Physiological and Molecular Responses of the Flag Leaf Under L-Phenylalanine Ammonia-Lyase Inhibition

The increasing challenge posed by climate change necessitates a deeper understanding of the plant metabolic pathways that influence productivity under varying environmental conditions. In this context, investigating the impact of L-phenylalanine ammonia-lyase (PAL) inhibition on flag leaf physiology in winter triticale provides valuable insights into mechanisms that may affect crop productivity. This study investigated the physiological and molecular responses of the flag leaf following the application of 4-hydroxybenzoic acid hydrazide (HBH), a PAL inhibitor. It was hypothesized that PAL inhibition would redirect carbon flow towards carbohydrate synthesis at the expense of phenolic production, accompanied by alterations in photosynthetic performance and antioxidant responses. HBH was detected in flag leaf tissues and caused a significant reduction in phenolic content, along with a marked increase in soluble carbohydrate levels. HBH treatment strongly activated the antioxidant system, as evidenced by elevated levels of non-enzymatic antioxidants and the increased activity of antioxidant enzymes. Changes were also observed in chlorophyll fluorescence parameters and in the accumulation of proteins associated with CO₂ fixation and the photosynthetic apparatus function. These findings demonstrate that PAL inhibition can substantially affect the redox balance and photosynthetic activity of the flag leaf during a critical period of plant development and yield formation.