VARIABILITY OF POTASSIUM CONCENTRATION IN THE NEEDLES OF DOUGLAS-FIR PROVENANCES

: Douglas-fir is one of the most common conifer species in the forest plantations of Europe. The provenance test model is based on the analysis of the properties (growth, anatom ical, physiological, chemical, mechanical and other properties) of Douglas-fir in order to justify the transfer of seeds from North America to the ecosystems of Serbia. This type of program has been implemented in Serbia on several locations using different Douglas-fir provenanc es. Given that the analysis of different physiological properties of trees is important for the introduction of specified provenances into habitats in Serbia, in this paper the variability of potassium concentration in young Douglas-fir needles of different provenances was examined in an experimental field in Serbia. Potassium is very important in the physiological processes of plants. It is an essential element involved in a number of biochemical and physiological pro cesses and plays a significant role in the adaptation of plants on biotic and abiotic stress factors. The highest potassium concentration was found in the “Oregon 205–14” provenance – the only provenance in which the potassium concentration was significantly higher than the average value, so it can be characterized as a superior provenance for the uptake and accumulation of this element. It was found that, at this stage of development, the differences in the potassium concentration in trees in the provenance test have no significant effect on growth parameters. Further studies should show whether the concentration of this biogenic element in any way affects other properties important for the selection of provenances during the introduction process, such as resistance to stress factors.


