Supplemental Sodium Nitroprusside and Spermidine Regulate Water Balance and Chlorophyll Pigments to Improve Sunflower Yield under Terminal Drought

Abstract

Drought is an inevitable environmental stress that drastically hampers the growth, productivity, and quality of food crops. Exogenous sodium nitroprusside and spermidine have decisive functions in the growth enhancement of plants; nevertheless, their specific role in mediating stress responses to improve drought tolerance in sunflowers at the reproductive stage (terminal drought) remains largely unknown. In the present study, we explored the positive effects of sodium nitroprusside and spermidine on physiological responses to increase in sunflower yield during periods of terminal drought. Initially, various doses (50, 100, 150, 200, 400 μM) for each sodium nitroprusside or spermidine were foliar sprayed to improve water content, chlorophylls, and biomass accumulation in sunflower seedlings under control (100% FC) and drought (60% FC) conditions. Optimized rates (100 μM for sodium nitroprusside) and (100 μM for spermidine) were further tested alone and in combination to assess drought tolerance potential and their ultimate impact on yield under drought stress. Drought exposure caused a marked reduction in relative water content (26%) and chlorophyll a (31%) and b (35%) contents; however, sodium nitroprusside and spermidine at 100 μM significantly improved the growth of sunflower (13%). Furthermore, combined use of sodium nitroprusside and spermidine at 100 + 100 μM markedly improved the achenes per head (16%), 1000-achene weight (14%), and ultimately grain (28%) and oil (21%) yields of sunflowers under drought stress. A strong association was found between the 1000-achene weight and the achene yield of sunflower. Hence, combined sodium nitroprusside and spermidine upregulate water balance and chlorophyll contents to increase sunflower yield under terminal drought.

Figure 1

Relative water content (a, b) of 28-day-old sunflower plants as affected by sodium nitroprusside (SNP) and spermidine (Spd) grown under control and drought conditions. The values are means ± SE (n = 3), and sharing different letter(s) indicates the significant differences following Tukey’s HSD test (P < 0.05). Drought stress: 100 and 60% FC; SNP: 50, 100, 150, 200, 400 μM; Spd: 50, 100, 150, 200, 400 μM.

Figure 2

Chlorophyll a (a, b) and chlorophyll b (c, d) contents of 28-day-old sunflower plants as affected by sodium nitroprusside (SNP) (a, c) and spermidine (Spd) (b, d) grown under control and drought conditions. The values are means ± SE (n = 3), and sharing different letter(s) indicates the significant differences following Tukey’s HSD test (P < 0.05). Drought stress: 100 and 60% FC; SNP: 50, 100, 150, 200, 400 μM; Spd: 50, 100, 150, 200, 400 μM.

Figure 3

Shoot length (a, b) and root length (c, d) of 28-day-old sunflower plants as affected by sodium nitroprusside (SNP) and spermidine (Spd) grown under control and drought conditions. The values are means ± SE (n = 3), and sharing different letter(s) indicates the significant differences following Tukey’s HSD test (P < 0.05). Drought stress: 100 and 60% FC; SNP: 50, 100, 150, 200, 400 μM; Spd: 50, 100, 150, 200, 400 μM.

Figure 4

Shoot fresh weight (a, b), shoot dry weight (c, d), and root dry weights (e, f) of 28-day-old sunflower plants as affected by sodium nitroprusside (SNP) and spermidine (Spd) grown under control and drought conditions. The values are means ± SE (n = 3), and sharing different letter(s) indicates the significant differences following Tukey’s HSD test (P < 0.05). Drought stress: 100 and 60% FC; SNP: 50, 100, 150, 200, 400 μM; Spd: 50, 100, 150, 200, 400 μM.

Figure 5

Clustered heat map of relative water content, chlorophyll pigments, and growth traits of sunflower following sodium nitroprusside (SNP) and spermidine (Spd) application. The change in colors indicates the significant differences at P < 0.05. RWC, relative water content; Chl. a, chlorophyll a; Chl. b, chlorophyll b; SL, shoot length; RL, root length; SFW, shoot fresh weight; SDW, shoot dry weight; RDW, root dry weight.

Figure 6

Plant height (a), head diameter (b), achenes/head (c), and 1000-achene weight (d) of sunflower plants as affected by sodium nitroprusside (SNP) and spermidine (Spd) grown under control and drought conditions. The values are means ± SE (n = 3), and sharing different letter(s) indicates the significant differences following Tukey’s HSD test (P < 0.05). Drought stress: control and drought at reproductive stage; SNP: 100 μM; Spd: 100 μM.

Figure 7

Achene yield (a), biological yield (b), oil yield (c), and harvest index (d) of sunflower plants as affected by sodium nitroprusside (SNP) and spermidine (Spd) grown under control and drought conditions. The values are means ± SE (n = 3), and sharing different letter(s) indicates the significant differences following Tukey’s HSD test (P < 0.05). Drought stress: control and drought at reproductive stage; SNP: 100 μM; Spd: 100 μM.

Figure 8

Pearson correlation coefficient (r) with density and frequency distribution, and values comprising significance (*) of correlation coefficients (r) for yield and yield traits of sunflower following sodium nitroprusside (SNP) and spermidine (Spd) application. PH, plant height; HD, head diameter; A/H, achenes/head; TAW, 1000-achene weight; AY, achene yield; BY, biological yield; OY, oil yield; HI, harvest index.