## 1 Introduction  
_Hylocreus_ ssp. are perennial succulents, growing as epiphytes. The genus originates from tropical and subtropical forests of Mexico and other areas within Central and South America (e.g., Guatema, Salvador, Panama, Costarica, Colombia or Brazil). From its native localities the plant has been spread to Near East and Australia. Nowadays, the industrial plantations of pitaya are mostly located in Asian countries, namely, Thailand, Malesia and Taiwan. The pitaya is a cactus genus characterized by long triangular green stems, equipped with thorns on the edges. The fruit is an elongated berry. The peel's characteristic feature is intense dark pink or reddish purple coloration with green scales resembling leaves. Pitaya fruit is mostly used for consumption and therefore the plant is cultivated for commercial purposes. It is a rich source both of vitamins (C--20.5-33 mg/100 g pulp, B3--0.2-2.8 mg/100 g pulp) and dietary fibre (69 g/100 g of dried pulp). Its pH value reaches 4.4-5.1. The predominating acid is malic acid [1; 2]. Full production of raw material falls on the fifth year of land use. It is possible to incessantly obtain fruit for 10-15 years. Depending on the region of cultivation, the yield from 1 ha plantation can reach as much as 16-80 tons of fruit annually [2; 3; 4; 5; 6; 7; 8; 9]. Plantlets for establishing a plantation can be acquired on generative way from seeds and vegetatively from in vivo or in vitro cultures. The seedlings raised from seeds grow slower and need a longer time to reach the reproductive phase, while the individual specimens obtainedare diverse. The second method, which is easier and cheaper, is vegetative propagation by cuttings. It consists in in vivo cutting of plants to acquire 15-60 cm-long segments (the longer the fragment the faster the regeneration time). Whereas using in vitro culture, one can accelerate the process of vegetative propagation and growth of the dragon fruit plants. Thanks to this method it is possible to acquire healthy pathogen-free plant material in a very short time [3; 5; 7]. So far research with the use of plant in vitro cultures in this species concentrated on acquisition of effective micropropagation and acclimatization [9]. From the perspective of different branches of bio-economy, modification of metabolic paths in plants finds a wide array of applications. Alterations to the chemical composition can be induced under in vivo and in vitro conditions. Increased production by plants of biologically active compounds can be applicable in industry. In the tissues of many _Hylocreus_ species it was possible to enhance the production of betacyans through triggering auto-polylploidization and also by using tyrosine or methyl jasmonate, whereas the synthesis of betlains was intensified with the use of elicitors, tyrosine or leucine as well as when raising the plants in red light conditions Table S1 [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. It was not only in species falling within the genus _Hylocreus_ that synthesis of valuable chemical compounds was effectively enhanced. Table S1 provides examples of species representing different plant genera and families for which such modifications have been reported [23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44]. Besides, in species belonging to the genus pitaya, namely _H. monacanthus_, _H. undatus_ and _Hylocreus polynrhizus_, as well as in representatives of other plant genera, such as _Andrographs paniculata_, _Phoenix dactylifera_ L. or _Artemisia argyi_, intensified production of flavonoids was successfully initiated Table S1. Consequences of the presence of plant growth regulators (PGR) and their possible impact on changes in the synthesis of volatile compounds in pitaya stems have not been analyzed as yet. So far modification to the profile of volatile compounds with the use of plant hormones was successfully performed in a few plant species representing other genera. Application of jasmonic acid in the narrow-leaved lvender (_L.