Douglas-fir [Pseudotsuga menziesii (Mirb.)
Franco] is a very tall, fast-growing conifer species native to North America. Under optimal conditions, this species reaches a height of over 100 m, a diameter of 4 m, and the age of 1300 years or more. In a natural environment, it is widespread in the western part of North America -from British Columbia (Canada) in the north to Mexico in the south (Fig. 1). Douglas-fir was introduced into Europe at the beginning of the 19 th century and nowadays it is grown in many countries. This conifer is one of the most economically important tree species for the production of wood forest products (H e r m a n n , L ave n d e r, 1990). Douglas-fir is an integral part of forest management in many countries due to its economic importance and the assumption that this species is able to resist climate change (Lavender, Hermann, 2014). This is one of the most important conifer species in plantations which are used for afforestation in France, Germany, Poland, the United Kingdom and New Abstract: Douglas-fir is one of the most common conifer species in the forest plantations of Europe. The provenance test model is based on the analysis of the properties (growth, anatomical, physiological, chemical, mechanical and other properties) of Douglas-fir in order to justify the transfer of seeds from North America to the ecosystems of Serbia. This type of program has been implemented in Serbia on several locations using different Douglas-fir provenances. Given that the analysis of different physiological properties of trees is important for the introduction of specified provenances into habitats in Serbia, in this paper the variability of potassium concentration in young Douglas-fir needles of different provenances was examined in an experimental field in Serbia. Potassium is very important in the physiological processes of plants. It is an essential element involved in a number of biochemical and physiological processes and plays a significant role in the adaptation of plants on biotic and abiotic stress factors. The highest potassium concentration was found in the "Oregon 205-14" provenance -the only provenance in which the potassium concentration was significantly higher than the average value, so it can be characterized as a superior provenance for the uptake and accumulation of this element. It was found that, at this stage of development, the differences in the potassium concentration in trees in the provenance test have no significant effect on growth parameters. Further studies should show whether the concentration of this biogenic element in any way affects other properties important for the selection of provenances during the introduction process, such as resistance to stress factors.
Key words: Douglas-fir, provenances, potassium, introduction, Serbia 800,000 hectares. The largest areas under this conifer are found in France, followed by Germany where it has quickly become the most widespread allochthonous tree species (S p i e c ke r et al., 2019).
Douglas-fir is a highly adjustable species, able to grow in different climate conditions. Although deep, moist and well-drained soils suit it better, it also grows on different substrates. It quickly occupies areas devastated by forest fires or other environmental disturbances; it tolerates shade, so it grows well under tree canopies. In its natural habitat, Douglas-fir covers a wide range of altitudes, from about 0 m on the Pacific coast up to 3,200 m in the Rocky Mountains (2019).
The process of introduction of Douglas-fir in Serbia has begun at the Institute of Forestry in Belgrade by setting up the provenance tests and by studying this conifer species and its characteristics (L ava d i n ov i ć , 1995, 2009; L ava d i n ov i ć , Ko p r i v i ca , 1996; L ava d i n o v i ć et al., 2018, 2018a). The introduction of a species is justified only if the introduced species has a better quality than the native tree species. Adaptability, productivity, as well as the suitability of introduced species, with a wide natural range, must be tested by provenance tests, which is also the aim of the study of Douglas-fir in Serbia.
The importance of potassium in the physiological processes of plants is considerable. This biogenic macronutrient activates more than 50 enzymes and as a cofactor participates in a series of enzymatic reactions which take place in photosynthesis, respiration and other processes. Potassium plays a significant role in the neutralization of excess organic acids in the cell, hydration of colloids of protoplasm, osmoregulation, as well as in the mechanism of opening and closing stomatal pores. It is important in the processes of synthesis, transport and accumulation of carbohydrates. It increases resistance to drought and other stress factors (low temperatures, salts, diseases, air pollution, etc.). Potassium plays a significant role in the regulation of active transport across cell membrane and the uptake of other essential elements, as well as in protection against oxidative stress, because it regulates the activity of enzymes involved in the detoxification of reactive oxygen (ROS) (Wang et al., 2013). The potassium concentration in leaves is normally in the range from 0.75% to 2.5% (Đ u n i s i j e v i ć -B o j o v i ć , 2019). Plants are considered to be exposed to acute deficiency if the potassium concentration in leaves is below 0.6%. The importance of potassium in plant metabolism is also indicated by the fact that plants absorb it most intensively in the earliest stages of their development. Potassium is mostly accumulated in meristematic zones and vacuoles. The amount of potassium in plant tissue depends on the species, organ and age of the plant. With the increase of potassium concentration, the uptake of SO 2 and the synthesis of carbohydrates and proteins also increase in plants, while in its absence the process of photosynthesis slows down, and respiration becomes more intense. Plants that are characterized by increased carbohydrate synthesis, which affect the synthesis of cellulose and hemicellulose and whose role is important in the construction of the cell walls and vessels, are called "potassium plants". Among the species of trees with this phenomena are beech, eastern black walnut, etc. (Nešković et al., 2003;Oljača et al., 2006).
Plant-available potassium in the soil is present in the form of water-soluble salts and in adsorbed form on the surface of colloidal particles. The decomposition of potassium primary minerals leads to the creation of these forms of potassium. There is a dynamic balance in soil between the water-soluble and adsorbed potassium. The potassium concentration in solution reduces by its adsorption, and the adsorbed potassium transfers from the adsorptive complex to the soil solution. The water-soluble potassium is more readily available than the absorbed one. This means that the uptake of potassium is more difficult from clay soils than from sandy soils.
The adsorbed potassium is a very common ion in the adsorptive complex and can be easily replaced by other cations by substitution. The antagonism between K + and Na + , NH4 + , Ca2 + , Mg2 + has been observed so far, and therefore their relationship in the nutrient environment is very important from the point of view of mineral nutrition. Plants quickly absorb it through their leaves, as well. The inclusion of essential macronutrients in nutrition is the most effective method for improving the quality and producing biomass of seedlings and forest trees, which can significantly shorten the rotation period (I n ge sta d , 1973). A good supply of basic biogenic elements prevents the occurrence of chlorosis in the post-transplant period and affects better survival, physiological vitality, as well as further plant growth and development (D j u k i c et al., 2004).
In this paper, the analysis of the potassium concentrations was conducted in needles of 20 Douglas-fir provenances, originating from a number of North American sites of different geographical latitudes, longitudes and elevations. Table 1 shows geographical characteristics of the provenances from which the seeds originate. The seed collection covers a part of the Douglasfir natural range from New Mexico in the south, along the Pacific coast and the Rocky Mountains, to British Columbia in the north of America.

