Putrescine attenuates heat stress by modulating membrane stability, antioxidant activity, and gaseous exchange in Brassica juncea L.

Rising temperatures severely stress plant growth and development by disrupting metabolic processes. Indian mustard (Brassica juncea L.) faces extreme temperatures during various growth phases. Early-sown Brassica encounter heat stress during germination and vegetative phases, while late-sown suffer higher yield losses due to increased evaporative demand during the reproductive stage, leading to faster maturation, increased senescence, and reduced productivity. Polyamines (PAs), small positively charged nitrogenous compounds, play important roles in plant growth and stress responses. Putrescine (Put), a key PA, is known to enhance plant growth and provide tolerance from stresses. This research aimed to explore how Put (1 mM) mitigates heat stress effects on early, timely, and late-sown genotypes RH-1707, RH-1708 (heat-sensitive), and RH-1566 and RH-1999–42 (heat-tolerant). RH-1999–42 showed the highest tolerance under early heat stress, while RH-1708 was the most sensitive. Conversely, RH-1707 experienced significant losses under late heat stress, whereas RH-1566 displayed tolerance under both early and late heat stress.Exogenous application of Put alleviated heat stress by reducing lipid peroxidation, hydrogen peroxide, electrolyte leakage, transpiration rate, stomatal conductance, and stomatal density, while boosting antioxidant enzyme activities, chlorophyll content, relative water content, proline content and overall yield. The most significant benefits were observed in RH-1707 and RH-1708, which were higher susceptible to cellular damage under heat stress. Anatomical examinations revealed enlarged xylem vessels in RH-1707 and RH-1708 under early sowing, while late-sown plants showed less developed vessels. Put application partially reduced vessel size under early heat stress, though its effects were minimal during late heat stress. This study demonstrated that 1 mM Put foliar spray effectively mitigates the harmful effects of heat stress at early and late-sown stages in Brassica juncea L.