RESUMO
Paddy field farming remains the dominant form of growing rice in modern times as the rice is the staple food for over half the world's population and is closely associated with food security and political stability of many countries. Record increase in rice production have been observed since the start of the Green Revolution. India is one of the largest paddy producer and exporter in the world. However, constant use of chemical herbicide like paraquat had shown adverse impact on the rice yield. Non-target organisms of the habitat including cyanobacterial paddy biofertilizer face the herbicide toxicity and are unable to perform efficiently their role as biofertilizer. Therefore, in the present study, an attempt has been made to enhance the paraquat resistance in rice biofertilizer (Microchaete sp. NCCU-342) by exogenous addition of salicylic acid. Paraquat showed toxicity in Microchaete sp. NCCU-342 in a dose-dependent manner. Concentration of paraquat >1.0 µM exhibited lethal effect since the beginning. Through successive narrow range experiment, LD50 value of paraquat was obtained as 0.6 µM. Biomass exposed to paraquat (LD50 value) and salicylic acid (0.3 mM) showed mitigation in free radical production (2.20 % MDA and 1.69 % H2O2) and enhancement in the activity of the antioxidant enzymes activity, i.e. SOD, CAT, APX (137.76 %, 87.45 %, 118 %, respectively) and osmolytes (3.8 % proline and 21.51% sucrose). Thus, for sustainable agricultural practice, especially for paddy field cyanobacterial biofertilizer, application of salicylic acid or organism with higher salicylic acid production ability may be an alternative to overcome the paraquat toxicity.