MATERIAL AND METHODS
The provenance test with 20 Douglas-fir provenances originating from North America was set on Mt. Juhor in central Serbia.
Two soil profiles were formed on the field plot for the purpose of soil analysis. The type of soil was determined, and the samples were taken for laboratory analyses. The plant-available potassium was determined by the Al method, according to Egner-Rihm (D ža m i ć et al., 1996). The textural composition was determined by the sedimentation using Na-pyrophosphate as a peptizing agent (R at z , 1971), while the active soil acidity and reserve soil acidity were determined using a potentiometer (Kn eževi ć, Ko š an i n , 2008).
Fresh Douglas-fir needles were sampled for the analysis of the potassium concentration at the site where the provenance test was set (L ava d i n ov i ć et al., 2018). The one-year-old needles from the upper part of the crown (the top third of the crown) were sampled and analyzed. The needles were dried to the absolutely dry state at 105°C, grounded to powder and burned in platinum vessels. The potassium concentration was determined by the ash analysis, after conversion to chlorides, using the flame photometric determination technique.
The deviation of potassium concentration in the needles of each Douglas-fir provenance was determined by the Z-test, according to the following formula: where: X̅ is the mean value of the potassium concentration in the needles of all Douglas-fir provenances; X is the potassium concentration in each Douglasfir provenance; σ is the standard deviation of the potassium concentration in the needles.
The correlation analysis was used to examine the dependence of the potassium concentration in the Douglas-fir needles and the mensurational parameters -mean diameter, mean height, basal area, volume and volume increment. The multiple regression analysis revealed the dependence of potassium concentration in the Douglas-fir needles, and geographical characteristics of the analyzed provenances (latitude, longitude and altitude) were used as independent variables. All statistical analyses were performed using the statistical software Statistica (StatSoft Inc., Tulsa, OK, 1997).

RESULTS AND DISCUSSION
The soil on which the provenance test was established is acid brown soil (dystric cambisol). Based on the textural composition, the soil in the first analyzed profile was sandy loam, and in the second one -loam in transition to sandy loam. The plant-available potassium in the humus-accumulative horizon is medium to good, while the cambic horizon is poor with this element in both profiles ( Table 2). The airy textural composition of the soil enables good nutrition with potassium, even if concentrations of this element are lower. In sandy soils plant-available potassium is in the form of easily soluble salts, while in clay soils it is adsorbed on the surface of clay minerals.
The potassium concentrations in the needles of the studied Douglas-fir provenances range from 0.83% (Oregon 205-38) to 1.40% . The average potassium concentration in the needles of the studied Douglas-fir provenances is 1.03%. The highest amount of potassium was found in the provenance "Oregon 205-14", which is the only provenance with the potassium concentration that significantly deviates from the average, i.e. where the Z-value is higher than the para1.96 standard deviations (Table 3; Graph 1). Since the differences in the concentrations of particular chemical elements in provenances may indicate their peculiarities in the adsorption of the elements, it can be concluded that provenance "Oregon 205-14" has a superior potassium

