Exogenously applied plant growth regulator protects rice from heat-induced damage by modulating plant defense mechanism

This study was undertaken to investigate the effect of biostimulants such as spermidine (1 mM, 2 mM), indole-3-acetic acid (10 –3 M, 10 –5 M), brassinolide (1 mg/L, 2 mg/L) and boron (50 mg/L, 100 mg/L) on protecting rice plants from heat-induced damage. Two rice genotypes, Nagina 22 (heat tolerant) and Manu Ratna (heat susceptible), primed with biostimulant solution were germinated, 14-day-old seedlings were further foliar sprayed with respective treatment solutions prior to the induction of heat stress (42 °C, 48 h). Heat stress-induced fatal damages to non-biostimulant-treated rice seedlings, whereas biostimulant-treated ones were protected from heat stress-induced damages, marked by lower levels of malondialdehyde (MDA), H 2 O 2 coupled with increased levels of seedling vigour index (SVI), chlorophyll and proline contents. Results showed that 1 mM spermidine (Spd) and 10 –5 M indole-3-acetic acid (IAA)-treated rice seedlings showed increased heat tolerance and were evaluated for its effect at reproductive stage stress in a pot culture study. At the panicle initiation stage, plants were pretreated with 1 mM Spd and 10 –5 M IAA followed by the induction of heat stress, keeping it in a temperature-controlled polyhouse (42.1 °C, till the time of harvest). Spd- and IAA-pretreated heat-exposed plants exhibited yield improvement over control by altering physiological, biochemical and molecular mechanisms. Therefore, we conclude from our results that foliar IAA or Spd applications can be regarded as an agronomic strategy for mitigating the adverse effects of heat stress conditions on rice plants.