The Productive Capacity and Quality of Several Walnut Cultivars ( Juglans regia L . ) Grown in North Oltenia , Romania

Romania has tradition in walnut culture and favorable climatic conditions. In order to establish the suitable cultivars for growing into the Northern Oltenia area several walnut cultivars of different origin were evaluated for productivity and fruits quality. Twelve Romanian cultivars and two foreign ones with terminal bearing and 10 cultivars with lateral bearing from France and U.S.A. were studied during 18 years of yielding with no irrigation. The group of lateral bearing cultivars proved to be more productive in the environmental conditions of the study area, the highest average yields were recorded for ‘Vina’ (3.64 t ha), ‘Hartley’ (3.59 t ha), ‘Ferjean’ (3.52 t ha), ‘Payne’ (3.45 t ha), ‘Pedro’ (3.39 t ha), etc. The most productive terminal bearing cultivars were: ‘Jupânești’ (2.88 t ha), ‘Valcor’ (2.77 t ha), ‘Franquette’ (2.71 t ha), ‘Valcris’ (2.62 t ha), etc. Although the lateral bearing cultivars emphasized good yields, their extensive culture into the areas with continental type of climatic conditions should be further analysed. The walnut kernel and oil content show that the cultivars are very similar regarding chemical composition when growing under same conditions in Northern Oltenia.


Introduction
Common walnut (Juglans regia L.), known also as Carpathian, Persian or English walnut, is grown in more than 60 countries all over the world from both hemispheres (Avanzato et al., 2014).
Most of the walnut production, which reached 3,462,731 tons in 2016, is obtained in China, U.S.A., Iran, Turkey, Ukraine, Chile, France, etc. Romania is on the 8 th place in the world rank and in the 2 nd place in the E.U., after France (FAO Stat Database, 2018).
Although Romania has tradition in walnut culture and favorable climatic conditions, the production is not constant and very variable from qualitative point of vue due to the fact that most of the walnuts are harvested from nongrafted individual trees and from small gardens.Organized walnut orchards cover only 1,673 ha (FAO Stat Database, 2018).
In the last years, the interest for establishing new walnut orchards increased a lot in Romania, but also in many other countries, as result of economic benefits and high demands of walnuts on the world market.
Productivity of walnut trees and nut quality are key parameters in assessing the behavior of walnut varieties.
The genetic and geographical origins of the cultivars and the environmental and technical conditions are responsible for increasing the productivity, fruit quality and economic suitability of walnut culture (Burke et al., 1976;Grimo, 1981;Botu and Achim, 1994;Botu, 1998;Botu et al., 2001;Aleta et al., 2014).
In order to achieve high yields and quality in-shell walnuts or kernels, the orchards should be planted with adequate cultivars grafted on suitable rootstocks only in the areas with most favorable environmental conditions.
In the last decades, at University of Craiova -SCDP Vâlcea, a walnut collection and trials have been set up, over 128 local and introduced varieties, biotypes and hybrids North Oltenia.Fourteen of the cultivars used in this study have terminal bearing; twelve of them are of Romanian origin, one from Oregon -U.S.A. ('Adams 10') and one from France ('Franquette').The ten remaining cultivars are either intermediate or lateral bearers and are originated in the U.S.A. and France (Table 1).
Over the 23 years from planting, 18 years of production were counted.In 2017, due to frost recorded during the blooming period (-2.1 °C on April 22 nd , 2017) most of the yield was affected.
The lateral bearing varieties achieved an average yield of 3.34 t ha -1 , with 0.91 t ha -1 higher than the terminal ones (2.43 t ha -1 ).
These yields were obtained in North Oltenia, in an area where during the study period the minimum temperature recorded was -22.7 °C.In the rest of the study period the low temperatures did not fall below -18 °C; -19 °C.Botu et al. (2017) mention that cultivation of lateral bearing walnut cultivars in continental climate areas is influenced by their susceptibility to low temperatures during winter.Terminal bearing cultivars from warmer climates like 'Franquette' are susceptible to winter temperatures too.Studies carried out by Aslamarz et al. (2010), Charrier et al. (2013), Gandev (2013), Aletà et al. (2014), all cited by Botu et al. (2017) confirm the susceptibility to low temperatures of the lateral bearing cultivars.
Such walnut cultivars should be planted only in favorable areas where the climate is milder during winter in order to perform well.Another issue to be taken into account is their susceptibility to diseases, especially to walnut blight (caused by Xanthomonas arboricola pv.juglandis).The lateral bearing cultivars proved more susceptible to blight on fruits and shoots then the terminal bearing ones (Botu et al., 2017).
In international trade the quality of in-shell and shelled walnut is very important, function of these, the prices vary a lot.The quality of walnuts is influenced by many characteristics, but also by the biometric features and chemical composition of kernels.
In case of the nut samples collected from the trial the nut size index (SI) was calculated: SI= (D + d + h)/3, where D is maximum diameter, d is minimum diameter and h is the nut height.being under evaluation.The purpose is to determine which the most valuable walnut cultivars are for the environmental conditions of Northern Oltenia in terms of productivity, quality and profitability.Also, a breeding program has been launched in the 1990s having the aim to release cultivars adapted to the local environment, productive, resistant or tolerant to main diseases and producing quality nuts.Till now, seven cultivars were released: 'Valcor' (1999), 'Valrex' (1999), 'Valmit' (2000), 'Unival' (2010), 'Valstar', 'Timval' and'Valcris' (2012) and one seedling rootstock, 'Portval' (2003).
Current paper presents the results achieved testing practically some of the most used walnut cultivars from the international assortment, along with Romanian cultivars in order to establish the most suitable cultivars to be grown in the Northern Oltenia.