VARIABILITY OF POTASSIUM CONCENTRATION IN THE NEEDLES OF DOUGLAS-FIR PROVENANCES
absorption and accumulation capacity. Slightly higher amounts of potassium than the average value were found only in Oregon provenances . The lowest potassium concentration in the needles was also detected in Oregon provenances . In these provenances, the deviation of potassium concentration in the needles was not significantly different from the average value for the studied provenances, but it was larger than one standard deviation. All provenances originating from Washington and New Mexico had smaller potassium concentrations in the needles than the average value, while their deviation from the average concentration was less than one standard deviation. The measured concentrations in the needles were within the normal range, and therefore they do not indicate a deficiency or surplus of this nutrient. According to the presented results of the analysis of the variability of Douglas-fir provenances in terms of potassium concentration in needles, there was no significant statistical dependence between the mensurational parameters (mean diameter, mean height, basal area, volume and volume increment) and the potassium concentration in the Douglas-fir needles (Table 4).
Although potassium plays a very important role in various physiological processes in plant cells and tissues, the results showed that there was no significant correlation between its concentration in the needles and the analyzed mensurational parameters of trees. Considering that all studied provenances are located in small experimental fields, with uniform habitat conditions, and that plant-available potassium compounds in the soil are very common, it can be concluded that potassium concentration in the needles varies due to different genetic properties of some Douglas-fir provenances in terms of their adsorption of this element.
Based on the multiple correlation analysis where potassium concentration in Douglas-fir needles was the dependent variable and the independent variables were geographical characteristics of the provenance natural habitats -the following regression equation was obtained: K = -1,6639 -0,0023 GŠ + 0,0022GD + 0,0001NV where: K is the potassium percentage in the needles of the studied provenances; GŠ is the geographical altitude of the natural habitats of the studied provenances; GD is the geographical longitude of the natural habitats of the studied provenances; and NV is the altitude of natural habitats of the studied provenances.
Multiple correlation analysis has shown that there is no statistically significant dependence between potassium adsorption from soil and the geographical characteristics of different Douglasfir provenances (Table 5; Graph 2).
Further studies should determine whether there are positive correlations between potassium adsorption and other provenance properties,  was not due to the higher resistance to frost, but to drought in the frost period. L a rs e n (1983) studied the effect of nitrogen and potassium supply on drought resistance in 2-year-old Douglas-fir seedlings from Snoqualmie (Washington), which were grown under experimental conditions and treated by 11 different supply levels of these nutrients. He found that potassium had a very positive effect on drought resistance in the tested plants.
On the other hand, the experiment performed by T i mmi s (1974) on one-year-old Douglas-fir seedlings from the Pacific coast showed that seedlings free of nitrogen, but rich in phosphorus and potassium, were not cold-resistant enough to survive winter conditions even in their natural habitats. The results of this study indicate that the resistance of the tested plants to frost is more dependent on the relationships between the nutrients than on the availability of an individual nutrient.

CONCLUSIONS
The highest potassium concentration was found in provenance "Oregon 205-14" -the only provenance in which potassium concentration was significantly higher than the average value, so it can be characterized as a superior provenance for the uptake and accumulation of this element. Oregon provenances have a greater variability of potassium concentrations in needles than other provenances, considering that the lowest potassium content was also found in an Oregon provenance (Oregon 205-38). The multiple regression analysis showed no significant correlation between the geographical characteristics of the provenances.
Although potassium plays a significant role in various physiological processes, the obtained results show that there is no significant correlation Graph 2. Potassium content recorded in the needles of the analyzed Douglas-fir provenances and the potassium value calculated according to the multiple regression equation between the potassium concentration in Douglasfir needles and mensurational parameters (i.e. mean diameter, height, basal area, volume, and volume increment). This is probably the result of favorable conditions of nutrition with this element in the habitat where the provenance experiment was set. Further studies should show whether there are positive correlations between potassium concentrations and other provenance properties, such as physiological vitality, resistance to diseases, drought, low temperatures, air pollution and other environmental stressors. The existence of differences in the concentrations of particular chemical elements in provenances may indicate their peculiarity in the adsorption of these elements. This is important because potassium concentration in Douglas-fir tissues is one of the factors that affect its resistance to stress factors (such as cold and drought) and it could determine the future selective application of suitable provenances in certain habitats in forestry practice.

Note:
The research is completed within the projects "The development of technological processes in forestry in order to realize the optimal forest cover" (No. 31070) and "Studying climate change and its influence on the environment: impacts, adaptation and mitigation" (No. 43007), financed by the Ministry of Science and Technological Development of the Republic of Serbia within integrated and interdisciplinary researches for the period 2011-2019.