Organic fragments of k-carrageenan, lipids and peptides plus K-rich inorganic fraction in Kappaphycus alvarezii biomass are responsible for growth stimulus in rice plant when applied both foliar and root pathway

Abstract

Kappaphycus alvarezii seaweeds regulate plant growth and development by modifying various metabolic functions. However, the substances responsible for the effects and relationship with the application pathway are poorly understood. This study aimed to structurally characterize biomass fractions and to evaluate the structure-bioativity relationship of different concentrations applied to the leaves and roots of rice plants (Oryza sativa L.). The aqueous extracts obtained from K. alvarezii were characterized by ATR-FTIR and 13C NMR CP/MAS, and their structures were compared by chemometrics. The composition was determined by CHN elemental analysis and atomic absorption. Bioactivity in plants was evaluated by foliar application (5 %, 10 %, 15 %, 20 % and 25 %) and root application (1 %, 2 %, 3 %, 5 % and 10 %), and the dry and fresh mass and root morphology were determined. The results show that the aqueous extract (KAlv-sapL) contains CAlkyl (26 %), CAlkyl-O,N (15 %), CAlkyl-O (23 %), CAlkyl-di-O (7 %), CAromatic-H,R (6 %), CAromatic-O,N (3 %), CCOOH-H,R (17 %) and CCO (3 %), belonging to fragments of k-carrageenan, lipids, and peptides; high amounts of K, P, Ca, Mg, Fe, Mn, Zn and Cu. KAlv-sapL applied via foliar treatment at 5 % and 10 % promoted increases in root and leaf dry mass by >30 % and 10 %, respectively, and 10 % of KAlv-sapL increased the area, diameter and number of roots by 11 %, 19 % and 40 %, respectively. Root application at 3 % (KAlv-sapL) stimulated the production of fresh and dry root mass by 60 % and 28 %, respectively, and when applied at 2 %, the root area, length, number and volume increased by 30 %, 50 %, 19 % and 31 %, respectively. In addition, the application at 2 % (KAlv-sapL) promoted intense mobilization and allocation of K+ in the leaves, increasing the uptake efficiency of this nutrient. The bioactivity of Kappaphycus seaweed occurs through joint action of carrageenan, lipids and peptides, and the inorganic fraction, rich in K+ and essential nutrients.