Radiation-mediated molecular weight reduction and structural modification in carrageenan potentiates improved photosynthesis and secondary metabolism in peppermint (Mentha piperita L.)

In an attempt to gain insights into the possible relationship between the irradiation-mediated molecular weight reduction and structural modification and the growth-promotion activity, characterization of the polysaccharide before and after irradiation was carried out through Fourier Transform Infrared (FT–IR), Ultraviolet-visible (UV–vis) and Nuclear Magnetic Resonance (NMR) spectroscopic studies. Moreover, graded concentrations of irradiated carrageenan (IC) were applied through foliage to assess the performance of peppermint (Mentha piperita L.). Among the various concentrations of IC [0 (control), un-irradiated carrageenan (UC), 40, 80, 120, 160 and 200 mg L−1], the effect of 80 mg L−1 IC established to be most favorable for most of the parameters studied. Rubisco and phenylalanine ammonia lyase activities were maximally enhanced by 65.9% and 35.6% by the application of 80 mg L−1 IC, respectively; as compared to the control and UC. A maximum enrichment in the content (32.8%) and yield (88.3%) of essential oil was noted by the application of 80 mg L−1 IC, respectively. Results of the gas chromatography revealed that the contents of menthol and 1, 8-cineole were increased; however, menthone and menthyl-acetate contents were decreased by the application of IC over the control and UC.