Insights into physiological and metabolic modulations instigated by exogenous sodium nitroprusside and spermidine reveals drought tolerance in Helianthus annuus L.

Drought is the most critical climatic factor instigating severe threats to crop production worldwide. As stress ameliorants, exogenous sodium nitroprusside (SNP) or spermidine (Spd) supply has positive responses in alleviating the drought adversities in crops, however, reports regarding their combined effects is still elusive. Here, the protective role of SNP and Spd to confer drought resistance in sunflower (Helianthus annuus L.) through up-regulation of physiological and metabolic processes was investigated. Plants were foliar sprayed with individual or combined SNP (100 μM) or Spd (100 μM). Drought was induced by keeping the soil at 100% (normal) and 60% (drought stress) field capacity levels. Drought exposure caused a marked decline in relative water content (RWC), excised leaf water retention (ELWR), net photosynthesis (P), transpiration rate (E), stomatal conductance (gs), and sub-stomatal conductance (Ci) with substantial increase in catalase (CAT), superoxide dismutase (SOD), and peroxidase (POX). SNP plus Spd exhibited a considerable increase in CAT, SOD, and POX activities under drought, and helped the plants to retain optimum water status and gas exchange attributes. Similarly, hydrogen peroxide (HO) and malondialdehyde (MDA) contents were increased significantly to drought; however, a notable decline was recorded in drought prone plants treated with exogenous SNP plus Spd. Moreover, addition of SNP plus Spd under drought caused a remarkable increase in chlorophyll a (Chl a), chlorophyll b (Chl b), chlorophyll total (Chl t), carotenoids (Car), and growth traits like shoot length (SL), root length (RL), shoot fresh weight (SFW), shoot dry weight (SDW), root dry weight (RDW). Combined SNP and Spd application could potentially alleviate the drought-induced damages in sunflower through increased water status (8-10%), antioxidant enzymes (17-28%), chlorophyll pigments (14-21%), and growth performance (12-22%) under drought stress.