## 1 Introduction  
Poland is characterised by a rich and high-quality raw material base, which makes it appreciated in the European Union countries. This is an ideal factor that creates the possibility of further development. Fruit production is of great importance to the agricultural economy. Apple trees (_Malus domestica_ Borkh.) are grown widely in different climate zones throughout the world, thus placing apples among the major fruits on the market [1]. Poland produces over 3.6 million tons of apples annually and ranks third in the world and first in Europe in terms of the production of these fruits [2]. All polyphenols have antioxidant properties. They scavenge free radicals and inhibit their production, stimulate the synthesis of antioxidant enzymes, and thus prevent oxidative stress from resulting in damage to the structural molecules of the body [3]. In this way, they help restore and maintain a favourable state of redox balance, whether in a plant cell or a human body [3]. The health effects of these compounds are dependent on their amount of daily intake and their bio-availability [4]. Several reports revealed that the plant material of apple trees is a rich source of phenolic compounds [1, 5, 6, 7, 8, 9, 10, 11, 12, 13]. For instance, [PERSON] et al. [5] reported high values of phenolic compounds in leaf extracts from thirteen cultivars. In extracts, the content of phenolic compounds was in the range of 54.68-106.81 mg/g of dried weight (DW), while phloridzin was found as a major component and had a concentration range of 46.43-98.51 mg/g DW [5]. Levels of individual phenolic compounds and total phenolics with diverse antioxidant properties vary in different parts of apple trees and also between apple cultivars [1; 14; 15]. Each plant accumulates polyphenols in all of its organs as protection against pests or UV radiation, but each morphological part of the plant contains a different amount of these compounds. Most of them are usually found in leaves, fruits, bark, flowers, and seeds. Research conducted by [PERSON] and [PERSON] [8] showed that leaves contain much more polyphenic compounds than fruits. Seven plants were analysed, among which quince leaves had the highest polyphenol content, containing over 4.5 times more of these compounds than quince fruit. Apple leaves also had seven times more polyphenols than apples fruit. The fruits contained only more procyanidins and anthocyanins, while the apple leaves contained almost 33 times more quercetin glycosides, 3.5 times more phenolic acids (acids: chlorogenic, neochlorogenic, cryptochlorogenic, and p-couamaric), and over 218 times more dihydrochalcones: phloretin and phloridzin (6331 mg/100 g of dry matter compared to less than 29 mg/100 g of dry matter in apple). The polyphenic concentration in fruits peaks early in the season and decreases during fruit development [16]. The most favourable time for harvesting apple leaves is determined by the activity of polyphenol oxidase, which increases at the end of September and reaches its maximum after the first frost. This enzyme breaks down the desired polyphenol compounds, and therefore, in order to obtain a raw material with the highest polyphenol content, the leaves should be collected from June to August. Then, apple leaves have the best composition, both in terms of the amount of phenolic compounds and other bioactive substances [17]. The influence of the external environment during plant cultivation also has a significant impact on the content of polyphenols in the plant raw material. Both unfavourable weather conditions and microbial attacks are stress factors for the plant, which, in response to them, increases the production of protective substances, i.e., phenolic compounds. Therefore, for example, excessive exposure to solar radiation or fungal infection increases the total polyphenol content [16; 18]. This is confirmed by the research of [PERSON] et al.