Plant material
The research has been carried out at Fruit Growing Research and Extension Station (S.C.D.P.) Vâlcea, belonging to the University of Craiova.The trial is located in Bujoreni, north of Râmnicu Vâlcea (45° 8'22.35"N and 24° 22'36.85"E), in the North of Oltenia Region, where the climate is of Cfb Köppen -Geiger type (Rubel and Kottek, 2010).The annual average temperature is 10.2 °C and annual rainfall of 715 mm in the study area.Taking into account the amount of rainfall, no irrigation of the trees was provided.The trial was set up in 1997, using planting distances of 9.0 by 8.0 meters (density of 139 trees ha -1 ), on an alluvial soil, having a pH of 6.8.
The chemical composition of the kernel, oil and residue for the walnut samples collected have been analyzed at the National Research and Development Institute for Cryogenics and Isotopic Technologies -ICSI Rm.Vâlcea (Popescu et al., 2016).
For the analysis of variance (ANOVA) the MS Excel software was utilized and DSAASTAT ver.1.514 (2015) for the Duncan's Multiple Range Test at significance level of p<0.05.DSAASTAT is an Excel VBA Macro developed by Onofri (2007).

Results and Discussion
The study conducted at SCDP Vâlcea aimed to establish the production capacity of several walnut cultivars in the intensive orchard under the environmental conditions from The nut size index is on average 37.39 mm for terminal bearing varieties and 36.05mm for lateral ones (Table 2).The size index fluctuates between 33.7 mm ('Sibișel precoce') and 42.5 mm ('Sibișel 44') for Romanian cultivars, and in case the foreign ones between 33.2 mm ('Fernor') and 40.0 mm ('Adams 10').
The shape of walnuts was determined using the index of roundness (IR).
IR= (D + d)/2h Differences on average are not major between the two groups of walnut cultivars, for the terminal bearing ones the mean of index of roundness was 0.82 while for the lateral ones 0.84.The closer the index of roundness gets to 1.0, the more round is the nut.Among the Romanian varieties there are some with round fruits ('Argeșan', 'Sibișel precoce', 'Valcor', 'Unival', 'Valcris' etc.), but also with elongated ones ('Jupâneşti', 'Sibișel 44' and 'Orăştie').
An important element for the shelled market the kernel percentage of the cultivars.It is desired that the kernel efficiency to overpass 50%.

Table 1 .
Cumulative and average walnut yield for cultivars grown in Northern Oltenia Means followed by the same letter do not differ significantly from one another (Duncan's multiple range test, p < 0.05)

Table 2 .
Nut characteristics of the walnut grown in Northern Oltenia Note: Means followed by the same letter do not differ significantly from one another (Duncan's multiple range test, p < 0.05)

Table 3 .
Nut sizes depending on diameter for the walnut cultivars grown in Northern Oltenia Note: Means followed by the same letter do not differ significantly from one another (Duncan's multiple range test, p < 0.05)

Table 4 .
Comparative chemical composition of kernel, oil and residues from walnut cultivars grown in Northern Oltenia