Nitric oxide stimulates digestion modifying the nutrient composition of the traps’ fluid of Nepenthes x ventrata

External digestion performed by autotrophs is a characteristic feature of carnivorous plants, such as those of the Nepenthes spp. These plants developed jug-shaped traps filled with digestive fluid that consists of water, various proteins (mostly enzymes), and nutrients. Moreover, the presence of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in the traps' fluid of N. ventrata has been demonstrated. RNS, among them nitric oxide (NO), accelerates digestion e.g. by the alteration of ROS levels. The aim of the study was to demonstrate the stimulation of external digestion by NO_x supplementation linked to the modulation of the nutrient composition of the trap fluid, digestive enzyme activity and gene transcription. Using the digestion fluid of N. ventrata mature traps we indicated that NO_x temporarily increases K, Fe, Cu and ammonia that may be involved in the modulation of free radicals content. The stimulatory effect of NO_x on the activities of enzymes responsible for digestion, and on the transcripts' levels of Nepenthesin I and II, Purple Acid Phosphatase, and S-like Ribonuclease was shown. The decrease in the level of carbonylated proteins (from food source) in the trap' fluid during digestion suggests their absorption by Nepenthes trap tissues. We also demonstrated the presence of carbonylated proteins in the trap fluid before feeding.