successful grafting (Avanzato and Atefi, 1997;Germain, 1998).
The study was conducted between 2014 and 2017, aiming to graft the walnut in controlled conditions.

Material and methods
For setting up 1 st field of nursery, a 20-meter-long, 10 m wide and 4.5 m high tunnel was used.(Figure 3) Walnuts for rootstock, after harvesting, were disinfected with fungicides, then layered in sand for eight weeks at 4-5 o C. Sprouted nuts were used and the sowing distance was 75 cm between the rows and 10 cm per row and the sowing depth was 5 cm.
After approximately 8 months, when the rootstocks were more than 100 cm tall and 14.2 mm thick at 10 cm from the ground, they were grafted.(Figure 5) Grafting method used chip budding in two grafting periods: August (bud sleeping) and April (with growing bud).
For binding the grafting area, the Buddy Tape was used.By removing graft leaves 20 days prior to grafting, it is possible to reduce the concentration of phenolic compounds in the tissues and their oxidation, together with the injury of grafting tissues (Hodaj B., Rama P., Hodaj Natasha, 2014).
For grafting in August, scions (grafting branches) were used, which were shaped at two different times: leaf removal and maintenance of a 2 cm petiole, carried out one month prior to the grafting of the branches, and, respectively, shaping at the time of harvesting the grafting branches.For grafting in April, branches were harvested late in autumn and kept in winter in warehouses at temperatures of 2-4 o C. Since temperature and humidity have a decisive effect on the process, they were recorded during grafting period, using the maximum and minimum thermometer and hygrometer (Figures 1 and 2).As reference indicators, the temperature was 27° C and the relative humidity was 40%.
During the vegetation period, specific maintenance works were carried out.
During the experiments, observations and determinations were made regarding: grafting success and graft growth.

Results and discussions
In greenhouse conditions the best results are obtained when the temperatures are maintained at 25° C on the day and 21° C at night and relative humidity of 39 and 70% respectively during the three weeks after grafting (Ebrahimi, A., Vahdati, K., Fallahi, E., 2006).
The optimum environmental conditions, grafting method and grafting period are of utmost importance for the success of walnut grafting, depending on the particularities of the region (Ahmed et al., 2012).
The April environmental conditions were closer to the benchmarks (27° C temperature, 40% humidity and light intensity 3000-4000 lux) and had a positive influence on grafting.Thus, grafting success was 51.8% in 2015 and 48.6% in 2017.
In the case of grafting in August (2014 and 2015), grafting success was lower, 8.3% (in 2014) and 16.0% (2015), as the recorded temperatures of 31-37° C and humidity of 55-75%, were higher than the benchmarks.
By removing graft leaves 20 days prior to grafting, it is possible to reduce the concentration of phenolic compounds in the tissues and their oxidation, together with the injury of grafting tissues (Hodaj B., Rama P., Hodaj Natasha, 2014).
High content of polyphenols in the plant negatively influences grafting.In the grafting section these phenolic compounds are oxidized by polyphenoloxidase and molecular oxygen, than transformed into a complex of brown or blackish molecules, resulting in tissues necrosis near the graft area (Karadeniz, 2005).
Data on the growth of grafted trees in 2 nd field of the nursery highlights the very good results obtained during the two grafting periods, the differences between them being insignificant.Thus, at the time of removal from the nursery, the grafted trees had over 180 cm high and 12 mm diameter at 80 cm height above the soil.(Table 1, Figures 4, 6, 8 and 9).

Conclusions
Controlling the temperature and humidity around optimum values under controlled conditions ensures good callus formation and therefore improves grafting success.
Chip budding is a successful method for walnut grafting, with grafting success percentage of 48.6-51.9%when grafting took place in mid-April and poor results when grafting in mid-August.
Establishing the first field in the solarium, by direct sowing and budding seedlings using chip budding method, can be one of the technological sequences that ensure the production of grafted material both in quantitative and qualitative terms, allowing easier control of environmental conditions in protected areas.